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9 Iron SUMMARY Iron functions as a component of a number of proteins, including enzymes and hemoglobin, the latter being important for the transport of oxygen to tissues throughout the body for metabolism. Factorial modeling was used to determine the Estimated Average Require- ment (EAR) for iron. The components of iron requirement used as factors in the modeling include basal iron losses, menstrual losses, fetal requirements in pregnancy, increased requirement during growth for the expansion of blood volume, and/or increased tissue and storage iron. The Recommended Dietary Allowance (RDA) for all age groups of men and postmenopausal women is 8 mg/day; the RDA for premenopausal women is 18 mg/day. The median dietary intake of iron is approximately 16 to 18 mg/day for men and 12 mg/day for women. The Tolerable Upper Intake Level (UL) for adults is 45 mg/day of iron, a level based on gastrointestinal distress as an adverse effect. BACKGROUND INFORMATION Almost two-thirds of iron in the body is found in hemoglobin present in circulating erythrocytes. A readily mobilizable iron store contains another 25 percent. Most of the remaining 15 percent is in the myoglobin of muscle tissue and a variety of enzymes necessary for oxidative metabolism and many other functions in all cells. A 75-kg adult man contains about 4 grams of iron (50 mg/kg) while a 290
IRON 291 menstruating woman has about 40 mg/kg of iron because of her smaller erythrocyte mass and iron store (Bothwell et al., 1979). Function Iron can exist in oxidation states ranging from â2 to +6. In biolog- ical systems, these oxidation states occur primarily as the ferrous (+2), ferric (+3), and ferryl (+4) states. The interconversion of iron oxidation states is a mechanism whereby iron participates in elec- tron transfer, as well as a mechanism whereby iron can reversibly bind ligands. The common biological ligands for iron are oxygen, nitrogen, and sulfur atoms. Four major classes of iron-containing proteins exist in the mammalian system: iron-containing heme proteins (hemoglobin, myoglobin, cytochromes), iron-sulfur enzymes (flavoproteins, heme- flavoproteins), proteins for iron storage and transport (transferrin, lactoferrin, ferritin, hemosiderin), and other iron-containing or activated enzymes (sulfur, nonheme enzymes). In iron sulfur enzymes, iron is bound to sulfur in one of four possible arrangements (Fe-S, 2Fe-2S, 4Fe-4S, 3Fe-4S proteins). In heme proteins, iron is bound to porphyrin ring structures with various side chains. In humans, the predominant form of heme is protoporphyrin-IX. Hemoglobin The movement of oxygen from the environment to the tissues is one of the key functions of iron. Oxygen is bound to an iron- containing porphyrin ring, either as part of the prosthetic group of hemoglobin within erythrocytes or as part of myoglobin as the facil- itator of oxygen diffusion in tissues. Myoglobin Myoglobin is located in the cytoplasm of muscle cells and increases the rate of diffusion of oxygen from capillary erythrocytes to the cytoplasm and mitochondria. The concentration of myoglobin in muscle is drastically reduced in tissue iron deficiency, thus limiting the rate of diffusion of oxygen from erythrocytes to mitochondria (Dallman, 1986a). Cytochromes The cytochromes contain heme as the active site with the iron-
292 DIETARY REFERENCE INTAKES containing porphyrin ring functioning to reduce ferric iron to ferrous iron. Cytochromes act as electron carriers. The 40 different proteins that constitute the respiratory chain contain six different heme proteins, six with iron sulfur centers, two with copper centers, and ubiquinone to connect nicotinamide adenine dinucleotide hydride to oxygen. Physiology of Absorption, Metabolism, and Excretion Absorption The iron content of the body is highly conserved. In the absence of bleeding (including menstruation) or pregnancy, only a small quantity is lost each day (Bothwell et al., 1979). Adult men need to absorb only about 1 mg/day to maintain iron balance. The average requirement for menstruating women is somewhat higher, approxi- mately 1.5 mg/day. There is, however, a marked interindividual variation in menstrual losses, and a small proportion of women must absorb as much as 3.4 mg/day. Towards the end of pregnancy, the absorption of 4 to 5 mg/day is necessary to preserve iron balance. Requirements are also higher in childhood, particularly during periods of rapid growth in early childhood (6 to 24 months), and adolescence. In the face of these varying requirements, iron balance is main- tained by the regulation of absorption in the upper small intestine (Bothwell et al., 1979). There are two pathways for the absorption of iron in humans. One mediates the uptake of the small quantity of heme iron derived primarily from hemoglobin and myoglobin in meat. The other allows for the absorption of nonheme iron, prima- rily as iron salts, that can be extracted from plant and dairy foods and rendered soluble in the lumen of the stomach and duodenum. Absorption of nonheme iron is enhanced by substances, such as ascorbic acid, that form low molecular weight iron chelates. Most of the iron consumed by humans is in the latter nonheme form. Heme iron is highly bioavailable and little affected by dietary fac- tors. Nonheme iron absorption depends on the solubilization of predominately ferric food iron in the acid milieu of the stomach (Raja et al., 1987; Wollenberg and Rummel, 1987) and reduction to the ferrous form by compounds such as ascorbic acid or a ferri- reductase present at the musosal surfaces of cells in the duodenum (Han et al., 1995; Raja et al., 1993). This bioavailable iron is then absorbed in a three-step process in which the iron is taken up by the enterocytes across the cellular apical membrane by an energy-
IRON 293 dependent, carrier-mediated process (Muir and Hopfer, 1985; Simpson et al., 1986), transported intracellularly, and transferred across the basolateral membrane into the plasma. The duodenal mucosal cells involved in iron absorption are formed in the crypts of Lieberkuhn. They then migrate up the villi becoming functional iron-absorbing cells only when they reach the tips of the villi. After a brief period of functionality, the cells are shed into the lumen together with iron that had entered the cell but had not been transferred to the plasma. In humans, mucosal cell turnover takes between 48 and 72 hours. Cells are programmed to regulate iron absorption when they reach tips of the villi by the amount of iron that they acquire from plasma during their early development. Recent studies by Cannone-Hergaux and coworkers (1999) strongly suggest that a metal transporter (divalent metal transporter [DMT-1] protein), which is a transmembrane protein and an isoform of natural resistance associated macrophage protein (NRAMP2), mediates the uptake of elemental iron into the duodenal cells. The quantity of this transport protein that is formed is inversely proportional to the iron content of the cell; synthesis is regulated by posttranscriptional modification of the DMT-1 mes- senger ribonucleic acid (mRNA) (Conrad and Umbreit, 2000). The regulatory mechanism involves the cellular iron response proteins (IRP) and the iron response element (IRE) on the mRNA (Eisenstein, 2000). The mechanism by which iron is transported through the entero- cyte has not been completely elucidated. Absorbed iron in the intra- cellular âlabile iron poolâ is delivered to the basolateral surface of enterocytes, becomes available for binding onto transferrin, and is then transported via transferrin in the plasma to all body cells. Ceruloplasmin, a copper-containing protein, facilitates the binding of ferric iron to transferrin via ferroxidase activity at the basolateral membrane (Osaki et al., 1966; Wollenberg et al., 1990). Heme is soluble in an alkaline environment and is less affected by intraluminal factors that influence nonheme iron uptake. Specific transporters exist for heme on the surface of rat enterocytes (Conrad et al., 1967; Grasbeck et al., 1982); however, rats do not absorb heme iron as efficiently as do humans (Weintraub et al., 1965). To date, no specific receptor/transporter for heme has been identified in humans. After binding to its receptor, the heme molecule is internalized and degraded to iron, carbon monoxide, and bilirubin IXa by the enzyme heme oxygenase (Bjorn-Rasmussen et al., 1974; Raffin et al., 1974). This enzyme is induced by iron deficiency (Raffin et al., 1974). It is thought that the iron that is liberated from
294 DIETARY REFERENCE INTAKES heme enters the common intracellular (enterocyte) pool of iron before being transported to plasma transferrin. Transport and Metabolism Iron movement between cells is primarily conducted via revers- ible binding of iron to the transport protein, transferrin. One atom of iron can bind to each of two binding sites on transferrin and will then complex with a highly specific transferrin receptor (TfR) located on the plasma membrane surfaces of cells. Internalization of transferrin in clathrin-coated pits results in an endosomal vesicle where acidification to a pH of approximately 5.5 results in the re- lease of the iron from transferrin. The movement of iron from this endosomal space to the cytoplasm is not completely understood at this time, but recent discoveries provide some clues. DMT1 (NRAMP2) has now been identified in endosomal vesicles (Gunshin et al., 1997). Although it is not a specific iron transporter and although it is capable of transporting other divalent metals, recent studies suggest that it may play a primary role in the delivery of iron to the cell. A second transporter, stimulator of iron transport (SFT), has been cloned and characterized as an exclusive iron transporter of both ferric and ferrous iron out of the endosome (Gutierrez et al., 1997). Iron entering cells may be incorporated into functional com- pounds, stored as ferritin, or used to regulate future cellular iron metabolism by modifying the activity of the two IRPs. The size of the intracellular iron pool plays a clear regulatory role in the syn- thesis of iron storage, iron transport, and iron metabolism proteins through an elegant posttranscriptional set of events (see review by Eisenstein and Blemings, 1998). Storage Intracellular iron availability is regulated by the increased expres- sion of cellular TfR concentration by iron-deficient cells and increased ferritin production when the iron supply exceeds the cellâs functional needs. Iron is stored in the form of ferritin or hemosiderin. The latter is a water-insoluble degradation product of ferritin. The iron content of hemosiderin is variable but generally higher than that of ferritin. While all cells are capable of storing iron, the cells of the liver, spleen, and bone marrow are the primary iron storage sites in humans.
IRON 295 Excretion In the absence of bleeding (including menstruation) or preg- nancy, only a small quantity of iron is lost each day (Bothwell et al., 1979). Body iron is therefore highly conserved. Daily basal iron losses are limited to between 0.90 and 1.02 mg/day in nonmenstru- ating women (Green et al., 1968). The majority of absorbed iron is lost in the feces. Daily iron losses from urine, gastrointestinal tract, and skin are approximately 0.08, 0.6, and 0.2 to 0.3 mg/day, respec- tively. These basal losses may drop to 0.5 mg/day in iron deficiency and may be as high as 2 mg/day in iron overload (Bothwell et al., 1979). Menstrual iron losses are quite variable. Studies on Swedish and British women demonstrated a mean iron loss via menses of 0.6 to 0.7 mg/day (Hallberg et al., 1966b). Clinical Effects of Inadequate Intake Important subclinical and clinical consequences of iron deficiency are impaired physical work performance, developmental delay, cog- nitive impairment, and adverse pregnancy outcomes. Several other clinical consequences have also been described. The bulk of exper- imental and epidemiological evidence in humans suggests that func- tional consequences of iron deficiency (related both to anemia and tissue iron concentration) occur only when iron deficiency is of a severity sufficient to cause a measurable decrease in hemoglobin concentration. Once the degree of iron deficiency is sufficiently severe to cause anemia, functional disabilities become evident. It is difficult to de- termine whether any particular functional abnormality is a specific consequence of the anemia per se, presumably due to impaired oxygen delivery, or the result of concomitant tissue iron deficiency. However, it has been shown that anemia and tissue iron deficiency exert independent effects on skeletal muscle (Davies et al., 1984; Finch et al., 1976). Anemia primarily affects maximal oxygen con- sumption. Endurance exercise is markedly impaired by intracellular iron deficiency in the muscle cells (Willis et al., 1988). From a prac- tical point of view, the distinction may be relatively unimportant since anemia and tissue iron deficiency develop simultaneously in humans who suffer from nutritional iron deficiency. Work Performance Various factors may contribute to impaired work performance
296 DIETARY REFERENCE INTAKES with iron deficiency. It has been shown that anemia and tissue iron deficiency exert independent effects on the function of organs such as skeletal muscle (Davies et al., 1984; Finch et al., 1976). Anemia primarily affects maximal oxygen consumption. Mild anemia reduc- es performance during brief but intense exercise (Viteri and Torun, 1974) because of the impaired capacity of skeletal muscle for oxida- tive metabolism. Endurance exercise is more markedly impaired by intracellular iron deficiency in skeletal muscle cells (Willis et al., 1988). In laboratory animals, the depletion of oxidative enzymes in skel- etal muscle occurs more gradually than the development of anemia (Dallman et al., 1982). The significant decrease in myoglobin and other iron-containing proteins in skeletal muscle of laboratory animals contributes significantly to the decline in muscle aerobic capacity in iron-deficiency anemia and may be a more important factor con- tributing to the limitation in endurance capacity (Dallman, 1986a; Siimes et al., 1980a). One study used 31P nuclear magnetic resonance spectroscopy to examine the functional state of bioenergetics in iron-deficient and iron-replete rat gastrocnemius muscle at rest and during 10 seconds of contraction (Thompson et al., 1993). Compared to controls, muscle from iron-deficient animals had a marked increase in muscle phosphocreatine breakdown and a decrease in pH and a slower recovery of phosphocreatine and inorganic phosphate concentra- tions after exercise. During repletion for 2 to 7 days with iron dextran, there was no substantial improvement in these indicators of muscle mitochondrial energetics. These authors concluded that âtissue factorsâ such as reduced mitochondrial enzyme activity, de- creased number of mitochondria, and altered morphology of the mitochondria might be responsible for impaired muscle function. Cognitive Development and Intellectual Performance Studies of iron deficiency anemia and behavior in the developing human and in animal models suggest persistent functional changes. Investigators have demonstrated lower mental and motor test scores and behavioral alterations in infants with iron deficiency anemia (Idjradinata and Pollitt, 1993; Lozoff et al., 1982a, 1982b, 1985, 1987, 1996; Nokes et al., 1998; Walter et al., 1989). In studies con- ducted in Guatemala and Costa Rica, infants with iron deficiency anemia were rated as more wary and hesitant and maintained closer proximity to caregivers (Lozoff et al., 1985, 1986). Several studies have shown an improvement in either motor or
IRON 297 cognitive development according to Bayleyâs scale of mental devel- opment after iron treatment of iron-deficient infants (Idjradinata and Pollitt, 1993; Lozoff et al., 1987; Oski et al., 1983; Walter et al., 1983). Other studies have failed to show an improvement in either motor or cognitive development scores after providing iron supple- ments to iron-deficient infants (Lozoff et al., 1982a, 1982b, 1987, 1996; Walter et al., 1989). Lower arithmetic and writing scores, poorer motor functioning, and impaired cognitive processes (mem- ory and selective recall) have been documented in children who were anemic during infancy and were treated with iron (Lozoff et al., 1991, 2000). Specific central nervous system processes (e.g., slower nerve con- duction and impaired memory) appear to remain despite correc- tion of the iron deficiency anemia. There is a general lack of speci- ficity of effect and of information about which brain regions are adversely affected. Recent data from Chile showed a decreased nerve conduction velocity in response to an auditory signal in for- merly iron-deficient anemic children despite hematologic repletion with oral iron therapy (Roncagliolo et al., 1998). This is strongly suggestive evidence for decreased myelination of nerve fibers, though other explanations could also exist. Current thinking about the impact of early iron deficiency ane- mia attributes some role for âfunctional isolation,â a paradigm in which the normal interaction between stimulation and learning from the physical and social environment is altered (Pollitt et al., 1993; Strupp and Levitsky, 1995). Adverse Pregnancy Outcomes Increased perinatal maternal mortality is associated with anemia in women when the anemia is severe (hemoglobin < 40 g/L) (Allen, 1997, 2000; WHO, 1992; Williams and Wheby, 1992). However, even moderate anemia (hemoglobin < 80 g/L) has been associated with a two-fold risk of maternal death (Butler and Bonham, 1963). The mechanisms associated with higher mortality of anemic women are not well understood. Heart failure, hemorrhage, and infection have been identified as possible causes (Fleming, 1968; Taylor et al., 1982). Several large epidemiological studies have demonstrated that ma- ternal anemia is associated with premature delivery, low birth weight, and increased perinatal infant mortality (see Table 9-1) (Allen, 1997; Garn et al., 1981; Klebanoff et al., 1991; Lieberman et al., 1988; Murphy et al., 1986; Williams and Wheby, 1992). Some of
298 DIETARY REFERENCE INTAKES TABLE 9-1 Association of Anemia and Iron Deficiency with Inadequate Weight Gain and Pregnancy Outcome Anemiaa Causes Other Than Iron Iron Outcome Total Deficiency Deficiency No Anemia Low birth weight Unadjusted, %b 17.1 25.9 15.9 12.2 AOR c 1.55 3.10 1.34 1.00 95% confidence interval 0.96â2.51 1.16â4.39 0.80â2.22 â Preterm delivery Unadjusted, % 26.2 44.4 23.5 18.4 AOR c 1.30 2.66 1.16 1.00 95% confidence interval 0.86â2.24 1.15â6.17 0.76â1.79 â Small for gestational age Unadjusted, % 11.1 8.3 11.5 7.5 AOR d 1.66 1.24 1.67 1.00 95% confidence interval 0.90â3.04 0.29â6.94 0.90â3.41 â Inadequate weight gain Unadjusted, % 31.0 40.0 29.9 24.6 AOR e 1.62 2.67 1.51 1.00 95% confidence interval 1.10â2.36 1.13â6.30 1.02â2.25 â a Anemia is defined as a hemoglobin concentration < 110 g/L (first trimester), < 105 g/ L (second trimester), < 110 g/L (third trimester), and a serum ferritin concentration < 12 Âµg/L (CDC, 1989; IOM, 1990). b Percent of anemic women at entry into study. c AOR = adjusted odds ratio. Adjusted for maternal age, parity, ethnicity, prior low- birth-weight or preterm delivery, bleeding at entry into study, gestation at initial blood draw taken at entry into study, number of cigarettes smoked per day, and prepregnancy body mass index. d Adjusted for maternal age, parity, prior low-birth-weight delivery, bleeding at entry into study, gestation at initial blood draw taken at entry into study, number of cigarettes smoked per day, and prepregnancy body mass index. e Adjusted for maternal age, parity, ethnicity, bleeding at entry into study, gestation at initial blood draw (entry), and prepregnancy body mass index. SOURCE: Scholl et al. (1992).
