Carbohydrates and Fiber
Carbohydrates, along with fat and protein, are the macrocomponents of the dietthe principal dietary sources of energy. Alcohol (ethanol) is the only other important source of energy. The principal dietary carbohydrates are sugars and complex carbohydrates. The sugars include monosaccharides, such as glucose and fructose, and disaccharides, such as sucrose (table sugar), maltose, and lactose (milk sugar). (Complex carbohydrates (polysaccharides) comprise starches and dietary fibers. Starches are polymers of glucose. Dietary fibersa are mainly indigestible complex carbohydrates in plant cell walls (cellulose, hemicellulose, and pectin) and a variety of gums, mucilages, and algal polysaccharides. Lignin is a noncarbohydrate component of dietary fiber in plant cell walls. Dietary fibers are converted to some extent into absorbable fatty acids by intestinal microorganisms. Pentoses and some carbohydrate-related compounds are present in the diet in smaller amounts. This category includes such substances as organic acids (e.g., citric and malic acids) and a number of polyols (e.g., sorbitol, xylitol), all of which have some energy value. Proximate analysis of foods commonly omits direct analysis of carbohydrate.
a Dietary fiber should not be confused with crude fiber, a nutritionally obsolete term that refers to the residue (primarily cellulose and lignin) remaining after food is treated with acid and alkali. Foods generally contain more dietary fiber than crude fiber, but no consistent quantitative relationship) exists between the two. Some tables of food composition and some food labels present fiber fiber content in terms of crude fiber. Little quantitative information is yet available on the individual components of dietary fiber in specific foods.
The value for carbohydrate content of foods given in compositional tables usually is ''carbohydrate by difference," i.e., the residual weight after subtracting amounts of water, protein, fat, and ash found by analysis; this moiety includes sugars, starches, fiber, and small amounts of other organic compounds.
In the United States, the average intake of carbohydrates by adults was 287 g for males (USDA, 1986) and 177 g for females in 1985 (USDA, 1987). Of the carbohydrates in individual diets, an average of 41% comes from grain products and 23% comes from fruits and vegetables (Anderson, 1982; Wotecki et al., 1982). About half of the total digestible carbohydrate intake is made up of monosaccharides and disaccharides. These are found in fruits (sucrose, glucose, fructose, pentoses) and milk (lactose). Sugars in soft drinks, candies, jams, jellies, and sweet desserts are mainly sucrose and high-fructose corn syrup. Complex carbohydrates, which constitute the other half of digestible carbohydrate intake, are starches found predominantly in cereal grains and their products (flour, bread, rice, corn, oats, and barley), potatoes, legumes, and a few other vegetables.
Sugars and starches together are the major source of energy in the diet. In 1985, they provided an average of 45.3% of the energy in the diet of adult men in the United States (USDA, 1986). The corresponding figures for adult women and children 1 to 5 years of age were 46.4 and 52.0%, respectively (USDA, 1987). Eleven percent of the total energy intake, representing almost one-quarter of total carbohydrate intake, is provided by added sweeteners, mostly sucrose and high-fructose corn syrup (Glinsmann et al., 1986).
Fructose intake in the United States increased after the introduction of high-fructose corn syrup into the food supply in 1970. The product is formed by the enzymatic conversion of some of the glucose in cornstarch to fructose. Its fructose content ranges from 40% to almost 100%. In 1985, high-fructose corn syrup accounted for 30% of the total sweetener supply in the United States (Glinsmann et al., 1986; IFT, 1986). In soft drinks, for example, the use of sucrose has been almost completely abandoned in favor of a high-fructose corn syrup product containing 55% fructose, approximately 40% glucose, and about 5% other sugars (Bailey et al., 1988; GAO, 1984). It is unknown whether this increased intake of free fructose has any health consequences (Reiser and Hallfrisch, 1987).
Sugar alcohols, except for xylitol, occur naturally in fruits. Because these sweet substances are slowly and incompletely absorbed from
the digestive tract and are less cariogenic than many other sugars, sugar alcohols such as sorbitol are useful in products intended for special diets and are often found in dietetic candies and chewing gum. In some people, however, sugar alcohols create a laxative effect due to their slow and incomplete absorption. Consumption of products containing an ounce or more of sorbitol, for example, may result in soft stools and diarrhea (IFT, 1986).
