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Total dietary fiber can be categorized as shown above. For example, total dietary fiber can be divided into
nonstarch polysaccharides and lignin. Nonstarch polysaccharides can be further divided into noncellulosic
polysaccharides and cellulose. Note: Cellulose, lignin, and most hemicelluloses are not soluble in water, whereas
pectins and other polysaccharides (such as gums and mucilages) are water soluble. Adapted from Asp and Johansson (1984).
Selvendran and Du Pont (1984), Southgate et al. (1978), and Theander (1981).
Pectins, gums, and mucilages are soluble in water, as are some hemicelluloses, whereas lignins, cellulose, and most hemicelluloses are insoluble in water. Although the physical and chemical properties of different dietary fiber components, such as viscosity, water-holding capacity, ion-exchange capacity, and binding capacity, have been studied, these properties do not adequately predict the physiological properties of specific dietary fibers and of high-fiber whole foods.
Physiological Effects of Dietary Fiber
Some physiological effects of dietary fiber are systemic, whereas others are localized in the gastrointestinal tract. Diets with a high fiber content are high-volume diets, requiring longer mastication and ingestion time (Heaton, 1980), and subjective assessments indicate that they may increase satiety (Bolton et al., 1981; Duncan et al., 1983; Kay and Stitt, 1978). Although guar gum and pectin, both viscous fibers, have been shown in some studies to delay gastric emptying (Holt et al., 1979; Tadesse, 1982; Wilmshurst and Crawley, 1980), other studies fail to confirm this (Rydning et al., 1985).
Some soluble dietary fiber components, such as oat bran, pectin, and guar gum, stimulate fecal excretion of bile acids. However, wheat bran has no such effect; it promotes a different composition of bile acids than does pectin (Hillman et al., 1986; Pomare and Heaton, 1973; Pomare et al., 1976). Some soluble fibers also lower serum cholesterol (Judd and Truswell, 1985). In short-term studies (a single meal or a few days), soluble fiber fed to healthy subjects enhanced glucose tolerance and increased insulin sensitivity, but the results of longer studies are conflicting (LSRO, 1987).
Dietary fiber affects colonic function and activities of the microflora. High fiber intakes promote increased bacterial mass but do not alter the microflora composition (Baird et al., 1977; Drasar et al., 1976; Finegold and Sutter, 1978). Colonic bacteria attack fermentable fiber components and degrade at least a portion of them to short-chain fatty acids and gases. The role and importance of these short-chain fatty acids in the colon have not been determined.
Insoluble fibers such as wheat bran decrease intestinal transit time and increase stool weight and volume. Increased stool volume observed after high intakes of dietary fiber is due in part to indigestible remnants of plant cell walls and in part to increased bacterial mass, but certain dietary fibers may also result in increased fecal nitrogen excretion. Most studies have not assessed the source of the increased nitrogen excretion. Stephen and Cummings (1980b) concluded that most of it can be attributed to bacterial mass and to mucosal cell debris and intestinal secretions.
Sources of Dietary Fiber
Quantifying total dietary fiber as well as the different fiber components in foods has been extremely difficult, because the complex nature of