Dietary Reference Intakes for Calcium, Phosphorus, Magnesium, Vitamin D, and Fluoride (1997) / Chapter Skim
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5 Phosphorus
5 Phosphorus
Pages 146-189
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From page 146... ...
The remaining 15 percent is clistributeci through the soft tissues (Diem, 1970~. Total phosphorus concentration in whole blood is 13 mmol/liter (40 mg/cil)
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From page 147... ...
Physiology of Absorption, Metabolism, and Excretion Food phosphorus is a mixture of inorganic and organic forms. Intestinal phosphatases hydrolyze the organic forms contained in ingested protoplasm, and thus most phosphorus absorption occurs as inorganic phosphate.
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From page 148... ...
Any reduction in absorption of phosphorus due to high amounts of clietary calcium were compensated for by parallel reductions in renal phosphorus excretion (DeVizia et al., 1985; Fomon and Nelson, 1993; Moya et al., 1992~. The least renal excretory work to maintain normal phosphorus homeostasis would be achieved with human milk as the major source of minerals during the first year of life.
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From page 149... ...
. Human milk-feci, compared with formula-feci, infants have a slightly lower plasma Pi (2.07 versus 2.25 mmol/liter or 6.4 versus 7.0 mg/cil)
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From page 150... ...
The relationship shown in Figure 5-1 holds only in adult inclivicluals with adequate renal function; that is, the slow rise of plasma Pi with rising phosphorus intake over most of the intake range applies only so long as excess absorbed phosphate can be spilled into the urine. However, in inclivicluals with recluceci renal function, phosphorus clearance remains essentially normal so long as GFR is at least 20 percent of mean adult normal values, largely because tubular reabsorption is reduced to match the reduction in filtered load.
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From page 151... ...
While outside of a mineralizing environment, ECF calcium and phosphorus concentrations are indefinitely stable at physiological pH and pCO2 ECF is supersaturated in the presence of the hyciroxyapatite crystal lattice. Hence, ECF supports calcium phosphate deposition only in the presence of a suitable crystal nucleus.
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From page 152... ...
(Ziegler and Fomon,1983~. Because infant formulas contain substantially greater amounts of phosphorus than human milk, the absorbed phosphorus from cow milk and soy formulas is twice that attained by human milk-feci infants (Moya et al., 1992~.
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From page 153... ...
Furthermore, the intake ratio, by itself, fails to take into account both differing bioavailabilities and physiological adaptive responses. For example, in term-born infants cluring the first year of life, a higher calcium content of soy-baseci formulas was found to recluce phosphorus absorption, but phosphorus retention was similar because of offsetting changes in renal phosphorus output (DeVizia et al., 1985~.
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From page 154... ...
Uncler such circumstances it would be inappropriate to conclude, simply on the basis of a departure from some theoretical Ca:P ratio, either that calcium intake should be elevated or, phosphorus intake recluceci. In balance studies in human adults, Ca:P molar ratios ranging from 0.08:1 to 2.40:1 (a 30-folci range)
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From page 155... ...
In infants, clietary intake of phosphorus spans a wicle range, clepenciing on whether the food is human milk, cow milk, aciapteci cow milk formula, or soy formula (see Table 5-2~. Moreover, the phosphorus concentration of human milk cleclines with progressing lactation, especially between 4 and 25 weeks of lactation (Atkinson et al., l995~.
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From page 156... ...
. b Phosphorus content of soy formula includes about 3 mmol/L, present as phytate phosphorus which is likely not to be bioavailable (DeVizia and Mansi, 1992)
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From page 157... ...
In full-term infants, severe hypophosphatemia from purely clietary causes is virtually unknown. It is likely to occur only in situations of poorly menage ci parenteral nutrition (in which intakes of phosphate are inadequate, with inappropriate administration of fluici and electrolyte therapy (which causes excessive renal phosphorus loss)
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From page 158... ...
And cluring senescence, if there is loss of bone or soft tissue mass, phosphorus balance will be negative. However, so long as plasma Pi remains within normal limits, these balances will reflect other changes occurring in the body and will not be an indicator of the adequacy of dietary phosphorus.
