Dietary fiber

Dietary fiber is a collective term for all food components derived from plant cell walls that are not digested by the endogenous secretions of the human digestive tract. It has no clearly defined composition. It may differ from foodstuff to foodstuff, and from diet to diet. Dietary fiber consists of pectic substances, hemicelluloses, plant gums and mucilages, algal polysaccharides, celluloses, and lignin. Further, tannins, indigestible proteins, plant pigments, waxes, siliceous materials, and phytic acid can be incorporated in the fiber matrix. These materials give bulk to the fecal matter, not only from their inherent mass, but also by their water-binding capacity. The amount of water bound can be four to six times the dry weight of the fiber.

Around 1970 it was suggested that dietary fiber is a protective factor against many diseases, prevalent in Western communities, e.g., colon cancer. This may be true, but harmful effects of overconsumption of fiber should not be overlooked. The various types of dietary fiber components have many reactive groups, including -COOH, -HPO3H, -OH, -SO3H and -NH2, to which metals, amino acids, proteins, and even sugars can be bound.

There are different ways of binding to dietary fiber. First, fiber components of many food products act like ion exchangers. Their binding capacity depends on pH and ionic composition of the bowel contents. Dietary fiber has the capacity to bind various metals, even if the phytic acid is removed. Disturbed Ca2+, Mg2+, Zn2+ and P balances have been observed in human subjects using diets rich in fiber in the form of whole wheat bread.

Secondly, amino acids and proteins are bound to dietary fiber. A diet containing 15% cellulose can cause a decrease in nitrogen absorption of as much as 8%. Carrageenans, which are highly indigestible, can cause a decrease in nitrogen absorption of about 16%. The interaction of dietary fiber with sugars does not result in a reduction of sugar absorption, but in a slow release of sugars into the bloodstream.

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