IRON 299 these studies have been criticized because maternal hemoglobin concentration was measured only at the time of delivery. Physiologi- cal factors cause the maternal hemoglobin concentration to rise shortly before delivery. Delivery, occurring early because of known or unknown factors unrelated to anemia, could therefore be ex- pected to show an association with a lower hemoglobin concentra- tion even though anemia played no causal role. Other surveys have shown the association to be present even when hemoglobin concen- tration was measured earlier in pregnancy. In one recent prospec- tive study, only anemia resulting from iron deficiency was associated with premature labor (Scholl et al., 1992). Furthermore, Goepel and coworkers (1988) reported that premature labor was four times more frequent in women with serum ferritin concentrations below 20 Âµg/L than in those with higher ferritin concentrations, irrespec- tive of hemoglobin concentration. High hemoglobin concentrations at the time of delivery are also associated with adverse pregnancy outcomes, such as the newborn infant being small for gestational age (Yip, 2000). Therefore, there is a U-shaped relationship between hemoglobin concentration and prematurity, low birth weight, and fetal death, the risk being in- creased for hemoglobin concentration below 90 g/L or above 130 g/L. The etiological factors are different, however, at each end of the spectrum. Iron deficiency appears to play a causal role in the presence of significant anemia by limiting the expansion of the maternal erythrocyte cell mass. On the other hand, elevated hemo- globin concentration probably reflects a decreased plasma volume associated with maternal hypertension and eclampsia. Both of the latter conditions have an increased risk of poor fetal outcome (Allen, 1993; Hallberg, 1992; Williams and Wheby, 1992). Fetal requirements for iron appear to be met at the expense of the motherâs needs, but the iron supply to the fetus may still be suboptimal. Several studies suggest that severe maternal anemia is associated with lower iron stores in infants evaluated either at the time of delivery by measuring cord blood ferritin concentration or later in infancy. The effect of maternal iron deficiency on infant status has been reviewed extensively by Allen (1997). While the observations relating iron status of the mother to the size of stores in infants (based on serum ferritin concentration) are important, it should be noted that the total iron endowment in a newborn infant is directly proportional to birth weight (Widdowson and Spray, 1951). Maternal iron deficiency anemia may therefore limit the infantâs iron endowment specifically through an associa- tion with premature delivery and low birth weight. Preziosi and
300 DIETARY REFERENCE INTAKES coworkers (1997) evaluated the effect of iron supplementation dur- ing pregnancy on iron status in newborn babies born to women living in Niger. The prevalence of maternal anemia was 65 to 70 percent at 6 months gestation. The iron status of the infants was also evaluated at 3 and 6 months of age. Although there were no differences between the supplemented and unsupplemented women in cord blood iron indexes at both 3 and 6 months of age, the children born to iron-supplemented women had significantly higher serum ferritin concentrations. Furthermore, it was reported that Apgar scores were significantly higher in infants born to supple- mented mothers. There were a total of eight fetal or neonatal deaths, seven in the unsupplemented group. Other Consequences of Iron Deficiency With use of in vitro tests and animal models, iron deficiency is associated with impaired host defense mechanisms against infec- tion such as cell-mediated immunity and phagocytosis (Cook and Lynch, 1986). The clinical relevance of these findings is uncertain although iron deficiency may be a predisposing factor for chronic mucocutaneous candidiasis (Higgs, 1973). Iron deficiency is also associated with abnormalities of the mucosa of the mouth and gastro- intestinal tract leading to angular stomatitis, glossitis, esophageal webs, and chronic gastritis (Jacobs, 1971). Spoon-shaped fingernails (koilonychia) may be present (Hogan and Jones, 1970). The eating of nonfood material (pica) or a craving for ice (pagophagia) are also associated with iron deficiency (Ansell and Wheby, 1972). Finally, temperature regulation may be abnormal in iron deficiency anemia (Brigham and Beard, 1996). SELECTION OF INDICATORS FOR ESTIMATING THE REQUIREMENT FOR IRON Functional Indicators The most important functional indicators of iron deficiency are reduced physical work capacity, delayed psychomotor development in infants, impaired cognitive function, and adverse effects for both the mother and the fetus as discussed above. As indicated earlier, these adverse consequences of iron deficiency are associated with a degree of iron deficiency sufficient to cause measurable anemia. A specific functional indicator, such as dark adaptation for vita- min A (see Chapter 4), is used to estimate the average requirement
IRON 301 for some nutrients. This is done by evaluating the effect on that functional indicator in a group of experimental subjects fed diets containing graded quantities of the nutrient. The effect of different levels of iron intake on the important functional indicators identi- fied above can not be measured in this way because of the difficulty inherent in quantifying abnormalities in these functional indica- tors, as well as the complexity of the regulation of iron absorption. Biochemical Indicators A series of laboratory indicators can be used to characterize iron status precisely and to categorize the severity of iron deficiency. Three levels of iron deficiency are customarily identified: â¢ depleted iron stores, but where there appears to be no limita- tion in the supply of iron to the functional compartment; â¢ early functional iron deficiency (iron-deficient erythropoiesis) where the supply of iron to the functional compartment is sub- optimal but not reduced sufficiently to cause measurable anemia; and â¢ iron deficiency anemia, where there is a measurable deficit in the most accessible functional compartment, the erythrocyte. Available laboratory tests can be used in combination to identify the evolution of iron deficiency through these three stages (Table 9-2). Storage Iron Depletion Serum Ferritin Concentration. Cellular iron that is not immediately needed for functional compounds is stored in the form of ferritin. Small quantities of ferritin also circulate in the blood. The concen- tration of plasma and serum ferritin is proportional to the size of body iron stores in healthy individuals and those with early iron deficiency. In an adult, each 1 Âµg/L of serum ferritin indicates the presence of about 8 mg of storage iron (Bothwell et al., 1979). A similar relationship is present in children in that each 1 Âµg/L of serum ferritin is indicative of an iron store of about 0.14 mg/kg (Finch and Huebers, 1982). When the serum ferritin concentration falls below 12 Âµg/L, the iron stores are totally depleted. Based on the Third National Health and Examination Survey (NHANES III), for adults living in the United States the median serum ferritin concentrations were 36 to 40 Âµg/L in menstruating
302 DIETARY REFERENCE INTAKES TABLE 9-2 Laboratory Measurements Commonly Used in the Evaluation of Iron Status Stage of Iron Deficiency Indicator Diagnostic Range Depleted stores Stainable bone marrow iron Absent Total iron binding capacity > 400 Âµg/dL Serum ferritin concentration < 12 Âµg/L Early functional Transferrin saturation < 16% iron deficiency Free erythrocyte protoporphyrin > 70 Âµg/dL erythrocyte Serum transferrin receptor > 8.5 mg/L Iron deficiency Hemoglobin concentration < 130 g/(male) anemia < 120 g/L (female) Mean cell volume < 80 fL SOURCE: Ferguson et al. (1992); INACG (1985). women and 112 to 156 Âµg/L in men (Appendix Table G-3). The median serum ferritin concentration was 27 Âµg/L for adolescent girls and 28 Âµg/L for pregnant women. These concentrations ex- ceed the cut-off concentration of less than 12 Âµg/L for adolescent girls and pregnant women (IOM, 1990; Table 9-2). However, direct correlation between the estimation of iron in- takes and iron status is low (Appendix Table H-5). Serum ferritin concentrations are known to be affected by factors other than the size of iron stores. Concentrations are increased in the presence of infections, inflammatory disorders, cancers, and liver disease because ferritin is an acute phase protein (Valberg, 1980). Thus, serum ferritin concentration may fall within the normal range in individuals who have no iron stores. Elevated serum ferritin concentrations are also associated with increased ethanol consumption (Leggett et al., 1990; Osler et al., 1998), increasing body mass index (Appendix Table H-3), and elevated plasma glucose concentration (Appendix Table H-4) (Tuomainen et al., 1997). Dinneen and coworkers (1992) reported high serum ferritin concentration in association with newly diagnosed diabetes mellitus. Analysis of the NHANES III database demonstrated a statistically significant direct correlation between body mass index and serum ferritin concentration in non- Hispanic white men over the age of 20 years, non-Hispanic black men and women aged 20 to 49 years, Mexican-American men aged
IRON 303 20 to 49 years, and Mexican-American women over the age of 50 years (Appendix Table H-3). An examination of the NHANES III database also showed that individuals in the highest quartile for plasma glucose concentration had higher serum ferritin concentra- tions than those in the lowest quartile for all gender and age groups (Appendix Table H-4). Similar findings were reported by Ford and Cogswell (1999). For these reasons and because of the variability in consumption of promoters and inhibitors of iron absorption, iron intake does not necessarily correlate with ferritin status. Despite the influence of various unrelated factors on serum ferritin concentration, this indicator is the most sensitive indicator of the amount of iron in the storage compartment. Total Iron-Binding Capacity. Iron is transported in the plasma and extracellular fluid bound to transferrin. This metalloprotein has a very high affinity for iron. Virtually all plasma iron is bound to trans- ferrin. Therefore it is convenient to measure plasma transferrin con- centration indirectly by quantifying the total iron-binding capacity (TIBC), which is the total quantity of iron bound to transferrin after the addition of exogenous iron to plasma. TIBC is elevated with storage iron depletion before there is evidence of inadequate delivery of iron to erythropoetic tissue. An increased TIBC (> 400 Âµg/dL) is therefore indicative of storage iron depletion. It is less precise than the serum ferritin concentration. About 30 to 40 per- cent of individuals with iron deficiency anemia have TIBCs that are not elevated (Ravel, 1989). TIBC is reduced in infectious, inflam- matory, or neoplastic disorders (Konijn, 1994). Early Iron Deficiency Early iron deficiency is signaled by evidence indicating that the iron supply to the bone marrow and other tissues is only marginally adequate. A measurable decrease in the hemoglobin concentration is not yet present and therefore there is no anemia. Serum Transferrin Saturation. As the iron supply decreases, the serum iron concentration falls and the saturation of transferrin is decreased. Levels below 16 percent saturation indicate that the rate of delivery of iron is insufficient to maintain the normal rate of hemoglobin synthesis. Low saturation levels are not specific for iron deficiency and are encountered in other conditions such as anemia of chronic disease (Cook, 1999), which is associated with impaired release of iron from stores.
304 DIETARY REFERENCE INTAKES The median serum transferrin saturation was 26 to 30 percent for men and 21 to 24 percent for women (Appendix Table G-2). The median serum transferrin saturation was 21 percent for pregnant women and 22 percent for adolescent girls. These values exceed the cut-off value of 16 percent (Table 9-2). Erythrocyte Protoporphyrin Concentration. Heme is formed in devel- oping erythrocytes by the incorporation of iron into protoporphy- rin IX by ferrochetalase. If there is insufficient iron for optimal hemoglobin synthesis, erythrocytes accumulate an excess of proto- porphyrin, which remains in the cells for the duration of their lifespans (Cook, 1999). An increased erythrocyte protoporphyrin concentration in the blood therefore indicates that the erythrocytes matured at a time when the iron supply was suboptimal. The cut off concentration for erythrocyte protoporphyrin concentration is greater than 70 Âµg/dL of erythrocytes. Erythrocyte protoporphyrin concentration is again not specific for iron deficiency and is also associated with inadequate iron delivery to developing erythrocytes (e.g., anemia of chronic disease) or impaired heme synthesis (e.g., lead poisoning). In iron deficiency, zinc can be incorporated into protoporphyrin IX, resulting in the formation of zinc protoporphyrin (Braun, 1999). The zinc protoporphyrin:heme ratio is used as an indicator of impaired heme synthesis and is sensitive to an insuffi- cient iron delivery to the erythrocyte (Braun, 1999). Soluble Serum Transferrin Receptor Concentration. The surfaces of all cells express transferrin receptors in proportion to their require- ment for iron. A truncated form of the extracellular component of the transferrin receptor is produced by proteolytic cleavage and released into the plasma in direct proportion to the number of receptors expressed on the surfaces of body tissues. As functional iron depletion occurs, more transferrin receptors appear on cell surfaces. The concentration of proteolytically cleaved extracellular domains, or soluble serum transferrin receptors (sTfR), rises in par- allel. The magnitude of the increase is proportional to the function- al iron deficit. The sTfR concentration appears to be a specific and sensitive indicator of early iron deficiency (Akesson et al., 1998; Cook et al., 1990). Furthermore, sTfR concentration is not affected by infectious, inflammatory, and neoplastic disorders (Ferguson et al., 1992). Because commercial assays for sTfR have become avail- able only recently, there is a lack of data relating iron intake to sTfR concentration, as well as relating sTfR concentration to functional outcomes. This indicator may prove to be very useful in identifying
IRON 305 iron deficiency, especially in patients who have concurrent infec- tions or other inflammatory disorders. Iron Deficiency Anemia Anemia is the most easily identifiable indicator of functional iron deficiency. As discussed above, physiological impairment occurs at this stage of iron deficiency both because of inadequate oxygen delivery during exercise and because of abnormal enzyme function in tissues. Hemoglobin Concentration and Hematocrit. The hemoglobin concen- tration or hematocrit is neither a sensitive nor a specific indicator of mild yet functionally significant iron deficiency anemia. Iron de- ficiency anemia is microcytic (reduced mean erythrocyte volume and mean erythrocyte hemoglobin). However, microcytic anemia is characteristic of all anemias in which the primary abnormality is impaired hemoglobin synthesis. Iron deficiency is only one of the potential causal factors. The diagnosis of iron deficiency anemia, based solely on the presence of anemia, can result in misdiagnosis in many cases. Garby and coworkers (1969) recognized this fundamental prob- lem. After supplemental iron tablets (60 mg/day) or a placebo were provided to a group of women with mild anemia for 3 months, the women were characterized as having iron deficiency anemia based on a change in hemoglobin concentration in response to the iron supplement that was greater than that which occurred with the placebo. There was a significant overlap between the distribution curves for the initial hemoglobin concentration of the responders (iron deficiency anemia) and the nonresponders (no iron deficiency anemia). A single hemoglobin concentration used as a discriminant value for detecting iron deficiency anemia therefore lacks precision. Based on NHANES III data (Appendix Table G-1), the median hemoglobin concentration for men was 144 to 154 g/L and 132 to 135 g/L for women. The median hemoglobin concentration was 132 g/L for adolescent girls and 121 g/L for pregnant women. The hemoglobin concentration for pregnant women approaches the cut- off concentration of 120 g/L (IOM, 1990). Erythrocyte Indexes. Iron deficiency leads to the formation of small erythrocytes. Mean corpuscular hemoglobin (MCH) is the amount of hemoglobin in erythrocytes. The mean corpuscular volume (MCV) is the volume of the average erythrocyte. Both MCH and
306 DIETARY REFERENCE INTAKES MCV are reduced in iron deficiency, but their values are not specific for it. They occur in all conditions that cause impaired hemoglobin synthesis, particularly the thalassemias (Chalevelakis et al., 1984). Surrogate Laboratory Indicators As discussed earlier, functional abnormalities occur only when iron deficiency is sufficiently severe to cause measurable anemia. Low iron storage does not appear to have functional consequences in most studies. This does not imply that all functional consequences of iron deficiency are mediated by anemia, but rather that cellular enzymes that require iron become depleted in concert with the development of anemia. There is extensive experimental evidence indicating that tissue iron depletion has significant physiological consequences that are independent of the consequences of anemia (Willis et al., 1988). Early anemia could nevertheless be chosen as the surrogate func- tional indicator. However, the significant overlap between the iron- sufficient and the iron-deficient segments of a population limit the sensitivity of this indicator. The precision of the laboratory diagnosis of iron deficiency anemia can be improved by combining hemoglo- bin measurements with one or more indicators of iron status. The Expert Scientific Working Group (1985) described two models or conceptual frameworks. The ferritin model employs a combination of serum ferritin concentration, erythrocyte protoporphyrin con- centration, and transferrin saturation. The presence of two or more abnormal indicators of iron status is indicative of iron deficiency. The MCV model uses MCV, transferrin saturation, and erythrocyte protoporphyrin concentration as indicators. Once again, when two or more indicators are abnormal, this is indicative of iron deficiency. The two models give similar results and improve the specificity of the hemoglobin concentration or hematocrit as an indicator of iron deficiency anemia. They were considered as potential surrogate lab- oratory indicators of functional iron deficiency for use in estimating requirements, but rejected because they were felt to lack sufficient sensitivity to provide an adequate margin of safety in calculating iron requirements. The sTfR concentration may, in the future, prove to be a sensitive, reliable, and precise indicator of early functional iron deficiency. At present, however, there are insufficient dose-response data to rec- ommend this indicator.
IRON 307 Methods Considered in Estimating the Average Requirement In light of the rationale developed in the previous section, the calculation of the Estimated Average Requirement (EAR) is based on the need to maintain a normal, functional iron concentration, but only a minimal store (serum ferritin concentration of 15 Âµg/L) (IOM, 1993). Two methods of calculation were consideredâfacto- rial modeling and iron balance. Factorial Modeling Because the distribution of iron requirements is skewed, the simple addition of the components of iron requirement (losses and accretion) cannot be done. Instead, the physiological requirement for absorbed iron can be calculated by factorial modeling of each of the components of iron requirement (basal losses, menstrual losses, and accretion). Total need for absorbed iron can be estimated through the summation of the component needs (losses and accre- tion) (see Chapter 1, âMethod for Setting the RDA when Nutrient Requirements Are Not Normally Distributedâ). Information about the distribution of values for the components of iron requirement, such as hemoglobin accretion, are modeled on the basis of known physiology. Since the distributions of some components are not nor- mally distributed (i.e., are skewed), simple addition is inappropriate. In this case, Monte Carlo simulation is used to generate a large theoretical population with the characteristics described by the com- ponent distributions. When the final distribution representing the convolution of components has been derived, then the median per- centile of the distribution can be used directly to estimate the aver- age requirement for absorbed iron and the ninety-seven and one- half percentile can be used for determining the Recommended Dietary Allowance (RDA). The EAR and RDA are then determined from this data set by dividing by the upper limit of iron absorption. Basal Losses. Basal losses refer to the obligatory loss of iron in the feces, urine, and sweat and from the exfoliation of skin cells. Attempts to quantify these iron losses by measuring the amount of each of individual component have yielded highly variable results because of the technical difficulties encountered in distinguishing between the small quantities of iron lost from the body and contam- inant iron in the samples collected. The only reliable quantitative data for basal iron losses in humans are derived from a single study (Green et al., 1968). However, a study by Bothwell and coworkers
308 DIETARY REFERENCE INTAKES (1979) on iron absorption derived from radioiron absorption tests provides collateral support for the accuracy of the measurements made by Green and coworkers (1968). The observations made by Green and coworkers (1968) were based on earlier experimental data demonstrating that all body iron compartments are in a constant state of flux and that uniform label- ing of all body iron could be achieved several months after the injection of a long-lived radiolabelled iron (55Fe, half life 2.6 years). After uniform labeling is achieved, the change in specific activity of a readily accessible iron compartment (circulating hemoglobin) could be used to calculate the physiological rate of iron loss, provided that iron balance is maintained during the period of observation. They also measured individual compartmental losses from skin and in sweat, urine, and feces separately in other volunteers. Results obtained by summing compartmental losses were similar to the whole body excretion studies. They reported an average calculated daily iron loss of 0.9 to 1.0 mg/day (â14 Âµg/kg) in three groups of men with normal iron storage status who lived in South Africa, the United States, and Venezuela (Table 9-3). While there is a need for more information associating body weight with basal iron losses, subsequent analyses of the data from South Africa (R. Green, Uni- versity of Witwatersrand, Johannesburg, South Africa, personal com- munication, 2000) showed that within the substudy groups, body weight was an important explanatory variable for basal iron loss; the other very important variable was magnitude of iron stores. TABLE 9-3 Total Body Iron Losses in Adults Body Weight Estimated Loss Study Site Ethnic Group n kg (SD)a mg/day (SD) Washington State Caucasian 12 78.6 (5.9) 0.98 (0.30) Venezuela Mestizo 12 67.6 (8.3) 0.90 (0.31) Durban (S. Africa) Indian 17 62.3 (9.2) 1.02 (0.22) Total (non-Bantu)b 41 68.6 (8.1) 0.96 (0.27) Johannesburg (S. Africa) Bantu 10 79.0 (6.9) 2.42 (1.09) Durban (S. Africa) Bantu 9 69.9 (7.5) 2.01 (0.94) a SD = standard deviation. b Bantu not included. They were selected on the basis of phenotypic iron overload. SOURCE: Green et al. (1968).
IRON 309 Menstrual Losses. Additional iron is lost from the body as a result of menstruation in fertile women. Menstrual iron losses have been estimated in a number of studies (Beaton, 1974) (see review by Hefnawi and Yacout, 1978) and in three large community surveys conducted in Sweden (Hallberg et al., 1966b), England (Cole et al., 1971), and Egypt (Hefnawi et al., 1980). There was a reasonable degree of consistency between the different studies. The median blood volume lost per period reported in the three largest studies was 20.3 mL (Egypt), 26.5 mL (England), and 30.0 mL (Sweden). Losses greater than 80 mL were reported in less than 10 percent of women. Accretion. The requirement for pregnancy and for growth in chil- dren and adolescents can also be estimated from known changes in blood volume, fetal and placental iron concentration, and the in- crease in total body erythrocyte mass. Balance Studies Chemical balance is the classical method for measuring nutrient requirements through the estimation of daily intake and losses. While this direct approach is conceptually appealing, its use in mea- suring iron requirements presents several major technical obstacles (Hegsted, 1975). For instance, it is difficult to achieve a steady state with nutrients such as iron that are highly conserved in the body. Because the fraction of the dietary intake that is absorbed (and excreted) is very limited, even small errors in the recovery of unab- sorbed food iron in the feces invalidate the results. Thirteen adult balance studies were evaluated (Table 9-4). All of these studies yielded values that exceed the daily iron loss calculat- ed on the basis of the disappearance of a long-lived iron radio- isotope after uniform labeling of body iron (Green et al., 1968). One might therefore conclude that all of the subjects were in posi- tive balance during the period of observation. Moreover, the mag- nitude of estimated positive balance in most cases predicted the relatively rapid accumulation of body iron. Neither of these conclu- sions is compatible with numerous other experimental observations. Therefore, balance studies were not considered in estimating an average requirement.