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From page 159... ...
The fact that growth and epiphyseal cartilage maturation in children are abnormal at even adult normal levels of Pi supports the assumption that subnormal Pi values are not adequate to sustain optimal tissue function. Therefore, in what follows, the requirement will be baseci on the intake associated with maintenance of serum Pi at the bottom end of the normal range.
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From page 160... ...
Using the term-born infant feci human milk as the model, the target range for serum Pi (the most appropriate biochemical indicator of dietary phosphorus adequacy during early life) , is 2.42 to 1.88 mmol/liter (7.5 to 5.8 mg/cil)
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From page 161... ...
Balance. Mean phosphorus intakes from human milk of 102 mg (3.3 mmol)
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From page 162... ...
Al Summary: Ages 7 through I2 months The basis of the AI for this age group is the average intake of phosphorus from human milk plus that obtained from infant foocis. For infants over 6 months of age, there are no available ciata on clietary phosphorus intakes from the combination of human milk and solici foocis.
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From page 163... ...
However, it must be emphasized that this database specifically exclucleci infants feci human milk, and thus the formula or cow milk in the infants' cliets would elevate the population distribution and mean phosphorus intakes. AI for Infants O through 6 months 100 mg (3.2 mmol)
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From page 164... ...
The value derived for phosphorus accretion in lean and osseous tissue is supported by estimates of phosphorus retention, 10 g (323 mmol) /kg body weight gained, cleriveci from balance studies in children aged 4 to 12 years (Fomon et al., 1982)
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From page 165... ...
Phosphorus intakes in excess of this amount would simply leaci to increased urinary loss. When urinary excretion is added to the accrued phosphorus of 54 mg (1.74 mmol)
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From page 166... ...
. The EAR value will provide for the calculated physiological need for phosphorus accretion in lean and bone mass, accounting for the expected urinary phosphorus loss at that dietary intake.
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From page 167... ...
No ciata are available on serum phosphorus or phosphorus balance at various phosphorus intakes in children age ci 4 through 8 years. To estimate tissue accretion of phosphorus, the compositions of lean and osseous tissue were calculated baseci on body weight of children growing from 4 through 8 years and known content of phosphorus in these tissues.
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From page 168... ...
/clay, for phosphorus intakes of children aged 4 through 8 years (see Appendix D)
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From page 169... ...
Phosphorus intakes necessary to meet the neecis for the aciclition of bone and soft tissue cluring this period of rapid growth can be calculated and acljusteci for by urinary output and absorptive eff~ciency. The main limitation of this approach for this age category is that tissue accretion values are not available for adolescents beyond 14 years; thus, preclicteci neecis for older adolescents may not be optimal to support any growth spurts beyond this age.
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From page 170... ...
Assuming a phosphorus content of bone mineral of 19 percent and a phosphorus content of soft tissue of 0.23 percent (Pennington, 1994) , daily phosphorus needs during peak growth would approximate 200 mg (6.5 mmol)
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From page 171... ...
/ciay for both boys and girls. Although these phosphorus intakes cover tissue accumulation neecis of the observed average adolescent, these intakes may not be optimal at the peak of the adolescent growth spurt.
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From page 172... ...
Although accretion ciata are not available for ages 14 through 18 years, it seems reasonable to maintain this EAR value for the older adolescent since a similar value was obtained using the serum phosphorus curve extrapolateci from adults. It should be noted, however, that this age range (9 through 18 years)
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From page 173... ...
The relationship between serum Pi and absorbed intake, as presented in Figure 5-1, allows estimation of the intakes associated with Pi values within the range typically consiclereci normal. The extrapolation from absorbed intake to ingested intake shown in Table 5-6 is baseci on an absorption efficiency for phosphorus of 60 to 65 percent, the value typically observed in studies of adults on mixed cliets (Heaneyanci Recker, 1982; Stanbury, 1971; Wilkinson, 1976~.
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From page 174... ...