Glucose absorbed in the intestine or produced by the liver is an important energy source for most tissues. Other dietary hexoses (fructose and the galactose moiety of the disaccharide lactose) are converted to glucose in the liver. Most amino acids, the glycerol component of fat, and some organic acids can be converted to glucose. Therefore, there is no absolute dietary requirement for carbohydrates, at least under most circumstances. In the absence of dietary carbohydrates, however, lipolysis of stored triglycerides and the oxidation of fatty acids increase and ketone bodies accumulate. A carbohydrate-free diet also is generally associated with an accelerated breakdown of dietary and tissue protein, loss of cations (especially sodium), and dehydration. These effects produced by low-carbohydrate diets or by fasting can be prevented by the daily ingestion of 50 to 100 g of carbohydrates (Calloway, 1971). Because of the desirability of limiting the intake of fat (see Chapter 5) and perhaps of protein (Chapter 6), the subcommittee recommends that more than half the energy requirement beyond infancy be provided by carbohydrates. One gram of carbohydrate yields 4 kcal. Thus, for people consuming as little as 2,000 kcal/day, the recommended intake would be at least 250 g. Similar recommendations, which emphasize an increased intake of complex carbohydrates rather than added sugars, have been made by other groups (see NRC, 1989).
Sugars, including sucrose in foods, provide the substrate for the microorganisms in the mouth that are responsible for tooth decay, particularly in children. Caries-producing potential depends on the ability of the carbohydrates to adhere to the tooth surface and the frequency of consumption, both of which affect the length of time carbohydrates are available as a bacterial substrate. Ingestion of other foods may inhibit the increase in oral hydrogen ion concentration associated with ingestion of sucrose and, hence, its cariogenicity. Thus, snacks of candies and other sweets may be more cariogenic than sugars consumed as part of a meal. An adequate intake of fluoride also inhibits tooth decay produced by dietary sugars.
Dietary fiber is the subject of considerable recent interest and extensive reviews (see, for example, LSRO, 1987; NRC, 1989; Vahouny and Kritchevsky, 1986). Because they are hygroscopic, dietary fibers soften the stool and, hence, promote normal elimination. Fiber-rich diets may also increase satiety. Some fiber components, including oat bran and pectin, also lower plasma cholesterol levels, either by binding bile acids or by other mechanisms.
The consumption of diets rich in plant foods (and therefore fiber) is inversely related to the incidence of cardiovascular disease, colon cancer, and diabetes (see NRC, 1989). Because an increase in dietary fiber consumption is almost invariably associated with a change in other dietary constituents, it is difficult to establish a clear relationship with dietary fiber alone. One plausible mechanism for an anticarcinogenic effect is the rapid passage of the digestive mass through the colon, thereby reducing the possibility that potential carcinogens have an opportunity to interact with the mucosal surface (NRC, 1982). In addition, the increased mass of the softer stool may dilute carcinogens.
Fibers may also bind to mineral elements. In this way, wheat bran may interfere with mineral absorption, but neither wheat bran nor other fibers, at the levels consumed in this country, appear to have an appreciable effect on the absorption of minerals (NRC, 1989).
In the United States, mean fiber intake is estimated to be approximately 12 g/day (Lanza et al., 1987). Over the last decade, many health organizations have recommended increasing the intake of complex carbohydrates in general or dietary fiber in particular (see, e.g., AHA, 1986; 1)H HS, 1986). The subcommittee recommends that a desirable fiber intake be achieved not by adding fiber concentrates to the diet, but by consumption of fruits, vegetables, legumes, and whole-grain cereals, which also provide minerals and vitamins (1)HHS, 1986; NRC, 1982).
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Reiser, S., and J. Hallfrisch. 1987. Metabolic Effects of Dietary Fructose. CRC Press, Boca Raton, Fla.
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Wotecki, C.E., S.D. Welch, N. Raper, and R.M. Marston. 1982. Recent trends and levels of dietary sugars and other caloric sweeteners. Pp. 1-27 in S. Reiser, ed. Metabolic Effects of Utilizable Dietary Carbohydrates. Marcel Dekker, New York.