310 DIETARY REFERENCE INTAKES TABLE 9-4 Iron Balance Studies in Adults Average Iron Average Balance Reference Study Group Duration Intake (mg/d) Data (mg/d) Kelsay et al., 12 men, 26 d 21.8 (low fiber) 3.8 1979 37â58 y 26.4 (high fiber) 4.6 Johnson 8 men, 40 d 18.8 1.8â2.3 et al., 1982 21â28 y Snedeker 9 men, 24 y 12 d 17.4 0.56 et al.,1982 Andersson 5 men and 24 d 14.9 (white bread) 1.73 et al., 1983 1 woman, 14.2 (brown bread) 0.7 25â55 y 14.1 (whole meal) 1.56 Mahalko 27 men, 28 d 15.52 3.42 et al., 1983 19â64 y 16.31 5.34 Van Dokkum 10 men, 23 y 20 d 14.4 (high fat) 3.0 et al., 1983 14.8 (low fat) 3.0 Behall et al., 11 men, 4 wk 16.6 2.5 1987 23â62 y Hallfrisch 20 men, 1 wk 18.93 3.01 et al., 1987 23â56 y duplicate 11.83 0.13 19 women, food 21â48 y record Holbrook 19 men, 7 wk 14.8â16.3 â0.9â2.3 et al., 1989 21â57 y Hunt et al., 11 women, 5.5 wk 16.3 6.3 1990 22â36 y 13.7 3.9 Turnlund 8 women, 41â21 d 11.5â12.8 2.9 et al., 1991 21â30 y (animal protein) 20â23 3.7â4.4 (plant protein) Ivaturi and 24 men and 14 d 10.59 0.524 Kies, 1992 women 10.59 (sucrose) 0.677 10.59 (fructose) â1.715 10.1 1.02 11.3 (sucrose) 0.62 11.3 (fructose) 0.79 Coudray et al., 1997 9 men, 21 y 28 d 11.6 (control) 2.52 11.5 (inulin) 1.77 12.3 (beet fiber) 2.21
IRON 311 FACTORS AFFECTING THE IRON REQUIREMENT The proportion of dietary iron absorbed is determined by the iron requirement of the individual. Absorption is regulated by the size of the body iron store in healthy humans (percentage absorp- tion is inversely proportional to serum ferritin concentration) (Cook et al., 1974). There is a several-fold difference in absorption from a meal between an individual who is iron deficient and some- one with sizeable iron stores. The calculation of dietary require- ments must be based on the maintenance of a well-defined iron status. This has been accomplished by setting the need for the main- tenance of a minimal iron store (serum ferritin concentration cut- off of 15 Âµg/L) as the surrogate indicator of functional adequacy. The other major factor to take into account when computing dietary iron requirements is iron bioavailability based on the com- position of the diet. Iron is present in food as either part of heme, as found in meat, poultry, and fish, or as nonheme iron, present in various forms in all foods. As previously discussed, the absorption mechanisms are different. Heme iron is always well absorbed and is only slightly influenced by dietary factors. The absorption of non- heme iron is strongly influenced by its solubility and interaction with other meal components in the lumen of the upper small intes- tine. Gastric Acidity Decreased stomach acidity, due to overconsumption of antacids, in- gestion of alkaline clay, or pathologic conditions such as achlorhydria or partial gastrectomy, may lead to impaired iron absorption (Conrad, 1968; Kelly et al., 1967). Nutrient-Nutrient Interactions: Enhancers of Nonheme Iron Absorption Ascorbic Acid. Ascorbic acid strongly enhances the absorption of nonheme iron. In the presence of ascorbic acid, dietary ferric iron is reduced to ferrous iron which forms a soluble iron-ascorbic acid complex in the stomach. Allen and Ahluwalia (1997) reviewed vari- ous studies in which ascorbic acid was added to meals consisting of maize, wheat, and rice. They concluded that iron absorption from meals is increased approximately two-fold when 25 mg of ascorbic acid is added and as much as three- to six-fold when 50 mg is added.
312 DIETARY REFERENCE INTAKES There appears to be a linear relation between ascorbic acid intake and iron absorption up to at least 100 mg of ascorbic acid per meal. Because ascorbic acid improves iron absorption through the re- lease of nonheme iron bound to inhibitors, the enhanced absorp- tion effect is most marked when consumed with foods containing high levels of inhibitors, including phytate and tannins. Ascorbic acid has been shown to improve iron absorption from infant wean- ing foods by two- to six-fold (Derman et al., 1980; Fairweather-Tait et al., 1995a). Other Organic Acids. Other organic acids including citric acid, lactic acid, and malic acid have not been studied as thoroughly as ascorbic acid, but they also have some enhancing effects on nonheme iron absorption (Gillooly et al., 1983). Animal Tissues. Meat, fish, and poultry improve iron nutrition both by providing highly bioavailable heme iron and by enhancing non- heme iron absorption. The mechanism of this enhancing effect on nonheme iron absorption is poorly described though it is likely to involve low molecular weight peptides that are released during digestion (Taylor et al., 1986). Nutrient:Nutrient Interactions: Inhibitors of Nonheme Iron Absorption Phytate. Phytic acid (inositol hexaphosphate) is present in legumes, rice, and grains. The inhibition of iron absorption from added iron is related to the level of phytate in a food (Brune et al., 1992; Cook et al., 1997). The absorption of iron was shown to increase four- to five-fold when the phytic acid concentration was reduced from 4.9 to 8.4 mg/g, to less than 0.1 mg/g in soy protein isolate (Hurrell et al., 1992). Genetically modified, low-phytic acid strains of maize have been developed. Iron absorption with consumption of low-phytic acid strains was 49 percent greater than with consumption of wild type strains of maize (Mendoza et al., 1998). Still, the overall avail- ability of iron remained quite low and generally under 8 percent, even for subjects with marginal iron status. The absorption of iron from legumes such as soybeans, black beans, lentils, mung beans, and split peas has been shown to be very low (0.84 to 1.91 percent) and similar to each other (Lynch et al., 1984). Because phytate and iron are concentrated in the aleurone layer and germ of grains, milling to white flour and white rice reduces the content of phytate
IRON 313 and iron (Harland and Oberleas, 1987), thereby increasing the bio- availability of the remaining iron (Sandberg, 1991). Polyphenols. Polyphenols markedly inhibit the absorption of non- heme iron. This was first recognized when tea consumption was shown to inhibit iron absorption (Disler et al., 1975). Iron binds to tannic acid in the intestinal lumen forming an insoluble complex that results in impaired absorption. The inhibitory effects of tannic acid are dose-dependent and reduced by the addition of ascorbic acid (Siegenberg et al., 1991; Tuntawiroon et al., 1991). The re- sponse to iron supplementation was shown to be significantly greater for Guatemalan toddlers who did not consume coffee (which con- tains tannic acid) than for those who did (Dewey et al., 1997). Polyphenols are also found in many grain products, other foods, herbs such as oregano, and red wine (Gillooly et al., 1984). Vegetable Proteins. Soybean protein has an inhibitory effect on non- heme iron absorption that is not dependent on the phytate effect (Lynch et al., 1994). Bioavailability is improved by fermentation, which leads to protein degradation. The iron bioavailability from other legumes and nuts is also poor. Calcium. Calcium inhibits the absorption of both heme and non- heme iron (Hallberg et al., 1991). The mechanism is not well under- stood (Whiting, 1995); however, calcium has been shown to inhibit iron absorption, in part by interfering with the degradation of phytic acid. Furthermore, it has been suggested that calcium inhibits heme and nonheme iron absorption during transfer through the mucosal cell (Hallberg et al., 1993). Calcium has a direct dose-related inhib- iting effect on iron absorption such that absorption was reduced by 50 to 60 percent at doses of 300 to 600 mg of calcium added to wheat rolls (Hallberg et al., 1991). Inhibition may be maximal at this level. When preschool children consumed mean calcium in- takes of 502 or 1,180 mg/day, no difference was observed in the erythrocyte incorporation of iron (Ames et al., 1999). Despite the significant reduction of iron absorption by calcium in single meals, little effect has been observed on serum ferritin concentrations in supplementation trials with supplement levels ranging from 1,000 to 1,500 mg/day of calcium (Dalton et al., 1997; Minihane and Fairweather-Tait, 1998; Sokoll and Dawson-Hughes, 1992).
314 DIETARY REFERENCE INTAKES Algorithms for Estimating Dietary Iron Bioavailability Despite the complexity of the food supply, the various inter- actions, and the lack of long-term bioavailability studies, attempts have been made to develop an algorithm for estimating iron bio- availability based on nutrients and food components that improve and inhibit iron bioavailability. Monsen and coworkers (1978) devel- oped a model that was based on the level of dietary meat, fish, or poultry and ascorbic acid. Most recently, an algorithm has been developed and validated for calculating absorbed heme and nonheme iron by the summation of absorption values derived from single-meal studies to estimate the iron absorption from whole diets (Hallberg and Hulthen, 2000). This algorithm involves estimating iron absorption on the basis of the meal content of phytate, polyphenols, ascorbic acid, calcium, eggs, meat, seafood, soy protein, and alcohol. Reddy and coworkers (2000) have developed another algorithm based on the animal tis- sue, phytic acid, and ascorbic acid content of meals. It is also impor- tant to note that single-meal studies may exaggerate the impact of factors affecting iron bioavailability. Cook and coworkers (1991) compared nonheme iron bioavailability from single meals with that of a diet consumed over a 2-week period. There was a 4.5-fold dif- ference between maximally enhancing and maximally inhibiting single meals. The difference was only two-fold when measured over the 2-week period. The determination of an Estimated Average Requirement (EAR) depends on a precise assessment of the physiological requirement for absorbed iron and the estimation of the maximum rate of absorption that can be attained by individuals just maintaining the level of iron nutriture considered adequate to ensure normal func- tion. As discussed earlier, normal function is preserved in individuals with a normal functional iron compartment provided that the dietary iron supply is secure and of sufficiently high bioavailability. There appears to be no physiological benefit to maintaining more than a minimal iron store (Siimes et al., 1980a, 1980b). The EAR is therefore set to reflect absorption levels in individuals with a nor- mal complement of functional iron, but only minimal storage iron as indicated by a serum ferritin concentration of 15 Âµg/L (IOM, 1993). The selection of this criterion for adequate iron balance is critical to determining the EAR because iron absorption is con- trolled primarily by the size of iron stores. As iron stores rise, the percentage of dietary iron absorption and apparent bioavailability fall (Cook et al., 1974).
IRON 315 The second factor that is critical to determining the EAR is dietary iron bioavailability. Although much is known about the factors that enhance and inhibit iron absorption, the application of specific algorithms based on these factors to complex diets remains imprecise. Based on the general properties of the major dietary enhancers, the FAO/WHO (1988) identified three levels of bioavailability and the associated compositional characteristics of such diets. The typical diversified U.S. and Canadian diets containing generous quantities of flesh foods and ascorbic acid were judged to be 15 percent bio- available. Constrained vegetarian diets, consisting mainly of cereals and vegetable foods with only small quantities of meat, fish, and ascorbic acid, were judged to be 10 percent bioavailable; very re- stricted vegetarian diets were judged to be 5 percent bioavailable. These levels of absorption were predicted for individuals who were not anemic, but had no storage iron. A mixed American or Canadi- an diet would therefore be predicted to allow the absorption of about 15 percent of the dietary iron in an individual whose iron status was selected as a basis for calculating the EAR (serum ferritin concentration of 15 Âµg/L). Hallberg and Rossander-Hulten (1991) suggested that the bio- availability of iron in the U.S. diet may be somewhat higher than 15 percent: approximately 17 percent. Some support for this conten- tion was provided by the observation of Cook and coworkers (1991) who measured nonheme iron absorption over a 2-week period in free-living American volunteers eating their customary diets. After correcting nonheme iron values (to a serum ferritin concentration of 15 Âµg/L), the bioavailability of nonheme iron in self-selected diets was 16.8 percent ([34 Âµg/L Ã· 15 Âµg/L] Ã 7.4 percent). Heme constitutes 10 to 15 percent of iron in the adult diet (Raper et al., 1984) and the diet of children (see Appendix Table I-2) and is always well absorbed. Based on a conservative estimation for overall heme absorption of 25 percent (Hallberg and Rossander-Hulten, 1991) and again a conservative estimate for the proportion of dietary iron that is in the form of heme (10 percent), estimated overall iron bioavailability in the mixed American or Canadian diet is approximately 18 percent: Overall iron absorption = (Fraction of nonheme iron [0.9] Ã proportion of nonheme iron absorption [0.168]) + (Fraction of heme iron [0.1] Ã proportion of heme iron absorption [0.25]) Ã 100 = 17.6 percent. For these reasons, 18 percent bioavailability is used to estimate
316 DIETARY REFERENCE INTAKES the average requirement of iron for children over the age of 1 year, adolescents, and nonpregnant adults consuming the mixed diet typ- ically consumed in the United States and Canada. The diets of most infants aged 7 through 12 months contain little meat and are rich in cereals and vegetables, a diet that approximates a medium bio- availability of 10 percent (Davidsson et al., 1997; Fairweather-Tait et al., 1995a; FAO/WHO, 1988; Skinner et al., 1997). FINDINGS BY LIFE STAGE AND GENDER GROUP Infants Ages 0 through 6 Months Method Used to Set the Adequate Intake No functional criteria of iron status have been demonstrated that reflect response to dietary intake in young infants. Thus, recom- mended intakes of iron are based on an Adequate Intake (AI) that reflects the observed mean iron intake of infants principally fed human milk. At birth, the normal full-term infant has a considerable endow- ment of iron and a very high hemoglobin concentration. Because the mobilization of body iron stores is very high, the requirement for exogenous iron is virtually zero. After birth, an active process of shifts in iron compartments takes place. Fetal hemoglobin concen- tration falls, usually reaching a nadir when the infant is between 4 and 6 months of age, and adult hemoglobin formation begins be- cause hematopoiesis is very active. Some time between 4 and 6 months, exogenous sources of iron are used and after 6 months, it can be assumed that the stores endowed at birth have been utilized and that the physiological norm is to meet iron needs from exoge- nous rather than endogenous sources as erythropoiesis becomes more active. Thereafter, the hemoglobin concentration rises slowly but continuously (1 to 2 g/L/year) through at least puberty (longer in males) (Beaton et al., 1989). This normal physiological sequence of events complicates the estimation of iron requirements. It is widely accepted that the iron intake of infants exclusively fed human milk must meet or exceed the actual needs of almost all of these infants and that the described pattern of utilization of iron stores is physiologically normal, not indicative of the beginning of iron deficiency. For this age group, it is assumed that the iron pro- vided by human milk is adequate to meet the iron needs of the infant exclusively fed human milk from birth through 6 months. Therefore, the method described in Chapter 2 is used to set an AI
IRON 317 for young infants based on the daily amount of iron secreted in human milk. The average iron concentration in human milk is 0.35 mg/L (Table 9-5). Therefore, the AI is set at 0.27 mg/day (0.78 L/ day Ã 0.35 mg/L). Since there is strong reason to expect that iron intake and iron requirement are both related to achieved body size and growth rate (milk volume relating to energy demand), it is assumed that a cor- relation between intake and requirement exists. This allows the group mean intake to be lower than the ninety-seven and one-half percentile of requirements (Recommended Dietary Allowance). Therefore, there should be no expectation that an intake of 0.27 mg/day is adequate to meet the needs of almost all individual in- fants and therefore should be applied with extreme care. Iron AI Summary, Ages 0 through 6 Months AI for Infants 0â6 months 0.27 mg/day of iron Special Considerations The iron concentration in cow milk ranges between 0.2 and 0.3 mg/L (Lonnerdal et al., 1981). Although the iron content in human milk is lower, iron is significantly more bioavailable in human milk (45 to 100 percent) compared to infant formula (10 percent) (Fomon et al., 1993; Lonnerdal et al., 1981). Casein is the major iron-binding protein in cow milk (Hegenauer et al., 1979). Because of the poor absorption of iron, in the United States cow milk is not recommended for ingestion by infants until after 1 year of age; in Canada it is not recommended until after 9 months of age. In addi- tion, the ingestion of cow milk by infants, especially in the first 6 months of life, has been associated with small amounts of blood loss in the stool. The cause of the blood loss is not well understood, but is assumed to be an allergic-type reaction between a protein in cow milk and the enterocytes of the gastrointestinal tract. Because the early, inappropriate ingestion of cow milk is associated with a higher risk of iron deficiency anemia, it would be prudent to monitor iron status of any infants ingesting cow milk. If anemia is detected, it should be treated with an appropriate dose of medicinal iron. The American Academy of Pediatrics (AAP, 1999) and Canadian Paediatric Society (1991) reviewed the role of commercial formulas in infant feeding. Their conclusion was that infants who are not, or only partially, fed human milk should receive an iron-fortified formula.
318 DIETARY REFERENCE INTAKES TABLE 9-5 Iron Concentration in Human Milk Milk Estimated Maternal Concen- Iron Intake Study Intake Stage of tration of Infants Reference Group (mg/d) Lactation (mg/L) (mg/d)a Picciano and 50 women Not reported 6â12 wk 0.202 0.15 Guthrie, 1976 Vaughan 38 women, Not reported 1â3 mo 0.49 0.38 et al., 1979 19â42 y 39.3 4â6 mo 0.43 0.34 47.1 7â9 mo 0.42 0.25 40.8 10â12 mo 0.38 0.23 65.5 13â18 mo 0.39 0.23 16.4 19â31 mo 0.42 0.25 Lemons 7 women Not reported 1 wk 0.77 0.60 et al., 1982 2 wk 0.98 0.76 3 wk 0.80 0.62 Mendelson 10 women Not reported 3â5 d 1.11 0.86 et al., 1982 8â10 d 0.99 0.77 15â17 d 0.81 0.63 28â30 d 0.88 0.68 Dewey and 20 women Not reported 1 mo 0.31 0.20 Lonnerdal, 2 mo 0.22 0.17 1983 3 mo 0.25 0.21 4 mo 0.22 0.18 5 mo 0.20 0.13 6 mo 0.21 0.13 Garza et al., 6 women, Not reported 6 mo 0.029 0.02 1983 26â35 y 7 mo 0.042 0.03 8 mo 0.050 0.03 Lipsman 7â13 teens 15.0 (mean 1 mo 0.4 0.31 et al., 1985 at 7 mo) 2 mo 0.3 0.23 3 mo 0.4 0.31 4 mo 0.35 0.27 5 mo 0.3 0.23 6 mo 0.25 0.15 7 mo 0.22 0.13 12â17 Not reported 1 mo 0.30 0.23 adults 2 mo 0.22 0.17 3 mo 0.25 0.19 4 mo 0.22 0.17 5 mo 0.20 0.16 6 mo 0.22 0.17 continued
IRON 319 TABLE 9-5 Continued Milk Estimated Maternal Concen- Iron Intake Study Intake Stage of tration of Infants Reference Group (mg/d) Lactation (mg/L) (mg/d)a Butte et al., 45 women 16.2 1 mo Not 0.19 1987 14.1 2 mo reported 0.15 13.9 3 mo 0.13 13.5 4 mo 0.12 Anderson, 7 women Not reported Up to 5 mo 0.26 0.20 1993 a Iron intake based on reported data or concentration (mg/L) Ã 0.78 L/day for 0â6 months postpartum and concentration (mg/L) Ã 0.6 L/day for 7â12 months postpar- tum. Infants Ages 7 through 12 Months Evidence Considered in Estimating the Average Requirement For older infants the approach to estimation of requirements is parallel to that of other age and gender groups. Although body iron stores decrease during the first 6 months (and this is seen as physiologically normal), it is appropriate to make provision for the maintenance and development of modest iron stores in early life, even though requirements for older children, adolescents, and adults do not make provision for iron storage as a part of require- ment. For infants over the age of 6 months, it becomes both feasible and desirable to model the factorial components of absorbed iron re- quirements to set the Estimated Average Requirement (EAR) and Recommended Dietary Allowance (RDA) (see âSelection of Indica- tors for Estimating the Requirement for IronâFactorial Model- ingâ). The major components of iron need for older infants are: â¢ obligatory fecal, urinary, and dermal losses (basal losses); â¢ increase in hemoglobin mass (increase in blood volume and increase in hemoglobin concentration); â¢ increase in tissue (nonstorage) iron; and â¢ increase in storage iron (as noted earlier, building a small re- serve in very young children is seen as important).
320 DIETARY REFERENCE INTAKES A number of these component estimates can be linked to achieved size and growth rate. Dibley and coworkers (1987) provided data on both estimates. Median body weights at 6 and 12 months were 7.8 and 10.2 kg for boys and 7.2 and 9.5 kg for girls (Dibley et al., 1987) and the body weight at the midpoint between 7 and 12 months (0.75 years) were 9 and 8.4 kg for male and female infants, respec- tively. These weights are similar to the reference weights provided in Table 1-1. Approximate normality was assumed and the standard deviation (SD) estimates for infants fed human milk were used as an indicator of likely variability in body size (WHO, 1994). These were taken to present a coefficient of variation (CV) of about 10 percent for this age group. Basal Losses. The estimated basal loss of iron in infants is taken as 0.03 mg/kg/day (Garby et al., 1964). On the assumption that the variability of these losses is proportional to the variability of weight, the accepted estimates of basal losses at 6 and 12 months are 0.22 Â± 0.02 (SD) mg/day at 6 months and 0.31 Â± 0.03 (SD) mg/day at 12 months for both genders; the midrange estimate is 0.26 Â± 0.03 (SD) mg/day. Increase in Hemoglobin Mass. The rate of hemoglobin formation, and hence iron needed for that purpose, is a function of rate of growth (weight velocity). The median or average growth rate is esti- mated as 13 g/day (2,400 g/180 days) for boys and 12.7 g/day (2,300 g/180 days) for girls, suggesting 13.0 g/day (0.39 kg/month) for both genders (Dibley et al., 1987). The World Health Organiza- tion Working Group on Infant Growth (WHO, 1994) gathered data on growth velocity from limited longitudinal studies of infants fed human milk. The reported means and SDs for 2-month weight in- crements at ages 8 to 20 months were 0.27 Â± 0.14 kg/month (9 g/ day) for boys and 0.26 Â± 0.12 kg/month (8.6 g/day) for girls. The observed CV was 45 to 52 percent. Although skewing of the distribu- tions would be expected, no information was provided. For the pur- poses of this report, the median weight increment is taken as 13 g/ day for both genders, and the SD is taken as 6.5 (CV, 50 percent). If blood volume is estimated to be 70 mL/kg (Hawkins, 1964), the median hemoglobin concentration as 120 g/L, and the iron content of hemoglobin as 3.39 mg/g (Smith and Rios, 1974), then the amount of iron utilized for increase in hemoglobin mass can also be estimated:
IRON 321 Weight gain (0.39 kg/month) Ã blood volume factor (70 mL/kg) Ã hemoglobin concentration (0.12 mg/mL) Ã iron concentration in hemoglobin (3.39 mg/g) Ã· 30 days/month = 0.37 mg/day. The CV of iron utilization for this function is taken as the CV for weight gain, and thus the estimate becomes 0.37 Â± 0.195 (SD) mg/ day. Increase in the Nonstorage Iron Content of Tissues. The nonstorage iron content of tissues has been estimated as 0.7 mg/kg body weight for a 1-year-old child (Smith and Rios, 1974). On the assumption that this estimate can be applied at age 7 months as well, the aver- age tissue iron deposition would be Weight gain (13.3 g/day) Ã nonstorage iron content (0.7 mg/kg) = 0.009 mg/day. Applying the CV accepted for weight gain (50 percent) gives a mod- eling estimate of tissue iron deposition of 0.009 Â± 0.0045 (SD) mg/ day. Increase in Storage Iron. The desired level of iron storage is a matter of judgment rather than physiologically definable need. In this report, it is assumed that body iron storage should approximate 12 percent of total iron deposition (Dallman, 1986b), or (Increase in hemoglobin iron [0.37 mg/day] + Increase in nonstorage tissue iron [0.009 mg/day]) Ã (Percent of total tissue iron that is stored [12 percent] Ã· Percent of total iron that is not stored [100 â 12 percent]) = 0.051 mg/day. The variability would be proportional to the combined variability of hemoglobin deposition and nonstorage iron deposition. Total Requirement for Absorbed Iron. Median total iron deposition (hemoglobin mass + nonstorage iron + iron storage) is 0.43 mg/day (0.37 + 0.009 + 0.051) and basal iron loss is 0.26 Â± 0.03 (SD) mg/ day. Therefore, the median total requirement for absorbed iron is 0.69 Â± 0.145 (SD) mg/day (Table 9-6).