, all males aged 19 through 50 years have phosphorus intakes above the EAR of 580 mg (18.7 mmol) /day (see Appendix D)
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From page 175... ...
. All of the men, age ci 51 through 70 years, haci phosphorus intakes in amounts above the EAR.
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From page 176... ...
2D concentrations also will lead to increased intestinal absorption of phosphorus. Balance studies in 24 pregnant women demonstrated positive phosphorus balance, which increaseci with length of pregnancy (Heaney and Skillman, 1971~.
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From page 177... ...
/day. Serum phosphorus concentrations cluring pregnancy are within the normal range at mean ciaily intakes of approximately 1,550 mg (50 mmol)
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From page 178... ...
Serum Pi. Despite the loss of phosphorus in milk, serum phosphorus concentrations in lactating women are in the high-normal or above-normal range, and they are higher in lactating women than in nonlactating women (Figure 5-2)
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From page 179... ...
The circles represent the serum phosphorus levels of lactating women (Bryne et al., 1987; Chan et al., 1982b; Cross et al., 1995a; Dobnig et al., 1995; Kalkwarf et al., 1996; Kent et al., 1990; Lopez et al., 1996; Specker et al., 1991a)
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From page 180... ...
a suggestion that high phosphorus intakes could reduce calcium absorption by complexing calcium in the chyme. Concern about high phosphorus intake has been raised in recent years because of a probable population-level increase in phosphorus intake through such sources as cola beverages and food phosphate additives.
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From page 181... ...
and to correspondingly elevated PTH concentrations (salvo and Heath, 1988; Silverberg et al., 1986~. It has been proposed that these adjustments in circulating calcium regulating hormones Associated with any elevation of serum Pi, even though within the usual normal range)
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From page 182... ...
In aciclition, with respect to high phosphorus intakes, chronic aciministration of 2 g (65 mmol) /clay phosphorus in men for at least 8 weeks procluceci no effect on calcium balance or calcium absorption relative to a cliet containing only 806 mg (26 mmol)
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From page 183... ...
However, in the first month of life, some infants exhibit unusual sensitivity to phosphorus intakes above those associated with human milk. In the past, the clinical syndrome of late neonatal hypocalcemic tetany was observed when infants were feci whole evaporated cow milk with a very high phosphorus content (DeVizia and Mansi, 1992~.
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From page 184... ...
Saturation of ECF with respect to calcium and phosphorus almost never occurs in inclivicluals with normal renal function, mainly because urine phosphate excretion rises in direct proportion to dietary intake. As Figure 5-1 shows, the upper limit of the normal adult range for serum Pi typically occurs at absorbed intakes above 2.2 g (71 mmol)
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From page 185... ...
Finally, given the evidence cited above that high phosphorus intakes in humans do not leaci to negative calcium balance or to increaseci bone resorption, it seems likely that the bone disease in other animals is more a consequence of low effective calcium intake than of high phosphorus intake per se. Interference with Calcium Absorption As noted, some concerns have been expressed that a high phosphorus intake could interfere with calcium nutrition by complexing calcium in the chyme and reducing its absorption (salvo and Heath, 1988; Calvo and Park, 1996~.
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From page 186... ...
A UL can be clefineci as an intake associated with the upper boundary of adult normal values of serum Pi. No reports exist of untoward effects following high clietary phosphorus intakes in humans.
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From page 187... ...
Uncertainty Assessment. No benefit is evident from serum Pi values above the usual normal range in adults.
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From page 188... ...
/day Special Considerations It is recognized that population groups such as professional athletes, military trainees, or those whose level of energy expenditure exceeds 6,000 kcal/day, may have dietary phosphorus intakes whose distributions overlap these limits. In such inclivicluals with phosphorus intakes above the UL, no harm is known to result.
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From page 189... ...
RESEARCH RECOMMENDATIONS · The model that relates absorbed phosphorus intake to serum phosphorus must be evaluated in clinical studies using oral phosphorus intakes, and investigated in children and adolescents as well as adults. · Bone mineral mass as a function of dietary phosphorus intake should be investigated at all stages of the life cycle.
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