322 DIETARY REFERENCE INTAKES TABLE 9-6 Summary Illustration of Median Absorbed Iron Requirements for Infants and Young Children Estimated Hemoglo Estimated Change in Basal Iron Surface Hemoglobin Lossd Depositio Age (y) Weighta (kg) Areab (m2) Mass c (g/y) (mg/d) (mg/d) Infants 6â12 moh 8.7 â â 0.26 0.37 Males 1.5 11.6 0.5340 30.2 0.29 0.28 2.5 13.6 0.6064 19.8 0.33 0.18 3.5 15.5 0.6700 22.7 0.36 0.21 4.5 17.5 0.7353 21.8 0.39 0.20 5.5 19.6 0.7996 26.2 0.43 0.24 6.5 21.9 0.8675 26.7 0.47 0.25 7.5 24.7 0.9422 29.9 0.51 0.28 8.5 26.8 0.9980 35.7 0.54 0.33 Females 1.5 10.8 0.5104 33.5 0.27 0.31 2.5 12.8 0.5842 28.4 0.31 0.26 3.5 14.7 0.6486 22.5 0.35 0.21 4.5 16.8 0.7166 24.4 0.39 0.23 5.5 19.0 0.7845 20.7 0.42 0.19 6.5 21.3 0.8524 19.7 0.46 0.15 7.5 23.8 0.9209 29.9 0.49 0.28 8.5 26.9 0.9986 27.0 0.54 0.25 a Representative anthropometry for modeling, based on Frisancho (1990). b Computed by equation of Haycock et al. (1978). c Derived from Table 9-7. d Based on 0.538 mg/m2/d, extrapolated from Green et al. (1968). e Based on assumed 0.7 g/kg body weight gain (Smith and Rios, 1974). f Calculated as 12 percent of total iron deposition through 3.0 years of age then falling; no provision for storage at 9.0 years of age. Dietary Iron Bioavailability. During the second 6 months of life, it is assumed that complementary feeding is in place. The primary food introduced at this time is infant cereal, most often fortified with low-bioavailable iron (Davidsson et al., 2000); this cereal is the pri- mary source of iron (see Appendix Table I-1). Feeding with human milk and infant formula (possibly fortified with iron), may continue. Iron absorption averaged 14.8 percent in human milk (Abrams et al., 1997). A study on food intakes of infants showed that by 1 year of age, over half of the infants consumed cereals and fruits, but less
IRON 323 ron Total Iron Needg Hemoglobin Increase Increase Basal Iron in Tissue in Storage 97.5th Lossd Deposition Irone Ironf Median Percentile (mg/d) (mg/d) (mg/d) (mg/d) (g/d) (g/d) 0.26 0.37 0.009 0.051 0.69 1.07 0.29 0.28 0.004 0.038 0.62 1.24 0.33 0.18 0.004 0.023 0.54 1.23 0.36 0.21 0.004 0.025 0.61 1.36 0.39 0.20 0.004 0.021 0.63 1.45 0.43 0.24 0.004 0.019 0.70 1.60 0.47 0.25 0.004 0.015 0.74 1.71 0.51 0.28 0.004 0.011 0.81 1.86 0.54 0.33 0.004 0.006 0.81 2.01 0.27 0.31 0.004 0.038 0.64 1.25 0.31 0.26 0.004 0.032 0.63 1.30 0.35 0.21 0.004 0.026 0.59 1.32 0.39 0.23 0.004 0.023 0.65 1.45 0.42 0.19 0.004 0.016 0.64 1.52 0.46 0.15 0.004 0.011 0.66 1.61 0.49 0.28 0.004 0.011 0.79 1.83 0.54 0.25 0.004 0.005 0.80 1.92 g Estimates derived from simulated population that take into account impact of skewing and are the basis for the Estimated Average Requirement (EAR) and Recommended Dietary Allowance (RDA). h Requirements for infants and young children were estimated by different methods. Upper limit of absorption for infants and children is 10 and 18 percent, respectively. See text for methods used for infants and children. than half consumed meat or meat mixtures (Skinner et al., 1997). Only 32 percent of infants consumed beef at 12 months of age. Therefore, a moderate bioavailability of 10 percent is used to set the EAR at 6.9 mg/day (0.69 Ã· 0.1). Iron EAR and RDA Summary, Ages 7 through 12 Months The EAR has been set by modeling the components of iron re- quirements, estimating the requirement for absorbed iron at the
324 DIETARY REFERENCE INTAKES fiftieth percentile, with use of an upper limit of 10 percent iron absorption and rounding (see Appendix Table I-3). EAR for Infants 7â12 months 6.9 mg/day of iron The RDA has been set by modeling the components of iron requirements, estimating the requirement for absorbed iron at the ninety-seven and one-half percentile, with use of an upper limit of 10 percent iron absorption and rounding (see Appendix Table I-3). RDA for Infants 7â12 months 11 mg/day of iron Children Ages 1 through 8 Years Evidence Considered in Estimating the Average Requirement The EAR for children 1 through 8 years is determined by factorial modeling of the median components of iron requirements (see âSelection of Indicators for Estimating the Requirement for Ironâ Factorial Modelingâ). The model is presented for males and females though gender is ignored in deriving the EAR for young children because the gender differences are sufficiently small. The major components of iron need for young children are: â¢ basal iron losses; â¢ increase in hemoglobin mass; â¢ increase in tissue (nonstorage iron); and â¢ increase in storage iron. A fundamental influence on body iron accretion is the rate of change of body weight (growth rate). Because variability in body weight is needed for calculating the distribution of basal losses, the reference weights in Table 1-1 were not used. Median change in body weight was estimated as the slope of a linear regression of reported median body weights on age (weight = 7.21 + 2.29 Ã age, for pooled gender) (Frisancho, 1990) (Table 9-6). The fit was satis- factorily close for the purpose of modeling. Inclusion of gender in the model demonstrated that boys typically weighed more than girls, but the interaction term was insignificant, statistically and biologi- cally. A median rate of weight change of 2.3 kg/year or 6.3 g/day was assumed for both sexes.
IRON 325 For each component discussed above, the data for children 1 through 3.9 years and 4 through 8.9 years were used for modeling the iron needs for children 1.5 through 3.5 years and 4.5 through 8.5 years, respectively. The midpoints for these age ranges are 2.5 and 6.5 years, which were used to estimate the total requirements for absorbed iron. Basal Losses. Basal iron losses for children, aged 1.5 to 8.5 years, were derived from the total body iron losses directly measured from adult men (Green et al., 1968) (see âSelection of Indicators for Estimating the Requirement for IronâFactorial Modelingâ). Rather than assuming a linear function of body weight, estimated losses were adjusted to the childâs body size on the basis of estimated surface area (Haycock et al., 1978). Body surface area was used rather than body weight because it is directly related to dermal iron losses (Bothwell and Finch, 1962) and because it is a predictor of metabolic size. On this basis, the adult male basal loss was computed as 0.538 mg/m2/day (Green et al., 1968). The derived values are presented in Table 9-6. Garby and coworkers (1964) found that iron lost from the gastro- intestinal tract alone was 0.03 mg/kg in infants, an amount that would yield higher estimated basal losses than were determined by extrapolating from the data of Green and coworkers (1968). There- fore, the basal losses of children 1 through 8 years of age may be underestimated. Nonetheless, the data of Green coworkers (1968) were used because of the greater number of study subjects (n = 41 versus n = 3 studied by Garby and coworkers ), as well as the finding that basal losses are related to body size (Bothwell and Finch, 1962; R. Green, University of Witwatersrand, Johannesburg, South Africa, personal communication, 2000). Increase in Hemoglobin Mass. Median increase in hemoglobin mass was estimated as Hemoglobin mass (g) = blood volume (mL/kg) Ã hemoglobin concentration (g/L). During growth, both blood volume and hemoglobin concentration change with age. Although blood volume is a function of body weight, the actual relationship between blood volume and weight appears to change with age. Hawkins (1964) estimated blood vol- ume at specific ages by averaging estimates obtained by several calculations based on body weight or body surface area. Hawkinsâ
326 DIETARY REFERENCE INTAKES estimates are presented in Table 9-7. Age- and gender-specific hemoglobin concentration is estimated from the equations of Beaton and coworkers (1989) using 119 + 1.4 g/L/year in males and 121 + 1.1 g/L/year in females. Estimated blood volume and hemoglobin mass are shown in Table 9-7. Change in hemoglobin mass was esti- mated between mass at successive ages (Table 9-6). Iron needs were computed from the estimated change of hemoglobin mass and its expected iron content (3.39 mg/g). Thus, for example, from Table 9-7, the increase in hemoglobin mass between ages 7 and 8 years was 29.9 g (from 231.8 to 261.7). That represents 101.4 mg of iron (29.9 g Ã 3.39 mg/g) per year or 0.28 mg/day, as shown for in- crease in hemoglobin mass at 7.5 years in Table 9-6. TABLE 9-7 Estimates of Blood Volume and Hemoglobin Mass, by Age and Gender Blood Hemoglobin Hemoglobin Weight Volume Concentration Mass Age (y) (kg)a (L)b (g/L)c (g) Males 1 9.8 0.70 120.4 84.3 2 12.5 0.94 121.8 114.5 3 14.3 1.09 123.2 134.3 4 16.6 1.26 124.6 157.0 5 18.5 1.42 126.0 178.9 6 20.7 1.61 127.4 205.1 7 23.0 1.80 128.8 231.8 8 25.7 2.01 130.2 261.7 9 28.5 2.26 131.6 297.4 Females 1 9.2 0.66 122.1 80.5 2 11.9 0.92 123.2 113.3 3 13.8 1.06 124.3 131.7 4 16.1 1.23 125.4 154.2 5 18.3 1.41 126.5 178.4 6 20.2 1.56 127.6 199.1 7 22.4 1.70 128.7 218.8 8 25.3 1.91 129.8 247.9 9 27.9 2.10 130.9 274.9 a Body weights are estimated by Frisancho (1990). b Blood volume estimates based on Hawkins (1964). c Hemoglobin concentrations estimates from Beaton et al. (1989).
IRON 327 Increase in the Nonstorage Iron Content of Tissues. Iron deposition was derived with use of the estimate of body weight change (0.7 mg/kg) (Smith and Rios, 1974) and the median rate of weight change (2.29 kg/year). The estimated deposition is estimated to be 0.004 mg/ day (2.29 kg/year Ã 0.7 mg/kg Ã· 365 days/year) for all age groups (Table 9-6). Increase in Storage Iron. Similar to the calculation described for older infants, increase in storage iron was computed as Increase in hemoglobin mass (mg/day) + Increase in tissue iron (mg/day) Ã Portion of total tissue iron that is stored (12 percent). This calculation was used for estimating an increase in iron stores for children up to 3 years old and was based on an estimated 12 percent of iron that enters storage (Dallman, 1986b). Beyond age 3 years, this percent progressively falls to no provision of iron stores by 9 years of age. The iron storage allowance for each age group is shown in Table 9-6. Total Requirement for Absorbed Iron. Total requirement for absorbed iron for children 1 through 8 years is based on the higher estimates derived for males. Median total iron deposition (hemoglobin mass + nonstorage iron + iron storage) is 0.21 mg/day (0.18 + 0.004 + 0.023) and basal iron loss is 0.33 mg/day for children aged 1 through 3 years. Therefore, the median total requirement for ab- sorbed iron is 0.54 mg/day (Table 9-6). The median total iron deposition is 0.27 mg/day (0.25 + 0.004 + 0.015) and basal iron loss is 0.47 mg/day for children 4 through 8 years. Therefore, the median total requirement for absorbed iron is 0.74 mg/day (Table 9-6). Dietary Iron Bioavailability. Based on a heme iron intake of 11 per- cent of total iron for children 1 to 8 years old, the upper limit of absorption is 18 percent (see âFactors Affecting the Iron Require- mentâAlgorithms for Estimating Dietary Iron Bioavailabilityâ and Appendix Table I-2). The derived estimates of dietary requirements are shown in Table 9-8. Representative values are selected for tabulated EARs and RDAs. The derived distributions of requirements for children 1 year of age and older are skewed and are tabulated in Appendix Table I-3. Estimation of the Variability of Requirements. For the estimation of
328 DIETARY REFERENCE INTAKES TABLE 9-8 Derived Estimates of the Estimated Average Requirement (EAR) and Recommended Dietary Allowance (RDA) for Young Children Requirement for Absorbed Dietary Reference Intakesa Iron (mg/d) (mg/d) Age (y) Median 97.5th Percentile EAR RDA Males 1.5 0.62 1.24 3.4 6.9 2.5 0.54 1.23 2.9 6.8 3.5 0.61 1.36 3.4 7.6 4.5 0.63 1.45 3.5 7.9 5.5 0.70 1.60 3.9 8.1 6.5 0.74 1.71 4.1 9.5 7.5 0.81 1.86 4.5 10.3 8.5 0.81 2.01 4.5 11.2 Females 1.5 0.64 1.25 3.4 6.9 2.5 0.63 1.30 2.7 7.2 3.5 0.59 1.32 3.3 7.3 4.5 0.65 1.45 3.4 8.1 5.5 0.64 1.52 3.4 8.4 6.5 0.66 1.61 3.6 8.9 7.5 0.79 1.83 4.3 10.2 8.5 0.80 1.92 4.4 10.7 a Based on 18 percent upper limit of absorption. variability of requirements, it is necessary to have an estimate of the variability of weight velocity. The Infant Growth Study (WHO, 1994) offers an estimate of the variability of 2-month weight gains at 10 to 12 months. The apparent CV was 62.5 percent in boys and 63.6 percent in girls. The report on weight velocity standards for the United Kingdom (Tanner et al., 1966) seems to suggest a CV of 25 to 30 percent for 1-year weight velocities in children in the age group examined at each year of age. Given that the relative variability (CV) increases as the duration of the increment interval decreases, it was judged appropriate to accept a somewhat higher estimate of the variability in biologically meaningful intervals. A CV of 40 per- cent for weight velocity in boys and girls at ages 1 through 8 years is estimated. In all likelihood, the actual distribution of weight veloci- ties is skewed, but no estimates of the actual distribution character- istics have been identified.
IRON 329 The variabilities of both hemoglobin iron deposition and tissue iron deposition were assigned the CV for weight gain (40 percent). Basal iron loss is estimated on the basis of surface area. The logical variability would be proportional to the variability of surface area. To obtain an estimate of variability of basal losses, these were com- puted for weights and heights reported in the U.S. Department of Agriculture (USDA) Continuing Survey of Food Intakes by Individ- uals (CSFII) (self-reported weight and height), and the variability of estimate within 1-year age intervals was examined. CVs of square root-transformed data for individual age-sex groups were examined, as well as the linear scale, and all showed appreciable departure from normality, but the square root transformation was empirically the best fit. CVs for individual age-sex groups ranged from 29 per- cent in 8-year-old boys to 47.4 percent in 4-year-old boys. With use of a statistical model that took into account age and gender effects, an overall CV of the basal iron loss was estimated as 38 percent. That CV was applied to the square root of the median basal losses shown in Table 9-6. Iron EAR and RDA Summary, Ages 1 through 8 Years The EAR has been set by modeling the components of iron requirements, estimating the requirement for absorbed iron at the fiftieth percentile, and with use of an upper limit of 18 percent iron absorption and rounding (see Table 9-8 and Appendix Table I-3). EAR for Children 1â3 years 3.0 mg/day of iron 4â8 years 4.1 mg/day of iron The RDA has been set by modeling the components of iron requirements, estimating the requirement for absorbed iron at the ninety-seven and one-half percentile, and with use of an upper limit of 18 percent iron absorption and rounding (see Table 9-8 and Appendix Table I-3). RDA for Children 1â3 years 7 mg/day of iron 4â8 years 10 mg/day of iron
330 DIETARY REFERENCE INTAKES Children and Adolescents Ages 9 through 18 Years Evidence Considered in Estimating the Average Requirement The EAR for children and adolescents ages 9 through 18 years is determined by factorial modeling of the median components of iron requirements (see âSelection of Indicators for Estimating the Requirement for IronâFactorial Modelingâ). The major compo- nents of iron need for children are: â¢ basal iron losses; â¢ increase in hemoglobin mass; â¢ increase in tissue (nonstorage iron); and â¢ menstrual iron losses in adolescent girls (aged 14 through 18 years). In this model, no provision was made for the development of iron stores after early childhood. It is accepted that all recognized func- tions of iron are met before significant storage occurs and that stores are a reserve against possible future shortfalls in intake rather than a necessary functional compartment of body iron. Because most individuals in this age group in the United States and Canada are believed to consume iron at levels above their own require- ment, it can be assumed that most will accumulate some stores. The major physiological event occurring in this age group is puberty. The associated physiological processes that have major impacts on iron requirements are the growth spurt in both sexes, menarche in girls, and the major increase in hemoglobin concentrations in boys. Because the growth spurt and menarche are linked to physiological age, the secular age at which these events occur varies among indi- viduals. The factorial model distorts this by using averages. Since the growth spurt and menarche can be detected in the individual, provision is made for adjustments of requirement estimates when counseling specific individuals. These are addressed later under âSpecial Considerationsâ. Estimation of the variability of requirements in this age range is complicated because of the physiological changes that occur. In this report, median requirements for absorbed iron are estimated for each year of age, but the variability of requirement and the requirement for absorbed iron at the ninety seven and one-half percentile are estimated at the midpoint for children 9 through 13 years (11 years) and adolescents 14 through 18 years (16 years). For modeling, the entire age range is treated as a continuum; for
IRON 331 description, the conventional age intervals of the DRIs are used. Although requirement estimates have been developed for individual ages, these should be interpreted with care. Unsmoothed data have been used and year-by-year fluctuations may not be meaningful. In addition to achieved size, it is necessary to estimate growth rates (weight velocities). After fitting linear regressions to median weights for segments of the age range, the regression slopes were taken as estimates of median weight velocities for the age interval. The estimates used are shown in Table 9-9. Basal Losses. Basal iron loss estimates are based on the study of Green and coworkers (1968) (see âSelection of Indicators for Esti- mating the Requirement for IronâFactorial Modelingâ). Observa- tions in adult men were extrapolated to adolescents on the basis of 14 mg/kg median weight and the losses for each age group are shown in Table 9-10. Increase in Hemoglobin Mass. Estimation of the net iron utilization for increasing hemoglobin mass necessitates estimation of the rate of increase in blood volume and estimation of the rate of change in hemoglobin concentration. Blood volume is taken as approximate- ly 75 mL/kg in boys and 66 mL/kg in girls (Hawkins, 1964). The average yearly weight gains for boys and girls are shown in Table 9-9. The rate of change in hemoglobin concentration has been directly estimated as the coefficients of the linear regression models applied to hemoglobin versus age for Nutrition Canada data by Beaton and coworkers (1989). The rate of change in hemoglobin concentration and the average hemoglobin concentrations for boys and girls are shown in Table 9-11. The iron content of hemoglobin is 3.39 mg/g (Smith and Rios, 1974), therefore the daily iron need for increased hemoglobin mass can be calculated as follows: TABLE 9-9 Growth Velocity for Boys and Girls Boys Girls Age (y) (kg/y) Age (y) (kg/y) 9â12 4.87 9â11 4.77 13â14 10.43 12â13 7.24 15â17 2.75 14â17 1.63 18 0 18 0 SOURCE: Tanner et al. (1966).
332 DIETARY REFERENCE INTAKES TABLE 9-10 Summary Illustration of Median Absorbed Iron Requirements for Children and Adolescents, Aged 9 through 18 Yearsa Components of Iron Needs Median Change in Basal Tissue Weightb Hemoglobin Loss Deposit Storage Menses Age (y) (kg) Mass (mg/d) (mg/d) (mg/d) (mg/d) (mg/d)c Boys 9 32.5 0.45 0.48 0.002 0 0 10 36.6 0.50 0.49 0.002 0 0 11 40.0 0.62 0.50 0.002 0 0 12 48.1 0.65 0.51 0.002 0 0 13 52.3 0.75 1.05 0.004 0 0 14 61.2 0.75 1.18 0.004 0 0 15 62.0 0.80 0.43 0.001 0 0 16 66.5 0.78 0.44 0.001 0 0 17 69.9 0.84 0.46 0.001 0 0 18 68.3 0.81 0.16 0 0 0 Girls 9 31.9 0.45 0.40 0.002 0 0b 10 35.8 0.50 0.41 0.002 0 0b 11 44.0 0.63 0.42 0.002 0 (0.45)b 12 46.3 0.65 0.63 0.003 0 (0.45)b 13 53.5 0.75 0.64 0.003 0 (0.45)b 14 53.4 0.75 0.14 0.001 0 0.45 15 56.9 0.80 0.14 0.001 0 0.45 16 55.6 0.78 0.14 0.001 0 0.45 17 60.0 0.83 0.15 0.001 0 0.45 18 58.0 0.81 0.10 0 0 0.45 a Summation of the median iron components and dividing by 18 percent bioavailability does not yield values that are equivalent to the 50th and 97.5th percentile data shown in Appendix Table I-3. This is because the summation of the median of non-normal distri- butions (above) do not yield the median of simulation models that represent normal- ized data. b Third National Health and Nutrition Examination Survey data (not demographically weighted). c The model assumes that all girls are menstruating at age 14.0 years and after; it also assumes that no girls reach menarche before age 14. This is not a valid assumption. In working with individuals, menstrual status can be ascertained and an adjustment can be made.
IRON 333 TABLE 9-11 Equations Used to Estimate Hemoglobin Concentration and Increase in Hemoglobin (Hb) Concentration Age (y) Boys Girls 8â13 Hba = 119 + (1.4 Ã age) âHbb = 1.4 Hb = 121 + (1.1 Ã age) âHb = 1.1 14â18 Hb = 94.3 + (3.4 Ã age) âHb = 3.4 Hb = 131 + (0.28 Ã age) âHb = 0.28 a Hb = g/L. b âHb = g/L/y. SOURCE: Beaton et al. (1989). Boys ([Weight (kg) Ã increase in hemoglobin concentration (kg/L/year)] + [Weight gain (kg/year) Ã hemoglobin concentration (g/L)]) Ã blood volume (0.075 L/kg) Ã hemoglobin iron (3.39 mg/g) Ã· 365 days/year. Girls ([Weight (kg) Ã increase in hemoglobin concentration (kg/L/year)] + [Weight gain (kg/year) Ã hemoglobin concentration (g/L)]) Ã blood volume (0.066 L/kg) Ã hemoglobin iron (3.39 mg/g) Ã· 365 days/year. For example, the medium daily need for increased hemoglobin mass for a 16-year-old girl would be ([55.6 Ã 0.28] + [1.63 Ã 135]) Ã 0.066 Ã 3.39 Ã· 365, or 0.14 mg/day. Increase in the Nonstorage Iron Content of Tissues. Nonstorage tissue iron concentration (myoglobin and enzymes) (Table 9-10) can be calculated when the average weight gain for boys and girls and the iron content in muscle tissue are known. The iron deposition is approximately 0.13 mg/kg total weight gain (0.26 mg/kg muscle tissue) (Smith and Rios, 1974). The median need for absorbed iron associated with increase in weight in both sexes is Tissue iron = Weight gain (kg/year) Ã nonstorage tissue iron (0.13 mg/kg) Ã· 365 days/year, or weight gain Ã 0.00036 mg/day.
334 DIETARY REFERENCE INTAKES For example, the median daily need for nonstorage iron for a 16- year-old boy is 0.001 mg/day (2.75 Ã 0.13 Ã· 365) after rounding. No provision is made for iron storage after the age of 9 years. It is not a component of requirement though it can be expected to occur when intake exceeds actual requirement. Menstrual Losses. Iron losses in the menses can be calculated when the average blood loss, the average hemoglobin concentration, and concentration of iron in hemoglobin (3.39 mg/g) (Smith and Rios, 1974) are known. It was deemed appropriate to use the blood losses reported by Hallberg and coworkers (1966a, 1966b) with additional information from Hallberg and Rossander-Hulthen (1991) and, more specifically, to use the blood loss estimates for 15-year-old girls. These losses were lower than those reported for older ages. Several important features of these and other data related to men- strual blood loss were recognized in developing models to predict requirements: â¢ Menstrual losses are highly variable among women and the dis- tribution of losses in the population shows major skewing, with some women having losses in excess of three times the median value. â¢ Menstrual losses are very consistent from one menstrual cycle to the next for an individual woman. â¢ Once the womanâs menstrual pattern is established after her menarche, menstrual losses are essentially unchanged until the onset of menopause in healthy women. Hallberg and coworkers (1966b) found very little difference in blood loss with age. Losses were lower in the 15-year-old group, but incomplete collection might have been a factor. Cole and coworkers (1971) reported a small effect of age that was attributed to two covariates, parity and infant birth weight. â¢ Contraceptive methods have a major impact on menstrual losses. Bleeding is significantly increased by the use of certain intrauterine devices and significantly decreased in individuals taking oral contra- ceptives. Age, body size, and parity were not considered to have an effect of sufficient magnitude on menstrual blood losses to include them as factors in the models for estimating iron requirements in femalesâ except with regard to the lower menstrual loss assumed for adoles- cents. The data on menstrual losses reported by Hallberg and coworkers
IRON 335 (1966a, 1966b) were used for all calculations in adolescent and adult females. This data set was selected for the following reasons: â¢ It is representative of the other survey data quoted above and can be considered generalizable to women living in countries other than that of the study, including the United States and Canada. â¢ Women were selected to fall into six age groups between 15 and 50 years, thus permitting estimates for all women. â¢ Although the original data were not available, comprehensive descriptions of the distribution of menstrual losses are available from a series of publications by Hallberg and colleagues. â¢ The survey was carried out before intrauterine devices and oral contraceptives were widely available. Only one woman in the study was using an oral contraceptive. None of them used an intrauterine device. The measurement can therefore reasonably be assumed to reflect âusual lossesâ. Blood losses per menstrual cycle were converted into estimated daily iron losses averaged over the whole menstrual cycle. The fol- lowing assumptions were made: â¢ Blood loss does not change with mild anemia and is therefore independent of hemoglobin concentration. â¢ In estimating hemoglobin loss (blood loss Ã hemoglobin con- centration), hemoglobin concentration was taken as a constant (135 Â± 9 g/L in adult women and based on age in adolescents) (Hallberg and Rossander-Hulthen, 1991) and variance was ignored. â¢ The iron content of hemoglobin is 3.39 mg/g (Smith and Rios, 1974). â¢ The duration of the average menstrual cycle is 28 days. Beaton and coworkers (1970) reported a cycle duration of 27.8 Â± 3.6 days in 86 self-selected healthy volunteers. Since the distribution of menstrual blood losses in the data re- ported by Hallberg is skewed, it was modeled as described previously (see âSelection of Indicators for Estimating the Requirement for IronâFactorial Modelingâ). Comparison of the observed and mod- eled values (Table 9-12) provides a way of visualizing the adequacy of the fit of the model. A log-normal distribution was fitted to the reported percentiles of the blood loss distribution (natural log of blood loss = 3.3183 Â± 0.6662 [SD]) to result in a median blood loss of 27.6 mL/estrous cycle. Blood losses of greater than 100 mL/ estrous cycle are observed at the ninety-fifth percentile (Table 9-12)
336 DIETARY REFERENCE INTAKES TABLE 9-12 Comparison of Reported and Modeled Distributions of Blood Loss Per Menstrual Cycle of Swedish Women Blood Loss (mL/estrous cycle) Percentile Observeda Modeled b 10 10.4 11.4 25 18.2 18.3 50 30.0 30.9 75 52.4 52.3 90 83.9 83.9 95 118.0 111.4 a n = 486. Data from Hallberg et al. (1966a), percentiles 10, 25, 50, 75, 90; Hallberg and Rossander-Hulthen (1991), percentile 95. b The predicted values were estimated from a fitted log normal distribution with mean and standard deviation = -3.4312 Â± 0.7783 (see text for methodology). and the distribution is highly skewed. Although these high menstrual losses were found in apparently healthy women, it would be diffi- cult to exclude unidentified hemostatic disorders (Edlund et al., 1996) or occult uterine disease as possible contributory factors. The investigators considered all the subjects they studied to be free of any condition that might affect menstruation. There are no criteria for identifying a subpopulation at risk for increased menstrual blood loss or for setting an upper limit for ânormalâ losses. Calculation of the EAR and RDA was therefore based on the complete set of obser- vations. Regression estimates of hemoglobin concentration and rates of change in hemoglobin concentration by age and gender have been derived by Beaton and coworkers (1989). Estimated hemoglobin concentration for females 14 to 20 years of age was 131 g/L + 0.28 Ã age (years). The above data were used to compute median menstrual iron loss as follows: (Blood loss [27.6 mL/28 days]) Ã (hemoglobin concentration [131 g/L] + [0.28 Ã age]) Ã iron content of hemoglobin (3.39 mg/g) Ã· 1,000. Thus for adolescent girls, the median iron loss would be 0.45 mg/ day (Table 9-10). Discussion on menstrual iron losses prior to 14
IRON 337 years is discussed under âSpecial Considerations.â (For a discussion of menstrual iron losses during oral contraceptive use, see the âSpecial Considerationsâ section following âLactationâ.) Total Requirement for Absorbed Iron. Because all components (basal iron loss, hemoglobin mass, and nonstorage iron) are not normally distributed (skewed), these components as shown in Table 9-10 can not be summed to accurately determine an EAR and RDA. After summing the components for each individual in the simulated pop- ulation, the estimated percentiles of distribution were tabulated and are shown in Appendix Tables I-3 and I-4. The modeled distribution of iron requirements are used to set the EAR (fiftieth percentile) and RDA (ninety-seven and one-half percentile) with the assump- tion of an upper limit of 18 percent for iron absorption. Dietary Iron Bioavailability. The upper limit of dietary iron absorp- tion was estimated to be 18 percent and used to set the EAR based on the fiftieth percentile of absorbed iron requirements (see âFactors Affecting the Iron RequirementâAlgorithms for Estimat- ing Dietary Iron Bioavailabilityâ). Estimation of the Variability of Requirements. While Table 9-10 shows an estimate of median requirement, it is a simple summation and does not reflect the distributions. The distribution of requirements must be modeled using Monte Carlo simulation before the EAR and RDA can be estimated. This necessitates estimation of variability for components of requirements. Basal or obligatory losses were derived from Green and coworkers (1968) with the assumption of proportionality to body surface area. To derive an estimate of variability of surface area, basal losses were computed with use of heights and weights reported in the USDA CSFII 1994â1996. Various transformations were then tested; a square root transformation approximated normality. The relative variability of surface area in this proxy data set was taken as an estimate of variability of basal iron loss. The observed CVs of proxy basal loss were 22.7 and 8.7 percent for boys aged 11 and 16, respec- tively, and 19.1 and 13.2 percent for girls aged 11 and 16, respec- tively. These CVs were applied to the square root of median iron loss, estimated on the basis of weight at ages 11 and 16 years (median loss shown in Table 9-10). Estimating iron associated with change in hemoglobin mass re- quires consideration of rate of increase in blood volume and in hemoglobin concentration. Blood volume estimates were based on
338 DIETARY REFERENCE INTAKES body size, and estimated median growth velocity is shown in Table 9-9. The algorithm for estimating iron need was presented earlier. For the purpose of modeling, blood volume as a proportion of body weight and rate of hemoglobin change as a function of age were taken as constants. The variability of iron need was attributed to variation in weight and weight velocity. Based on reported percentiles of body weight in the Third National Health and Nutrition Examination Survey (NHANES III), normal distributions were fitted at 11 and 16 years of age for boys and girls. The fit was approximate only but acceptable for the present purpose. The resultant body weight distributions (kg) were 42.96 Â± 12.47 and 70.30 Â± 12.70 for boys aged 11 and 16, respectively, and 44.96 Â± 9.96 and 61.36 Â± 12.88 for girls aged 11 and 16, respec- tively. The average weights differ from the median weights shown in Table 9-10. Estimates of weight velocity at ages 11 and 16 years were based on the analyses of longitudinal data reported by Tanner and coworkers (1966) (Table 9-9). Approximation of a normal distribu- tion was assumed. The resultant distributions of weight velocities (kg/year) were used for modeling: 4.87 Â± 1.65 and 2.75 Â± 2.27 for boys aged 11 and 16, respectively, and 4.77 Â± 2.06 and 1.63 Â± 1.63 for girls aged 11 and 16, respectively. The variability of tissue iron deposition was based on the variability of body weight. Values for iron hemoglobin concentration and altered hemoglobin concentration were estimated for these ages from the equations of Beaton and coworkers (1989), and variability in hemoglobin con- centration was ignored. Variability arising from menstrual loss was estimated from the fitted regression of blood loss (ln blood loss = 3.3183 Â± 0.6662 [SD]). Iron EAR and RDA Summary, Ages 9 through 18 Years The EAR has been set by modeling the components of iron re- quirements, estimating the requirement for absorbed iron at the fiftieth percentile, and with use of an upper limit of 18 percent iron absorption and rounding (see Appendix Tables I-3 and I-4). For the EAR and RDA for girls, it is assumed that girls younger than 14 years do not menstruate and that all girls 14 years and older do menstruate. EAR for Boys 9â13 years 5.9 mg/day of iron 14â18 years 7.7 mg/day of iron
IRON 339 EAR for Girls 9â13 years 5.7 mg/day of iron 14â18 years 7.9 mg/day of iron The RDA has been set by modeling the components of iron re- quirements, estimating the requirement for absorbed iron at the ninety-seven and one-half percentile, and with use of an upper limit of 18 percent iron absorption and rounding (see Appendix Tables I-3 and I-4). RDA for Boys 9â13 years 8 mg/day of iron 14â18 years 11 mg/day of iron RDA for Girls 9â13 years 8 mg/day of iron 14â18 years 15 mg/day of iron Special Considerations Adjustment for Growth Spurt. During the growth spurt, median rates of growth of boys might be double those seen in 11-year-olds; for girls the difference is smaller (about a 50 percent increase). The needs for absorbed iron associated with growth (increase in body weight) were estimated as 0.035 mg/g weight gained for boys and 0.030 mg/g weight gained for girls. The additional weight gain in the peak growth spurt years was estimated as the difference be- tween the maximum and average growth rate (Table 9-9), which is 15.2 g/day ([10.43 â 4.87 kg/year] Ã 1,000 g/kg Ã· 365 day/year) for boys and 6.76 g/day ([7.24 â 4.77 kg/year] Ã 1,000 g/kg Ã· 365 days/year) for girls. These represent demands of 0.53 mg/day of iron for boys and 0.20 mg/day for girls. Therefore, the increased requirement for dietary iron is 2.9 mg/day for boys identified as currently in the growth spurt, and for girls the increase is approxi- mately 1.1 mg/day. Menstruation Before Age 14 Years. In the United States, the average age of menarche is about 12.5 years. It is reasonable to assume that by age 14 almost all girls will have started to menstruate, and hence the estimates of iron requirements should include menstrual losses at that time. It would be unreasonable to assume that no girls are menstruating before age 14 years. For girls under age 14 who have started to menstruate, it would be appropriate to consider a median
340 DIETARY REFERENCE INTAKES menstrual loss of 0.45 mg/day of iron. Therefore, the requirement is increased by approximately 2.5 mg/day of iron. Adults Ages 19 Years and Older Method Used to Estimate the Average Requirement Factorial modeling was used to calculate the EAR and RDA for adult men and women (see âSelection of Indicators for Estimating the Requirement for IronâFactorial Modelingâ). Requirements for maintaining iron requirements were derived by estimating losses. No provision is made for growth beyond age 19 years, and therefore there is no allowance for deposition of tissue iron. Men. Basal iron loss was the only component used to estimate total needs for absorbed iron. Basal losses are based on the study by Green and coworkers (1968). Basal iron losses are taken as related to body weight (14 Âµg/kg/day), and for adult men, the require- ment for absorbed iron is equivalent to the basal losses: Basal losses (mg/day) = Weight (kg) Ã 0.014 mg/kg/day. (1) There are insufficient data for estimating variability of basal losses in adult men. Therefore, the median and variability for basal losses were calculated by using the median and variability values for body weight reported in NHANES III. Because variability in body weight is needed for calculating the distribution of basal losses, the refer- ence weights in Table 1-1 were not used. Recorded weights reason- ably yield a normal distribution based on the square root of the median weight for men: Weight 77.4 (kg)0.5 = 8.8 Â± 0.84 kg. (2) The distribution of basal losses, and therefore requirements in men, was obtained by combining equations (1) and (2). The esti- mated median daily iron loss in men living in the United Statesâ and therefore the median requirement for absorbed ironâis 1.08 mg/day (77.4 kg Ã 0.014 mg/kg/day). The ninety-seven and one- half percentile of absorbed iron requirements is 1.53 mg/day. The upper limit of dietary iron absorption was estimated to be 18 percent (see âFactors Affecting the Iron RequirementâAlgorithms for Estimating Dietary Iron Bioavailabilityâ). Using this value, the EAR is 6 mg/day (1.08 mg/day Ã· 0.18).
IRON 341 It is important to note that these calculations ignore the fact that men have higher iron stores than women. Moreover, the calcula- tions assume that this widely recognized observation has no biologi- cal importance, but is merely the consequence of a total intake of food energy and associated food iron that is typically higher in men than in women, coupled with a much lower iron need in men. Appendix Table I-3 provides the estimated percentiles of the distri- bution of iron requirements for adult men. Menstruating Women. Factorial modeling is again used to estimate the requirement for absorbed iron. Iron requirements for women were estimated by using the customary two-component model: Iron requirement = basal losses + menstrual losses. There are no direct measurements of basal iron losses, separated from menstrual iron loss, in women. Values for women have there- fore been derived from the observations made in men (Green et al., 1968) (see âSelection of Indicators for Estimating the Iron Require- mentâFactorial Modelingâ) by using a simple linear weight adjust- ment. The mean and variability in basal losses is based on the distri- bution of body weights recorded in NHANES III. Because variability in body weight is needed for calculating the distribution of basal losses, the reference weights in Table 1-1 were not used. The square root of reported weights yields a normal distribution reasonably closely: Weight 64 (kg)0.5 = 8.0 Â± 1.06 kg. Therefore the median basal iron loss was calculated as follows: Basal iron losses (mg/day) = Median weight (64 kg) Ã 0.014 mg/ kg/day = 0.896 mg/day. The ninety-seven and one-half percentile of the estimated ab- sorbed iron requirement is 1.42 g/day (101.6 kg Ã 0.014 mg/kg/ day). Menstrual blood (iron) losses have been estimated in many small studies (Beaton, 1974) and in two large community surveys, one in Sweden (Hallberg et al., 1966b) and the other in the United Kingdom (Cole et al., 1971). The findings of all of these studies were reasonably consistent. The factors and choice of data selection described for adolescent girls were also used for estimating men- strual losses in premenopausal women. Table 9-12 shows that the
342 DIETARY REFERENCE INTAKES modeled median blood lost per menstrual cycle is 30.9 mL. The average concentration of iron in hemoglobin is 3.39 mg/g (Smith and Rios, 1974). As determined by Beaton and coworkers (1989), the average hemoglobin concentration for nonanemic women is 135 g/L. Using the above information, the daily menstrual iron loss can be calculated as follows: Menstrual iron loss (mg/day) = blood loss/28 days (30.9 mL) Ã hemoglobin concentration (135 g/L) Ã iron concentration in hemoglobin (3.39 mg/g) Ã· 28 days = 0.51 mg/day. The simulated distribution of menstrual losses is shown in Table 9-13. Median total iron needs were derived by summing the compo- nent needs (basal loss [0.896] + menstrual losses [0.51] = 1.4 mg/ day). The upper limit of dietary iron absorption was estimated to be 18 percent (see âFactors Affecting the Iron RequirementâAlgorithms for Estimating Dietary Iron Bioavailabilityâ). By dividing the sum of absorbed requirements by 18 percent, a distribution of dietary re- quirements was derived (see Appendix Table I-4). Based on this calculation and rounding, the EAR and RDA are set at 8 and 18 TABLE 9-13 Estimated Distribution of Menstrual Losses and Absorbed and Dietary Iron Needs in Adult Womena Basal Iron Dietary Iron Percentile Losses Daily Iron Absorbed Iron Requirement of Women (mg/d) Loss (mg/d)b Needs (mg/d)c (mg/d)d 5 0.55 0.14 0.88 4.88 10 0.62 0.19 0.98 5.45 25 0.74 0.30 1.18 6.55 50 0.89 0.51 1.41 8.06 75 1.06 0.86 1.83 10.17 90 1.23 1.38 2.35 13.05 95 1.36 1.83 2.67 14.83 97.5 1.42 2.32 3.15 17.5 a Because the distribution of basal and menstrual iron losses are approximated from modeling, the sum of each for a specific percentile will not be equivalent to absorbed iron needs. b Menstrual iron losses, averaged over 28 days. c Menstrual + basal iron losses. d Based on 18 percent bioavailability.
IRON 343 mg/day, respectively, for menstruating women not using oral contraceptives. Postmenopausal Women. As for men, basal iron loss is the only com- ponent of iron needs for postmenopausal women and the physio- logical iron requirements and the EAR and RDA were derived by factorial modeling using the following equation: Basal losses (Âµg/day) = weight (kg) Ã 14 Âµg/kg. As was the case for men, the median and variability for basal losses was calculated using the median and variability values for body weight reported in NHANES III. Because variability in body weight is needed for calculating the distribution of basal losses, the refer- ence weights in Table 1-1 were not used. Recorded weights approx- imate a normal distribution based on the square root of weight: Weight 64 (kg)0.5 = 8.0 Â± 1.06 kg. The distribution of basal losses, and therefore requirements for postmenopausal women, was obtained by combining the equations relating weight to basal losses and describing the weight distribu- tion as outlined for men (Appendix Table I-3). The estimated median daily iron loss in postmenopausal women living in the United States, and therefore the median requirement for absorbed iron, is 0.896 mg/day (64 kg Ã 0.014 mg/kg/day). The ninety-seven and one-half percentile of estimated absorbed iron requirement is 1.42 g/day (101.6 kg Ã 0.014 mg/kg/day). The upper limit of dietary iron absorption was estimated to be 18 percent (see âFactors Affecting Iron RequirementâAlgorithms for Estimating Dietary Iron Bioavailabilityâ). Based on this value, the EAR is set at 5 mg/day (0.896 Ã· 0.18). It is assumed that basal losses, as a function of lean body mass, are essentially constant with age. Thus with increasing age, the only adjustment made to the EAR was the reduction associated with menopause. Iron EAR and RDA Summary, Ages 19 Years and Older The EAR has been set by modeling the components of iron re- quirements, estimating the requirement for absorbed iron at the fiftieth percentile, and with use of an upper limit of 18 percent iron absorption and rounding (Appendix Tables I-3 and I-4).
344 DIETARY REFERENCE INTAKES EAR for Men 19â30 years 6 mg/day of iron 31â50 years 6 mg/day of iron 51â70 years 6 mg/day of iron > 70 years 6 mg/day of iron EAR for Women 19â30 years 8.1 mg/day of iron 31â50 years 8.1 mg/day of iron 51â70 years 5 mg/day of iron > 70 years 5 mg/day of iron The RDA has been set by modeling the components of iron re- quirements, estimating the requirement for absorbed iron at the ninety-seven and one-half percentile, and with use of an upper limit of 18 percent iron absorption and rounding (Appendix Tables I-3 and I-4). RDA for Men 19â30 years 8 mg/day of iron 31â50 years 8 mg/day of iron 50â70 years 8 mg/day of iron > 70 years 8 mg/day of iron RDA for Women 19â30 years 18 mg/day of iron 31â50 years 18 mg/day of iron 51â70 years 8 mg/day of iron > 70 years 8 mg/day of iron Pregnancy Evidence Considered in Estimating the Average Requirement Factorial modeling is used to estimate median requirements of pregnant women (see âSelection of Indicators for Estimating the Requirement for IronâFactorial Modelingâ) with use of the equation: Requirement for absorbed iron = basal losses + iron deposited in fetus and related tissues + iron utilized in expansion of hemoglobin mass. Basal Losses. Using a body weight of 64 kg for a nonpregnant
IRON 345 woman and an average basal loss of 14 Âµg/kg (Green et al., 1968), basal iron losses were calculated to be 0.896 mg/day (64 kg Ã 0.014 mg/kg) or approximately 250 mg for the entire pregnancy (280 days). Fetal and Placental Iron Deposition. Numerous estimates of the iron content of the fetus and placental tissue exist. In the computation of the requirements, an estimate of 315 mg has been used (FAO/ WHO, 1988). Bothwell and coworkers (1979) and Bothwell (2000) offered an estimate of 360 mg/pregnancy (270 + 90), whereas Hytten and Leitch (1971) suggested a total of 450 mg/pregnancy (375 + 75) but noted that there were insufficient data to estimate deposition by trimester. Thus, while there is considerable disagree- ment regarding these estimates, there are no new data to deter- mine which estimate is more accurate. For this reason, the FAO/ WHO total of 315 mg of iron partitioned by trimester was used. Increase in Hemoglobin Mass. Although controversy continues, a general accepted value for iron needed to allow for expansion of hemoglobin mass is approximately 500 mg (FAO/WHO, 1988). Hemoglobin mass changes very little during the first trimester but expands greatly during the second and third trimesters. Informa- tion on the precise timing of the increase remains uncertain. For modeling, an equal division between the second and third trimes- ters is assumed in keeping with FAO/WHO (1988). The actual magnitude of hemoglobin mass expansion depends on the extent of iron supplementation provided (De Leeuw et al., 1966). Beaton (2000) suggested that for every 10 g/L difference in the final hemoglobin concentration in the last trimester of preg- nancy, there would be a difference of about 175 mg in the estimate of need for absorbed iron. It follows from this that the estimate of iron needs in pregnancy is directly dependent upon the cut-off that is used for hemoglobin concentration. In turn, that cut-off may depend on whether one believes that the iron needs of pregnancy can ever be met by diet alone. Evidence is needed concerning the functional significance of using a somewhat lower cut-off for final hemoglobin concentration. In this connection, it is to be recog- nized that by using a high hemoglobin concentration, the efficiency of dietary iron utilization is being targeted given that iron absorp- tion is strongly affected by body iron status (Beaton, 2000). At this time, the hemoglobin concentration implied by the reference curve portrayed in Figure 9-1 is accepted. With the above estimates, the total usage of iron throughout preg- nancy is 250 mg (basal losses) + 320 mg (fetal and placental deposi-
346 DIETARY REFERENCE INTAKES FIGURE 9-1 Hemoglobin concentrations in healthy, iron-supplemented (100â325 mg/day) pregnant women living in industrialized countries. The upper solid line represents the median hemoglobin concentration. The lower dashed curve repre- sents the fifth percentile of hemoglobin concentration. SOURCE: IOM (1993). tion) + 500 mg (increase in hemoglobin mass), or 1,070 mg. At delivery, actual loss of iron in blood, including blood trapped in the placenta, may be in the range of 150 to 250 mg. That implies that of the 500 mg allowed for erythrocyte mass expansion during preg- nancy, as much as 250 to 350 mg remains in the body to revert to maternal stores. The net cost of pregnancy could then be estimated as approximately 700 to 800 mg of iron (1,070 â [250 to 350]). This amount could be seen as the obligatory need for absorbed iron. Iron is not utilized at a uniform rate during pregnancy. The esti- mates of deposition of iron in the conceptus by stage of pregnancy are presented in Table 9-14. Dietary Iron Bioavailability. The upper limit of dietary iron absorp- tion is approximately 25 percent during the second and third tri- mesters (Barrett et al., 1994). This may be an underestimate of efficiency, coupled perhaps with the acceptance of too high a target for third trimester hemoglobin concentrations.
IRON 347 Table 9-15 presents a summary of the factorial model for estima- tion of median physiological needs, and Table 9-16 translates this to the median dietary iron requirement for pregnant women for each trimester. The iron requirement for women during the first tri- mester is less than that for premenopausal women because men- struation has ceased. TABLE 9-14 Estimated Deposition of Iron in Conceptus by Stage of Pregnancy Umbilicus and Stage of Pregnancy Fetus (mg) Placenta (mg) Total (mg) First trimester 25 5 30 Second trimester 75 25 100 Third trimester 145 45 190 Total 245 75 320 SOURCE: Based on Bothwell and Charlton (1981). TABLE 9-15 Summary of Absorbed Iron Requirements in Pregnant Adult Women Total Basal Erythrocyte Fetus and Absorbed Iron Stage of Losses Mass mg/d Placenta mg/d Requirement Gestation (mg/d) (mg/trimester) (mg/trimester) (mg/d) First trimester 0.896 â 0.27 (25) 1.2 Second trimester 0.896 2.7 (250) 1.1 (100) 4.7 Third trimester 0.896 2.7 (250) 2.0 (190) 5.6 TABLE 9-16 Dietary Iron Requirement During Pregnancy Absorbed Iron Absorption Requirement Stage of Gestation Requirement (mg/d) (%)a (mg/d) First trimester 1.2 18 6.4 Second trimester 4.7 25 18.8 Third trimester 5.6 25 22.4 a Absorption efficiency in the first trimester is as estimated for nonpregnant females; in the second and third trimesters, the efficiency is increased to 25 percent by the in- creased demand for iron as part of the physiological regulation of iron flux.
348 DIETARY REFERENCE INTAKES Estimation of the Variability of Requirements. Several approaches re- garding components of variation could be considered in estimating the CV for iron needs in pregnancy: â¢ variability of basal requirement based on prepregnancy body weight; this would then need to be matched with the estimates of basal losses in nonpregnant females; â¢ variability of iron in the fetus based on variation in fetal weight at term; basing variability on birth weight alone would be a conser- vative (low) approach; â¢ variability of blood iron based on variation in hemoglobin con- centration (SD of about 9 g/L) ignoring variation in blood volume; and â¢ variation based on the responses to level of iron supplementa- tion. The most conservative approach is based on variation in basal loss and assumes a CV of body weight of 21 percent (see âAdults Ages 19 Years and Olderâ) and a CV of hemoglobin concentration in iron- supplemented women during the third trimester of about 7 percent (9 g/L/135 g/L) (Beaton et al., 1989). When these assumptions are applied, with basal losses based on prepregnancy weight, the iron need for products of conception is 315 Â± 66.2 (SD), and the iron need for hemoglobin mass expansion is 500 Â± 35 (SD). For the total pregnancy, this model yielded an estimated requirement of 1,055 mg Â± 99.2 (SD) (CV, 9.4 percent). Table 9-16 summarizes the aver- age requirement for absorbed and dietary iron for each trimester. To estimate the needs of pregnant adolescents, the approach described above was followed with the notable exception that for adolescents the factorial model included basal losses and iron depo- sition in tissue as computed for adolescents. The fact that birth weights for adolescent mothers tend to be lower than for older women was ignored. In adolescents, the ninety-seven and one-half percentile of requirement was estimated for each trimester from simulation models rather than deriving one CV estimate and apply- ing it to all three trimesters. Iron EAR and RDA Summary, Pregnancy The EAR and RDA are established by using estimates for the third trimester to build iron stores during the first trimester of preg- nancy.
IRON 349 EAR for Pregnancy 14â18 years 23 mg/day of iron 19â30 years 22 mg/day of iron 31â50 years 22 mg/day of iron The RDA has been set by modeling the components of iron re- quirements, estimating the requirement for absorbed iron at the ninety-seven and one-half percentile, and using an upper limit of 25 percent iron absorption and rounding. RDA for Pregnancy 14â18 years 27 mg/day of iron 19â30 years 27 mg/day of iron 31â50 years 27 mg/day of iron Lactation Evidence Considered in Estimating the Average Requirement Components of Requirement. Until menstruation resumes, assumed to be after 6 months of exclusive breast feeding, median iron needs during lactation are estimated as the sum of iron secretion in human milk and basal iron losses calculated for nonpregnant, nonlactating women (0.896 mg/day). The derived estimate of iron secreted in mature human milk is 0.27 Â± 0.089 (SD) mg/day (0.35 mg/L Ã 0.78 L/day) (Table 9-5 and Chapter 2). Therefore, the median total requirement for absorbed iron is 1.17 mg/day (0.896 mg/day + 0.27 mg/day). For adolescent lactating mothers, the approach was identical to the one above except that in addition to basal losses (0.85 mg/day) and milk secretion (0.27 mg/day), provision was made for the deposition of iron in tissues (0.001 mg/day) and hemoglobin mass (0.14 mg/day) (see Table 9-10) as part of expected growth of the mother. Thus, the median requirement for absorbed iron is 1.26 mg/day (0.85 + 0.27 + 0.001 + 0.14). Again, a simulation model was used to derive the ninety-seven and one-half percentile of need. Dietary Iron Bioavailability. To estimate the total iron requirement for lactation, iron secreted in milk and basal iron loss must be add- ed by means of simulated distribution. The resultant distribution of iron needs, and assuming 18 percent absorption, results in the EARs and RDAs listed below.
350 DIETARY REFERENCE INTAKES Estimation of the Variability of Requirements. The variability of re- quirement was based on basal needs modeled as described for non- pregnant, nonlactating women and milk secretion modeled from the above distribution. Large breast-feeding studies suggest CVs be- tween 10 and 40 percent (Dewey and Lonnerdal, 1983; Vaughan et al., 1979). A CV of 30 percent was adopted and iron concentration of mature human milk, for the purpose of modeling, is taken as 0.35 mg/L assuming normality with a CV of 33 percent. Iron EAR and RDA Summary, Lactation EAR for Lactation 14â18 years 7 mg/day of iron 19â30 years 6.5 mg/day of iron 31â50 years 6.5 mg/day of iron The RDA for iron is set by determining the estimate of require- ments at the ninety-seven and one-half percentile. RDA for Lactation 14â18 years 10 mg/day of iron 19â30 years 9 mg/day of iron 31â50 years 9 mg/day of iron Special Considerations Use of Oral Contraceptives and Hormone Replacement Therapy It has been reported that approximately 17 percent of women in the United States use oral contraceptives (Abma et al., 1997), which are known to reduce menstrual blood loss. Although many studies have documented lower menstrual blood losses among women using oral contraceptives, only one study actually allowed estimation of the magnitude of reduction, compared to expected loss. A reanalysis of data from that study (Nilsson and Solvell, 1967) suggested that a reasonable estimate of effect would be the equivalent of a 60 per- cent reduction from expected loss. Therefore, the requirement at the fiftieth and ninety-seven and one-half percentile for adolescent girls taking oral contraceptives is 6.9 and 11.4 mg/day, respectively and 6.4 and 10.9 mg/day for premenopausal women (see Appendix Table I-4). Hormone replacement therapy (HRT), which provides estrogen and progesterone, is commonly practiced by postmenopausal
IRON 351 women. Some uterine bleeding can occur in some women during HRT, especially during the first year of therapy (Archer et al., 1999; MacLennan et al., 1993; Oosterbaan et al., 1995). Therefore, women on HRT who continue to menstruate may have higher iron require- ments than postmenopausal women who are not on HRT. Vegetarianism As previously discussed, iron is more bioavailable from meat than from plant-derived foods. Meat and fish also enhance the absorp- tion of nonheme iron. Therefore, nonheme iron absorption is lower for those consuming vegetarian diets than for those eating non- vegetarian diets (Hunt and Roughead, 1999). Serum ferritin con- centrations have been observed to be markedly lower in vegetarian men, women, and children than in those consuming a nonvegetarian diet (Alexander et al., 1994; Dwyer et al., 1982; Shaw et al., 1995). For these reasons, individuals who typically consume vegetarian diets may have difficulty consuming adequate intakes of bioavail- able iron to meet the EAR. Cook and coworkers (1991) compared iron bioavailability from single meals with that of a diet consumed over a 2-week period. There was a 4.4-fold difference between max- imally enhancing and maximally inhibiting single meals, but the difference was only two-fold when measured over the 2-week period. It is therefore estimated that the bioavailability of iron from a vegetarian diet is approximately 10 percent, rather than the 18 per- cent from a mixed Western diet. Hence the requirement for iron is 1.8 times higher for vegetarians. It is important to emphasize that lower bioavailability diets (approaching 5 percent overall absorp- tion) may be encountered with very strict vegetarianism and in some developing countries where access to a variety of foods is limited. Intestinal Parasitic Infection Intestinal parasites infect approximately 1 billion of the worldâs population. Some of the parasites, particularly hookworm, cause significant intestinal blood loss. These infections are prevalent in developing countries where the intake of bioavailable iron is often inadequate. When possible, the primary intervention should be elimination of the parasitic infection. In addition, an adequate intake of bioavailable dietary iron may be necessary to treat iron deficiency. When bioavailable dietary iron is not available, supplemental iron may be needed. Various regimens are provided for such groups at
352 DIETARY REFERENCE INTAKES risk of iron deficiency anemia (Stoltzfus and Dreyfuss, 1998; WHO/ UNICEF/UNU, 1998). Blood Donation An annual donation of 0.5 L of blood is equivalent to between 200 and 250 mg of iron, which represents approximately 0.6 to 0.7 mg/day. Blood donors have lower serum ferritin concentrations than nondonors (Milman and Kirchhoff, 1991a, 1991b). More fre- quent donations can be problematic, especially for women, result- ing in a need for supplemental iron (Garry et al., 1995). Increased Iron Losses in Exercise and Intense Endurance Training Many reviewers of the scientific literature conclude that iron status is marginal or inadequate in a large number of individuals, particu- larly females, who engage in regular physical exercise (Clarkson and Haymes, 1995; Raunikar and Sabio, 1992; Weaver and Rajaram, 1992). Dietary intake patterns of these individuals are frequently suboptimal with a reduced intake of a number of micronutrients. Weaver and Rajaram (1992) estimated that daily iron losses increase to 1.75 mg/day in male athletes and to 2.3 mg/day in female athletes with prolonged training. This is in contrast to a whole body loss of iron of approximately 1.08 mg/day in males beyond puberty and 1.45 mg/day in menstruating females. Ehn and coworkers (1980) demonstrated that highly trained, long distance runners have a bio- logic half-life of body iron of only approximately 1,000 days, a sig- nificantly shorter time than the 1,300 and 1,200 days, respectively, of male and female nonexercisers. Several reviewers of this topic conclude that increased fecal losses and perhaps sporadic hematuria contribute to depressed iron stores in athletic segments of the pop- ulation (Siegel et al., 1979; Stewart et al., 1984). There is a notable reduction in hematologic parameters that could be the result of increased intravascular hemolysis of erythrocytes. Many studies have found an increased rate of erythrocyte turnover and fragility in athletes (Lampe et al., 1991; Newhouse and Clement, 1995; Rowland et al., 1991). Thus, several mechanisms by which iron balance could be affected by intense physical exercise have been advanced (Fogelholm, 1995; Magnusson et al., 1984; Weight, 1993), including increased gastrointestinal blood losses after running, and hemoglobinuria as a result of erythrocyte rupture within the foot during running. For the above reasons, and based on the strong whole body iron loss data collected by Ehn and coworkers (1980),
IRON 353 the EAR for iron will conservatively be 30 percent greater for those who engage in regular intense exercise. If the estimate of Weaver and Rajaram (1992) is used, the EAR may be as much as 70 percent greater in the subpopulation of athletes. Validation of Requirement Estimates The theoretical and operational derivation of iron requirement estimates has been described for each life stage group. Require- ments have been based on the estimation of the amount of iron needed to meet body functions with minimal storage. This level of nutriture is marked by a serum ferritin concentration of about 15 Âµg/L in children, adolescents, and adults and by a somewhat lower concentration (10 to 12 Âµg/L) in infants. Percentiles of the simulat- ed distributions of requirement are presented in Appendix Tables I-3 and I-4. The prevalence of apparently inadequate intakes is estimated through an assessment of the estimated distribution of usual intakes and by applying risk tables (Appendix Tables I-5, I-6, I-7) derived from the estimated requirement distributions and compared with the estimated prevalence of inadequate iron status based on serum ferritin concentration (see Table 14-1). The data sets used in this comparison were USDA CSFII 1994â1996 for iron intake and NHANES III for serum ferritin concentration. Statistical procedures were used to derive estimates of the usual iron intake or usual serum ferritin concentration; the data were also adjusted, with use of reported weighting factors, to represent the U.S. population and to compensate for the fact that sampling weights were not identical in the two data sets. Table 9-17 presents the outcome of this compari- son. Considering that the dietary data do not include iron ingested as direct supplements and that no adjustment for alleged under- reporting has been made, the agreement between apparent dietary inadequacy and apparent biochemical deficiency is reasonable for most age groups. Children Ages 1 through 8 Years The estimated prevalence of inadequate intake is lower (less than 5 percent) than the estimated prevalence of inadequate iron status for children (Table 9-17). One reason for the lack of congruence between iron intake and iron status may be the lack of validation of cut-off concentrations for serum ferritin in young children. Although studies have confirmed the correlation between a lack of storage
354 DIETARY REFERENCE INTAKES TABLE 9-17 Comparison of Estimated Prevalence of Apparently Inadequate Iron Intakes and Serum Ferritin Concentrations Indicative of Apparent Iron Deficiency, Third National Health and Nutrition Examination Survey, 1988â1994 Prevalence of Biochemical Deficiency (%) Usual Intake Prevalence (mean Â± Apparently Ferritin Ferritin Age, standard Inadequate Concentration Concentration Gender deviation) Intakes (%) < 15 Âµg/L < 10 Âµg/L 1â3 y, both 10.9 Â± 4.0 <5 26 13 4â8 y, both 13.0 Â± 3.9 <5 6 9â13 y Male 17.9 Â± 5.7 <5 <5 Female 14.1 Â± 4.2 <5 8 14â18 y Male 20.1 Â± 6.9 <5 <5 Female 13.4 Â± 5.1 10 15 19â30 y Male 19.6 Â± 6.8 <5 15 Female 13.2 Â± 4.1 <5 13 31â50 y Male 19.6 Â± 6.8 <5 <5 Female 12.7 Â± 4.6 15â20 16 51â70 y Male 16.9 Â± 6.3 <5 <5 Female 12.3 Â± 4.1 <5 <5 71+ y Male 16.1 Â± 7.1 <5 <5 Female 12.4 Â± 4.9 <5 <5 NOTE: Data are limited to individuals who provided complete and reliable Day 1 dietary intake records. Breastfeeding infants and children were excluded from all analyses. The intake distributions for children 1â3 years of age are unadjusted. Percentiles for these groups were computed using SAS PROC UNIVARIATE . For all other groups, data were adjusted using the Iowa State University method using C-Side. SOURCE: ENVIRON International Corporation and Iowa State University Department of Statistics, 2000.
IRON 355 iron and low ferritin concentrations, such studies have not been conducted in children. Thus ferritin concentrations of 10 and 15 Âµg/L may not be indicative of low iron stores in children. Children and Adolescents Ages 9 through 18 Years When the predicted prevalence of inadequate intakes and the reported prevalence of iron deficiency are compared in those aged 9 through 18 years, agreement is not consistent (Table 9-17). For example, in girls aged 9 through 13 years, the prevalence of inade- quate intake is less than 5 percent, but the prevalence of low serum ferritin concentration is 8 percent. The lack of congruence of these results is likely due to the fact that a proportion of girls aged 12 and 13 years have reached menarche and have higher iron requirements than those who have not reached menarche. There is better con- gruence between dietary and biochemical estimates for 14- through 18-year-old girls whose iron requirements include menstrual iron losses. Among boys, the prevalences of inadequate intakes and low serum ferritin concentrations are both less than 5 percent. Adults Ages 19 Years and Older There is congruence between the prevalences of inadequate iron intakes and low serum ferritin concentrations for men and for pre- and postmenopausal women (Table 9-17). The prevalence of inade- quate iron intakes for premenopausal women is approximately 20 percent and the prevalence of low serum ferritin concentration is 13 to 16 percent, prevalences indicating that the additional iron requirements due to menstrual losses are not being met in this group of women. The overall pattern offers some degree of reassurance that the general model used to estimate requirements, the specific estimates of components of that model, and the assumed limits to bioavail- ability of dietary iron are reasonable. INTAKE OF IRON Food Sources The iron content of vegetables, fruits, breads, and pasta varies from 0.1 to 1.4 mg/serving. Because most grain products are forti- fied with iron, approximately one-half of ingested iron comes from bread and other grain products such as cereals and breakfast bars.
356 DIETARY REFERENCE INTAKES Some fortified cereals contain as much as 24 mg of iron per 1-cup serving. Heme iron represents only 7 to 10 percent of dietary iron of girls and women and only 8 to 12 percent of dietary iron for boys and men (Raper et al., 1984). Human milk provides approximately 0.27 mg/day (Table 9-5). Dietary Intake Data from nationally representative U.S. surveys are available to estimate iron intakes (Appendix Tables C-18, C-19, D-3, E-5). Data from these surveys indicate that the median daily intake of dietary iron by men is approximately 16 to 18 mg/day, and the median intake by pre- and postmenopausal women is approximately 12 mg/ day. Data from a survey done in two Canadian provinces showed that the dietary intake of iron by both men and women was slightly lower than intakes in the United States (Appendix Table F-2). The median intake of dietary iron by pregnant women was approximately 15 mg/day, which is less than the Estimated Average Requirement (EAR) of 22 mg/day, indicating the need for iron supplementation during pregnancy. Intake from Supplements Approximately 21 to 25 percent of women and 16 percent of men were reported to consume a supplement that contains iron (Moss et al., 1989; see Table 2-2). The median intake of iron from supple- ments is approximately 1 mg/day for men and women, an amount based on the difference in median iron intake from food plus sup- plements and food alone (Appendix Tables C-18 and C-19). The median iron intake from food plus supplements by pregnant women is approximately 21 mg/day. TOLERABLE UPPER INTAKE LEVELS The Tolerable Upper Intake Level (UL) is the highest level of daily nutrient intake that is likely to pose no risk of adverse health effects for almost all individuals. Although members of the general population should be advised not to routinely exceed the UL, intake above the UL may be appropriate for investigation within well- controlled clinical trials. Clinical trials of doses above the UL should not be discouraged, as long as subjects participating in these trials have signed informed consent documents regarding possible toxic- ity and as long as these trials employ appropriate safety monitoring
IRON 357 of trial subjects. In addition, the UL is not meant to apply to indi- viduals who receive iron under medical supervision. Hazard Identification Iron is a redox-active transition metal. In health, it is carried from one tissue to another bound to transferrin and stored in cells in the form of ferritin or hemosiderin. These proteins hold iron in the ferric state. Kinetic restrictions prevent the iron from being reduced by cellular reductants, and it is thus shielded from unwanted partici- pation in redox reactions (McCord, 1996). If the transport and storage mechanisms are overwhelmed, the free iron will immediately be chelated by cellular compounds, such as citrate or adenosyl diphosphate, that readily participate in redox reactions catalyzing the formation of highly toxic free radicals or the initiation of lipid peroxidation. Adverse Effects Acute Effects. There are reports of acute toxicity resulting from overdoses of medicinal iron, especially in young children (Anderson, 1994; Banner and Tong, 1986; NRC, 1979). Accidental iron over- dose is the most common cause of poisoning deaths in children under 6 years of age in the United States (FDA, 1997). Vomiting and diarrhea characterize the initial stages of iron intoxication. With increasing time after ingestion, at least five organ systems can be- come involved: cardiovascular, central nervous system, kidney, liver, and hematologic (Anderson, 1994). The severity of iron toxicity is related to the amount of elemental iron absorbed. Symptoms occur with doses between 20 and 60 mg/kg with the low end of the range associated primarily with gastrointestinal irritation while systemic toxicity occurs at the high end (McGuigan, 1996). These data, how- ever, are not used because acute intake data are not considered in setting a UL. Iron-Zinc Interactions. High intakes of iron supplements have been associated with reduced zinc absorption as measured by changes in serum zinc concentrations after dosing (Fung et al., 1997; Meadows et al., 1983; OâBrien et al., 2000; Solomons, 1986; Solomons and Jacob, 1981; Solomons et al., 1983). However, plasma zinc concen- trations are not considered to be good indicators of body zinc stores (Whittaker, 1998). Studies using zinc radioisotopes showed reduced zinc absorption when both minerals were administered in the fast-
358 DIETARY REFERENCE INTAKES ing state at an iron-zinc ratio of 25:1 but not at 1:1 or 2.5:1 (Sandstrom et al., 1985). When iron and zinc supplements were given with a meal, however, this effect was not observed. Other investigators have reported similar observations (Davidsson et al., 1995; Fairweather- Tait et al., 1995b; Valberg et al., 1984; Walsh et al., 1994; Yip et al., 1985). A radioisotope-labeling study by Davidsson and coworkers (1995) showed that fortifying foods such as bread, infant formula, and weaning foods with iron had no effect on zinc absorption. In general, the data indicate that large doses of supplemental iron inhibit zinc absorption if both are taken without food, but do not inhibit zinc absorption if they are consumed with food. Because there is no evidence of any clinically significant adverse effect asso- ciated with iron-zinc interactions, this effect is not used to deter- mine a UL for iron. Gastrointestinal Effects. High-dose iron supplements are commonly associated with constipation and other gastrointestinal (GI) effects including nausea, vomiting, and diarrhea (Blot et al., 1981; Brock et al., 1985; Coplin et al., 1991; Frykman et al., 1994; Hallberg et al., 1966c; Liguori, 1993; Lokken and Birkeland, 1979) (Table 9-18). Because GI effects are local, the frequency and severity of the effect depends on the amount of elemental iron released in the stomach (Hallberg et al., 1966c). The adverse effects of supplemental iron appear to be reduced when iron is taken with food (Brock et al., 1985). While most of the observed effects are relatively minor, some individuals have found them severe enough to stop further supple- mentation (Frykman et al., 1994). A single-blinded, 8-week study by Brock et al. (1985) reported âmoderate to severeâ GI effects in 50 percent of subjects taking 50 mg/day of elemental iron as ferrous sulfate. This finding is supported by other better-controlled, prospective studies showing GI effects at similar doses (Coplin et al., 1991; Frykman et al., 1994; Lokken and Birkeland, 1979). These data suggest a definite causal relation between high iron intake and GI effects. Secondary Iron Overload. Secondary iron overload occurs when the body iron stores are increased as a consequence of parenteral iron administration, repeated blood transfusions, or hematological dis- orders that increase the rate of iron absorption. Although the iron in patients with secondary iron overload tends to be stored initially in macrophages where it is less damaging, the typical pathological consequences of iron overload that are characteristic of hereditary hemochromatosis may eventually occur.
IRON 359 Whether an excessive iron intake alone can lead to secondary iron overload and associated organ damage is unknown. Some indi- viduals appear to control their rates of iron acquisition very effec- tively in the face of a high iron intake, but as yet there has been no study with a large number of experimental subjects and a sufficient duration to be certain of this conclusion. Individuals who are hetero- zygous for hemochromatosis manifest minor phenotypic expression, usually a slight to moderate increase in serum ferritin concentra- tions and transferrin saturation (Bulaj et al., 1996). Iron stores are modestly increased but do not continue to rise significantly with increasing age, and the pathological features of homozygous hemo- chromatosis do not occur. There is only one clear example of dietary iron overload. The high prevalence of iron overload in South African and Zimbabwean blacks is associated with the consumption of traditional beer with an average iron content of 80 mg/L (Bothwell et al., 1964). The iron is highly bioavailable and some people may consume several liters of the beer per day. Iron overload does not occur in members of the population who are not consuming large quantities of beer or iron. There is therefore little doubt that the high iron intake plays a major role in the pathogenesis of sub-Saharan iron overload. However, intake may not be the only factor. Gordeuk and coworkers (1992) collected evidence to suggest that there is also a genetic component involving a gene different from the HFE gene-linked hereditary hemochromatosis (Feder, 1999). Cardiovascular Disease. Sullivan (1981) first hypothesized that in- creased body iron plays a role in the development of coronary heart disease (CHD). This hypothesis was based on the difference in the prevalence of ischemic heart disease between men and postmeno- pausal women, on one hand, and between men and premenopausal women on the other. According to Sullivanâs hypothesis, the preva- lence of CHD is higher in men and increases after menopause in women as a result of higher body iron stores. Epidemiological support for this hypothesis was provided by Salonen and coworkers (1992). In a cohort study, they demonstrated a significant association between high serum ferritin concentrations and the risk of myocardial infarction (MI) among middle-aged men in Finland. Men with serum ferritin concentrations greater than 200 Âµg/L had a 2.2-fold greater risk of acute MI than men with levels less than 200 Âµg/L. The association was even stronger in those with high cholesterol concentrations. Their original conclusions were confirmed by a reanalysis of the same group of subjects after a
360 DIETARY REFERENCE INTAKES TABLE 9-18 Iron and Gastrointestinal (GI) Adverse Effects, by Increasing Dose Dose of Form of Iron (Fe) Suppleme Reference Sample Size Study Group Iron (mg Coplin et al., 1991 Ferrous sulfate 18â40 y 50 n = 38 women nonpregnant women Randomized double- blind, cross-over trial Bis-glycino Fe II 18â40 y 50 (chelated Fe)b nonpregnant women n = 38 women Brock et al., 1985 Ferrous sulfate 18â39 y 50 n = 272 women and menc nonpregnant women Single-blind, parallel group study Ferrous sulfate in wax matrix 18â39 y 50 prep (control group) nonpregnant women n = 271 women and menc Critical Study Frykman et al., 1994 n = 97 (total) Blood donors Controlled, double-blind Placebo crossover study n = 46 men 34â48 y n = 51 women 35â52 y Ferrous fumarate n = 23 men 34â45 y 60 n = 25 women 40â52 y
IRON 361 ects, Dose of Supplemental p Iron (mg/d) When Taken Duration Results/Comments 50 Before 2 wk 25/38 GI problemsa t women breakfast 7 abdominal pain 10 bloating 13 constipation 9 diarrhea 12 nausea 50 2 wk 23/38 GI problems t women 9 abdominal pain 9 bloating 13 constipation 7 diarrhea 9 nausea No placebo control 50 Before 8 wk 53 abdominal discomfortd t women breakfast 26 nausea d 5 vomiting 47 constipationd 26 diarrhead 50 Before 8 wk 25 abdominal discomfort t women breakfast 11 nausea 3 vomiting 18 constipation 13 diarrhea Wax matrix coating was used to help minimize GI distress rs Not indicated 4 wk GI side effects 14% all GI side effectsd 4% nausea 10% gastric pain 20% constipationd 19% diarrhea 60 Not indicated 4 wk GI side effects 25% all GI side effectsd 6% nausea 19% gastric pain 35% constipationd 37% diarrhea continued
362 DIETARY REFERENCE INTAKES TABLE 9-18 Continued Dose of Form of Iron (Fe) Suppleme Reference Sample Size Study Group Iron (mg Liguori, 1993 ITF 282 (iron protein 15â85 y 120 (60 m succinylate) 2Ã per Double-blind, n = 549 (64 men; 485 women) randomized, multicenter study Ferrous sulfate-controlled 15â88 y 105 release nonpregnant n = 546 (55 men; 491 women) Blot et al., 1981 Elemental iron 27.5 Â± 4.5 y 105 n = 132 pregnant women Lokken and Birkeland, Ferrous fumarate 18â28 y 120 1979 n = 19 Double-blind, cross-over Hallberg et al., 1966c (I) Placebo Blood donors n = 195 Ferrous sulfate Blood donors 222 n = 198 Hallberg et al., 1966c (II) Placebo Blood donors n = 119 Ferrous sulfate Blood donors 222 n = 120 Ferrous fumarate Blood donors 222 n = 118 Ferrous gluconate Blood donors 222 n = 120
IRON 363 Dose of Supplemental p Iron (mg/d) When Taken Duration Results/Comments 120 (60 mg Before 8.5 wk 63/546 (11%) GI side effects 2Ã per day) breakfast 31 epigastric pain and before 23 constipation dinner 32 abdominal pain 14 nausea 25 heartburn 105 Before 8.5 wk 127/549 (26%) GI side effectsd t breakfast 23 constipation 31 abdominal pain 14 nausea 33 heartburn No placebo control 105 90 d 14% had severe alimentary side effects No placebo control 120 8 wk 5/19 GI distress 1 diarrhea 1 epigastric pain 3 constipation 2/19 GI distress (placebo) 2 epigastric pain and constipation rs 14 d GI side effects 14% (6 men, 17 women) rs 222 14 d GI side effects 23% (6 men, 34 womend) rs 14 d GI side effects 14% (2 men, 16 women) rs 222 14 d GI side effects 28% (5 men, 26 womend) rs 222 14 d GI side effects 26% (4 men, 25 womend) rs 222 14 d GI side effects 31% (8 men, 27 womend) continued
364 DIETARY REFERENCE INTAKES TABLE 9-18 Continued Dose of Form of Iron (Fe) Suppleme Reference Sample Size Study Group Iron (mg Hallberg et al., 1966c (III) Placebo Blood donors n = 200 Ferrous sulfate Blood donors 180 n = 195 Ferrous glycine sulfonate Blood donors 180 n = 200 Ferrous gluconate Blood donors a Coplin et al. (1991) observed no statistically significant difference in frequency of side effects for the different preparations. b The iron glycine chelate has been shown to be more bioavailable than the sulfate. 5-year follow-up (Salonen et al., 1994). Another prospective cohort study reported an association between high serum ferritin concen- trations and carotid vascular disease (Kiechl et al., 1997). However, several other large prospective cohort studies failed to demonstrate a significant relationship between serum ferritin concentrations and increased risk for CHD (Aronow and Ahn, 1996; Frey and Krider, 1994; Magnusson et al., 1994; Manttari et al., 1994; Stampfer et al., 1993) (Table 9-19). The relationships between various other measures of iron status (e.g., serum transferrin saturation, serum iron concentration, and total iron-binding capacity) and CHD severity, incidence, or mortal- ity have been examined in other prospective cohort studies. Investi- gators reported that transferrin saturation (Liao et al., 1994), serum iron concentrations (Liao et al., 1994; Morrison et al., 1994; Reunanen et al., 1995), and total iron-binding capacity (Magnusson et al., 1994) were related to CHD (Tables 9-19 through 9-22). How- ever, some of these same studies and several other large prospective cohort studies failed to demonstrate any relationship with transferrin saturation (Baer et al., 1994; Reunanen et al., 1995; Sempos et al., 1994; Van Asperen et al., 1995) or total iron-binding capacity (Liao et al., 1994; Reunanen et al., 1995; Van Asperen et al., 1995).
IRON 365 Dose of Supplemental p Iron (mg/d) When Taken Duration Results/Comments rs 14 d GI side effects 12% (6 men, 16 women) rs 180 14 d GI side effects 26% (15 men, 30 women) rs 180 14 d GI side effects 24% (11 men, 33 women) rs GI side effects 27% (9 men, 39 women) c The 543 subjects evaluated in this study comprised 484 nonpregnant, premenopausal women and 59 men, aged 18 to 39 years. d Statistically significant difference (p < 0.05) in frequency of side-effects between the iron and the placebo group. Danesh and Appleby (1999) recently conducted a systematic assessment of 12 prospective epidemiological studies of iron status and CHD. They concluded that these studies do not support a strong association between iron status and CHD. There was no association between CHD and heterozygosity in two studies (Franco et al., 1998; Nassar et al., 1998). Two subsequent surveys from Europe demonstrated a two-fold increase in acute MI in heterozygous men (Tuomainen et al., 1999) and a 1.6-fold in- crease in overall CHD mortality in heterozygous women (Roest et al., 1999). In summary, the currently available data do not provide convincing support for an association between high body iron stores and increased risk for CHD. Taken as a whole, this body of evidence does not provide con- vincing support for a causal relationship between the level of dietary iron intake and the risk for CHD. However, it is also important to note that the evidence is insufficient to definitively exclude iron as a risk factor. Several studies suggest that the serum ferritin concen- tration is directly correlated with the risk for CHD. However, serum ferritin concentrations are affected by several factors other than dietary iron intake. The significance of the high serum ferritin con- centrations that have been observed in population surveys and the
366 DIETARY REFERENCE INTAKES TABLE 9-19 Serum Ferritin Concentration and Cardiovascular Disease Disease Other Ris Study Type of Study Follow-up Subjectsa Outcomeb Factors A Salonen et al., Prospective 3y 51 M MI Age, exam 1992 cohort (42, 48, 54, ischem or 60 y) G, HDL Stampfer et al., Nested About 10 y 238 M w/MI MI S, age, ot 1993 case-controle 238 M controls risk fac (40â84 y) Frey and Krider, Prospective 1â10 y 298 M MI (in 32 of Reviewed 1994 cohort (mean 5 y) (42â60 y)g 298 M) Ch, G, Magnusson Prospective 8.5 y 81 M MI Age, othe et al., 1994 cohort 18 W parame (25â74 y) HDL Manttari et al., Nested 5y 136 cases CHD Age, BM, 1994 case-control (M, 49 y) HDL, T 132 controls (M, 47 y) Salonen et al., Prospective 5y 83 M MI 1994 cohort Aronow and Prospective 3y 171 M New CHD Age, sex, Ahn, 1996 cohort 406 W events CHD (62â100 y) Kiechl et Prospective 5y 826 M/W Carotid Age, S, C al., 1997 cohort (40â79 y) athero- Alc, Hb sclerosis Hyp a M = men, W = women. b MI = myocardial infarction, CHD = coronary heart disease. c S = smoking, ECG = echocardiogram, BP = blood pressure, G = blood glucose, HDL = high density lipoprotein, TG = triglyceride concentration, AB = apolipoprotein B, L = blood leukocyte count, Fe = serum iron, Ch = total cholesterol, BM = body mass, Hb = hemoglobin concentration, Alc = alcohol intake, AA-1 = Apolipoprotein A-1, Hyp = hypertension. d Amount provided as mean concentrations. e The blood samples were collected in 1982 from 14,916 men aged 40â84 years without prior MI or stroke.
IRON 367 Ferritin ease Other Risk Concentration tcomeb Factors Assessedc (Âµg/L) d Association Age, exam year, S, > 200 Elevated ferritin concentration was a ischemic ECG test, BP, strong risk factor for acute MI G, HDL, TG, AB, L (relative risk [RR] = 2.2) compared to men with lower ferritin levels S, age, other coronary 250 cases No association was observed between risk factors 222 controls serum ferritin concentration and (p = 0.08)f risk of myocardial infarction (in 32 of Reviewed med. charts for 156 all No association of serum ferritin with 98 M) Ch, G, L, lipid profile 148 cases risk of MI Age, other Fe 198 (M) No association between serum parameters, BP, S, Ch, 91 (W) ferritin and MI, RR = 0.99 HDL D Age, BM, BP, Ch, Hb, 84 cases No association between serum HDL, TG, L, S 85 control ferritin concentration and risk of CHD â¥ 200 Elevated ferritin concentration was a strong risk factor for acute MI compared to men with lower ferritin levelsh w CHD Age, sex, S, prior 143 M-CHD No association with new CHD events vents CHD 146 M-no new events 122 W-CHD 128 W-no new events otid Age, S, Ch, BP, G, 185 with Serum ferritin was a strong predictor thero- Alc, Hb, AB, AA-1, athero- of atherosclerosis clerosis Hyp sclerosis 114 no athero- sclerosis f After adjusting for other coronary risk factors (aside from age and smoking), men with levels â¥ 200 Âµg/L had an relative risk of 1.1 compared with those having lower levels. g These patients were from two southern West Virginia counties with a high number of mine workers who had been exposed to mine dust. h This association remained statistically significant when the following risk factors were added to the model individually and together: systolic blood pressure, height, weight, body mass index, serum apolipoprotein B concentration, concentrations of triglycerides and HDL2 subfraction of high-density lipoprotein cholesterol, plasma fibrinogen con- centration, ischemia on exercise testing, maximal oxygen uptake, energy, and saturated fat intake.
368 DIETARY REFERENCE INTAKES TABLE 9-20 Transferrin Saturation and Coronary Heart Disease Follow-up Disease Adjusted Study Type of Study (y) Subjects a Outcomeb These Fac Baer et al., Retrospective 14 46,932 MI Age, race 1994 cohort (â¥ 30 y) educati history diabete systolic Liao et al., Follow-up 13 4,237 MI, CHD Age, S, C 1994 (40â74 y) educati Sempos et Cohort 13 4,518 CHD, MI Age, diab al., 1994 (47â74 y) educati Reunanen et Prospective 14 6,086 M Mortality S, BP, BM al., 1995 cohort 6,102 W from CHD history (45â64 y) disease Van Asperen Cohort 17 129 M Mortality Age, S, A et al., 1995 131 W from IHD Ch, BM (64â87 y) diabete a M = men, W = women. b MI = myocardial infarction, CHD = coronary heart disease, IHD = ischemic heart disease. TABLE 9-21 Serum Iron Concentration and Cardiovascular Disease Follow-up Disease Adjusted Study Type of Study Period (y) Subjectsa Outcomeb These Fac Liao et al., Prospective 13 1,827 M MI, CHD Age, systo 1994 cohort 2,410 W educati (40â74 y at baseline) Morrison et al., Cohort 15 10,000 MI Age, S, B 1994 diabete Reunanen et Prospective 15 6,086 M CHD Age, Ch, al., 1995 cohort 6,102 W diabete a M = men, W = women. b MI = myocardial infarction, CHD = coronary heart disease. c BP = blood pressure, Ch = total cholesterol, S = smoking.
IRON 369 Disease ease Adjusted for Intake tcomeb These Factorsc Values Association Age, race, S, Alc, Not provided Relative risk = 1.3 education, family The relative risk for subjects with history of CAD and increased iron stores (TS â¥ 62%) diabetes, G, Ch, BMI, was not statistically significant systolic BP CHD Age, S, Ch, systolic BP, Not provided Transferrin saturation inversely education related to CHD; not related to MI D, MI Age, diabetes, BP, S, Ch, Not provided Transferrin saturation is not related education to CHD or MI risk in men or women rtality S, BP, BMI, diabetes, Mean Fe Transferrin saturation was inversely om CHD history of heart intakes: but not significantly associated disease 17 mg (M) with CHD mortality 13 mg (W) rtality Age, S, Alc, systolic BP, Not provided No significant association between om IHD Ch, BMI, history of transferrin saturation and diabetes and IHD ischemic heart disease c S = smoking, Alc = alcohol intake, CAD = coronary artery disease, G = blood glucose, Ch = total cholesterol, BMI = body mass index, BP = blood pressure. ular ease Adjusted for tcomeb These Factorsc Relative Risk and Associations CHD Age, systolic BP, Ch, S, Inversely associated with MI and CHD in women education (0.82 and 0.86; p < 0.01) Inversely associated with CHD in men (0.92; p = 0.065) Age, S, BP, Ch, Serum iron significantly associated with risk of MI, diabetes status rate ratio = 2.18 (men); 5.53 (women) No association between risk of acute MI and dietary or supplemental iron intake D Age, Ch, BP, S, Risk for CHD mortality was highest in the lowest diabetes, obesity serum iron quartile
370 DIETARY REFERENCE INTAKES TABLE 9-22 Total Iron-binding Capacity and Cardiovascular Disease Follow-up Disease Adjusted Study Type of Study Period (y) Subjectsa Outcomeb These Fac Liao et al., Cohort 13 1,827 M CHD, MI Age, systo 1994 2,410 W educati Magnusson Prospective 8.5 2,036 M/W MI incidence Age, sex, et al., 1994 cohort (25â74 y) triglyce serum f (ferritin leukocy Reunanen Prospective 14 6,086 M CHD S, BP, BM et al., 1995 cohort 6,102 F mortality history (45â64 y at disease baseline) Van Asperen Prospective 17 129 M IHD Age, S, A et al., 1995 cohort 131 W mortality Ch, BM (64â87 y) of diab a M = men, W = women. b CHD = coronary heart disease, MI = myocardial infarction, IHD = ischemic heart disease. nature of the relationship between serum ferritin concentration and CHD risk remain to be determined. Cancer. The increased risk for hepatocellular carcinoma in indi- viduals with hereditary hemochromatosis and cirrhosis is well estab- lished (Powell, 1970). The evidence for an association between advanced hereditary hemochromatosis and other types of cancer is less certain. One large controlled study failed to demonstrate an increased incidence of extrahepatic malignancies (Niederau et al., 1985) whereas others have reported higher risk (Bomford and Williams, 1976; Hsing et al., 1995). Several epidemiological studies have reported a positive correla- tion between measures of iron status and cancer in the general population. Stevens and coworkers (1988) reported serum trans- ferrin saturation to be significantly higher among men who had cancer than among men who remained free of cancer. Further anal-
IRON 371 cular ease Adjusted for tcomeb These Factorsc Association D, MI Age, systolic BP, Ch, Total iron binding capacity not related to MI or CHD education, S incidence Age, sex, Ch, HDL, Total iron binding capacity was a significant triglycerides, BP, (p = 0.007) independent negative risk factor for serum ferritin, log CHD (ferritin), Fe, Hb, leukocyte count D S, BP, BMI, diabetes, No relationship between total iron binding capacity mortality history of heart and CHD mortality in men; an inverse disease (nonsignificant) association found in women D Age, S, Alc, systolic BP, No clear association between total iron binding mortality Ch, BMI, prevalence capacity and IHD of diabetes, IHD c BP = blood pressure, Ch = total cholesterol, S = smoking, HDL = high density lipo- protein cholesterol, Fe = serum iron, Hb = hemoglobin concentration, BMI = body mass index, Alc = alcohol intake. ysis of these data showed a significant positive correlation between transferrin saturation and cancer risk for both men and women (Stevens et al., 1994). However, these findings were not confirmed when follow-up was extended to 17 years and upon reanalysis of the data (Sempos et al., 1994). Selby and Friedman (1988) found a lower incidence of cancer in iron-depleted women, but the possible confounding effect of ciga- rette smoking was not eliminated in this study. Another prospective study found significantly higher serum iron concentrations in indi- viduals with colorectal cancer than in control subjects (Wurzelmann et al., 1996), but the differences in the serum iron concentrations were small and well within the normal range for the general popu- lation. The biological relevance of this finding is therefore ques- tionable. Nelson and coworkers (1994) reported an apparent association between serum ferritin concentrations and adenoma of the colon
372 DIETARY REFERENCE INTAKES in a case-control study of 264 men and 98 women. This association was independent of other risk factors including smoking, gender, and alcohol consumption. In a later study in heterozygous carriers of the gene for hemochromatosis, Nelson and coworkers (1995) found a small, but statistically significant increase in the apparent relative risk for colorectal cancer, hematological malignancy, colonic adenomas, and stomach cancer. There is no doubt that iron accumulation in the liver is a risk factor for hepatocellular carcinoma in patients with hemochroma- tosis. However, the evidence for a relationship between dietary iron intake and cancer, particularly colon cancer, in the general popula- tion is inconclusive. Identification of Distinct and Highly Sensitive Subpopulations Between 1 in 200 and 1 in 400 individuals of northern European descent are affected by an autosomal, recessive disorder known as hereditary hemochromatosis (Bacon et al., 1999). In populations of Celtic extraction, a single missense mutation of the hemochromatosis (HFE) gene (C282Y) is found in over 90 percent of affected individ- uals. A few patients with hemochromatosis are compound heterozy- gotes for C282Y and a second mutation, H63D, which is relatively common in the general population (Beutler et al., 2000), but on its own does not appear to cause iron overload (Worwood, 1999). The remaining patients lack an identified mutation suggesting evidence of other undiscovered genetic disorders. The clinical disorder is characterized by excessive absorption of food iron associated with the failure to store the additional iron in reticuloendothelial cells. The iron intake of these individuals is in the normal range. Iron accumulation occurs at a rate of about 2 mg/day with the develop- ment of clinical manifestations between the fourth and sixth decades of life. At this stage, the total body iron burden may reach 20 to 30 g. The additional iron is stored preferentially in parenchymal cells. Extensive organ damage is the result. If untreated, the disorder results in cirrhosis of the liver, primary liver cancer, myocardial injury with congestive cardiopathy and heart failure, and damage to endocrine organs, particularly the pancreatic islets and the anterior pituitary gland, with resultant diabetes and impotence or amenor- rhea (Bothwell and MacPhail, 1998; Walker et al., 1998). Arthritis and increased pigmentation of the skin are also characteristic find- ings (Bothwell et al., 1979; Olynyk et al., 1999). Individuals with hereditary hemochromatosis as described above (i.e., homozygotes for the HFE gene) are considered distinct and exceptionally sensi-
IRON 373 tive to the effects of iron overload; therefore, they were not consid- ered in deriving a UL for the general healthy population. Effective and widespread screening for early detection of hemochromatosis is needed so that studies investigating the adverse effects of dietary iron in individuals with this disorder will be useful in setting a UL for this subpopulation. Summary Gastrointestinal side effects were selected as the critical adverse effects on which to base the UL for iron. Although gastrointestinal distress is not a serious side effect when compared with the possible risk for vascular disease and cancer, the other side effects consid- ered (impaired zinc absorption, increased risk for vascular disease and cancer, and systemic iron overload) did not permit the deter- mination of a UL. Gastrointestinal distress is primarily observed in individuals who have consumed high levels of supplemental iron on an empty stomach. Large doses of iron supplements may inhibit zinc absorption when both are consumed in the fasting state, but zinc absorption is not impaired when supplementary iron is taken with meals. The relationship between iron intake and both vascular disease and cancer is unclear at the present time. With the possible exception of individuals living in Southern Africa who suffer from sub-Saharan iron overload, iron overload has not been shown to result solely from a high dietary iron intake. Moreover, no differ- ences were found in the serum ferritin concentrations between indi- viduals who fell in the lower and upper quartiles for total dietary iron intake in the Third National Health and Nutrition Examina- tion Survey (NHANES III) (Appendix Table H-5). Heterozygous carriers of the C282Y mutation most commonly associated with hereditary hemochromatosis could be at increased risk for accumu- lating harmful amounts of iron, but there are no direct observa- tions to confirm this suspicion. Homozygotes and individuals with other iron-loading disorders may not be protected by the UL and are addressed under âSpecial Considerationsâ. Dose-Response Assessment Adults Data Selection. The data on GI effects following supplemental in- takes of iron salts were used to derive a UL for iron for apparently healthy adults.
374 DIETARY REFERENCE INTAKES Identification of a No-Observed-Adverse-Effect Level (NOAEL) and a Lowest-Observed-Adverse-Effect Level (LOAEL). A LOAEL of 60 mg/day of supplemental iron salts was identified on the basis of a controlled, double-blind study by Frykman and coworkers (1994). They evaluated GI effects in 97 Swedish adult men and women after intake of either a nonheme iron supplement (60 mg/day as iron fumarate), a sup- plement containing both heme iron and nonheme iron (18 mg/ day, 2 mg from porcine blood and 16 mg as iron fumarate), or a placebo. The groups were similar with respect to gender, age, and basic iron status. The frequency of constipation and the total incidence of all side effects were significantly higher among those receiving nonheme iron than among those receiving either the com- bination of heme and nonheme iron or the placebo (Table 9-18). Although most of the reported GI effects were minor, five individuals found them to be severe enough to stop taking the medication. Four of these withdrawals occurred during the nonheme-containing iron treatment and one occurred just after changing from the nonheme-containing iron treatment to the placebo. To estimate a LOAEL for total iron intake, the LOAEL for supple- mental ferrous fumarate intake of 60 mg/day for Swedish men and women was added to 11 mg/day, the estimated mean iron intake from food in women from six European countries (Van de Vijver et al., 1999) and in men from Denmark (Bro et al., 1990). The LOAEL for total intake is therefore approximately 70 mg/day (11 + 60). It was not possible to identify a NOAEL based on the data on GI effects. Therefore, the LOAEL of 70 mg/day was used to derive a UL. There is supportive evidence for a LOAEL of 50 to 120 mg/day of supplemental iron salts from several other prospective studies (Brock et al., 1985; Coplin et al., 1991; Liguori, 1993; Lokken and Birkeland, 1979). However, these studies either failed to include a placebo control or contained fewer subjects than the study by Frykman and coworkers (1994). Uncertainty Assessment. An uncertainty factor (UF) of 1.5 was selected to account for extrapolation from a LOAEL to a NOAEL. Because of the self-limiting nature of the observed GI effects, a higher UF was not justified. Derivation of a UL. The LOAEL of 70 mg/day was divided by a UF of 1.5 to obtain a LOAEL and UL value of 45 mg/day of iron, after rounding.
IRON 375 UL = LOAEL = 70 mg/day â 45 mg/day UF 1.5 Iron UL Summary, Ages 19 Years and Older UL for Adults â¥ 19 years 45 mg/day of iron Pregnancy and Lactation Data are limited on GI effects in pregnant and lactating women. Rybo and Solvell (1971) compared the side effects of ferrous sul- fate, sustained release iron, and placebo in pregnant women. They found that the frequency of severe nausea or vomiting, or both, was significantly higher when 200 mg/day of elemental iron as ferrous sulfate was given than when placebo was given. The lack of data involving doses less than 100 mg/day in pregnant women presents uncertainty as to what dose constitutes a NOAEL for pregnant women. In the absence of data from studies involving lower doses, the UL for nonpregnant and nonlactating adult women (45 mg/day) was specified for pregnant and lactating women as well. Iron UL Summary, Pregnancy and Lactation UL for Pregnancy 14â18 years 45 mg/day of iron 19â50 years 45 mg/day of iron UL for Lactation 14â18 years 45 mg/day of iron 19â50 years 45 mg/day of iron Infants, Children, and Adolescents Data Selection. Data from several studies in infants and young chil- dren (Burman, 1972; Farquhar, 1963; Fuerth, 1972; Reeves and Yip, 1985) were judged appropriate for use in deriving a UL for infants and children, and in aggregate they define a dose-response rela- tionship. Identification of a NOAEL and a LOAEL. No adverse GI effects were reported when 1-month-old infants were supplemented with 5 mg/ day of nonheme iron for up to 1 year (Farquhar, 1963) and when 3-
376 DIETARY REFERENCE INTAKES month-old infants were supplemented with 10 mg/day of nonheme iron for up to 21 months (Burman, 1972) (i.e., no adverse effects as compared with infants supplemented with a placebo). Using a higher dose of supplemental nonheme iron (30 mg/day) for 18 months, Farquhar (1963) reported no adverse GI effects in 132 infants. Similarly, no significant adverse GI effects were reported when 124 infants 11 to 14 months of age were supplemented with 3 mg/kg body weight/day (approximately 30 mg/day) of nonheme iron for 3 months (Reeves and Yip, 1985). The median intake of iron for infants, aged 11 to 14 months, is approximately 10 mg/day. Thus, the above human data suggest that an intake of 40 mg/day would be a NOAEL for infants and young children. Uncertainty Assessment. There is little uncertainty regarding the range of intakes that is likely to induce GI effects in infants and young children. Therefore a UF of 1 is specified. Derivation of a UL. The NOAEL of 40 mg/day was divided by a UF of 1, resulting in a UL of 40 mg/day of supplemental nonheme iron for infants and young children. Because the safety of excess supple- mental nonheme iron in children aged 4 through 18 years has not been studied, a UL of 40 mg/day is recommended for children 4 through 13 years of age, and the adult UL of 45 mg/day is recom- mended for adolescents. Iron UL Summary, Ages 0 through 18 Years UL for Infants 0â12 months 40 mg/day of iron UL for Children 1â3 years 40 mg/day of iron 4â8 years 40 mg/day of iron 9â13 years 40 mg/day of iron UL for Adolescents 14â18 years 45 mg/day of iron Special Considerations Individuals with the following conditions are susceptible to the adverse effects of excess iron intake: hereditary hemochromatosis; chronic alcoholism, alcoholic cirrhosis, and other liver diseases;
IRON 377 iron-loading abnormalities, particularly thalassemias; congenital atransferrinemia; and aceruloplasminemia (Fairbanks, 1999). These individuals may not be protected by the UL for iron. A UL for subpopulations such as persons with hereditary hemochromatosis can not be determined until information on the relationship be- tween iron intake and the risk of adverse effects from excess iron stores becomes available. A body of experimental evidence suggests that intermittent dosing (once or twice per week) of iron supplements may be an effective means of controlling iron deficiency in developing countries (Beaton and McCabe, 1999). Under these circumstances, individuals receiv- ing intermittent doses of iron supplements may exceed the UL. The effects of intermittent dosing on gastrointestinal side effects has not been studied adequately. Intake Assessment Based on distribution data from NHANES III (Appendix Table C-19), the highest median reported intake of iron from food and supplements for all life stage and gender groups, excluding preg- nancy and lactation, was approximately 19 mg/day. This was the median intake reported by men 31 through 50 years of age. The highest intake from food and supplements at the ninetieth percen- tile reported for any life stage and gender group, excluding preg- nancy and lactation, was approximately 34 mg/day for men 51 years of age and older. This value is below the UL of 45 mg/day. Between 50 and 75 percent of pregnant and lactating women consumed iron from food and supplements at a level greater than 45 mg/day, but iron supplementation is usually supervised in pre- and postnatal care programs. Risk Characterization Based on a UL of 45 mg/day of iron for adults, the risk of adverse effects from dietary sources appears to be low. Gastrointestinal dis- tress does not occur from consuming a diet containing naturally occurring or fortified iron. Individuals taking iron salts at a level above the UL may encounter gastrointestinal side effects, especially when taken on an empty stomach. Twenty-five percent of men aged 31 to 50 years in the United States have ferritin concentrations greater than 200 Âµg/L (Appendix Table G-3), which may be a risk factor for cardiovascular disease (Sullivan, 1981). This prevalence is higher in men older than 50 years. However, the significance of
378 DIETARY REFERENCE INTAKES these high ferritin concentrations and their relationship to dietary iron intake is uncertain. Nevertheless, the association between a high iron intake and iron overload in sub-Saharan Africa makes it prudent to recommend that men and postmenopausal women avoid iron supplements and highly fortified foods. Currently, doses equal to or greater than the UL are used for the treatment of iron deficiency anemia. The UL is not meant to apply to individuals who are being treated with iron under close medical supervision. RESEARCH RECOMMENDATIONS FOR IRON â¢ Determination of the significance of high ferritin concentra- tion. â¢ Investigation of the effect of iron absorption and dietary iron on phenotypic expressions in individuals with hereditary hemo- chromatosis. â¢ Research to distinguish between hereditary hemochromatosis and iron overload. â¢ Study of the effect of limited iron intake during pregnancy on infant iron status during the first 6 months of life. â¢ Bioavailability of supplemental iron. â¢ Concurrence on valid indicators for assessing the effect of iron deficiency anemia on cognitive development and function. â¢ The risk of cardiovascular disease for those with high stores of body iron. â¢ The relationship between high iron stores in men and the bio- availability of dietary iron and impaired regulation of iron balance. â¢ The relationship between iron consumption and oxidative cellular damage. â¢ Integrative mechanisms of iron transporter proteins that influ- ence gastrointestinal absorption in various dietary conditions and physiologic states. REFERENCES AAP (American Academy of Pediatrics). 1999. Iron fortification of infant formulas. Pediatrics 104:119â123. Abma JC, Chandra A, Mosher WD, Peterson LS, Piccinino LJ. 1997. Fertility, family planning, and womenâs health: New data from the 1995 National Survey of Family Growth. Vital Health Stat 23:1â114. Abrams SA, Wen J, Stuff JE. 1997. Absorption of calcium, zinc, and iron from breast milk by five- to seven-month-old infants. Pediatr Res 41:384â390.
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