Dietary fiber (fibre), sometimes called roughage, is the indigestible portion of plant foods having two main components - soluble (prebiotic, viscous) fiber that is readily fermented in the colon into gases and physiologically active byproducts, and insoluble fiber that is metabolically inert, absorbing water throughout the digestive system and easing defecation. It acts by changing the nature of the contents of the gastrointestinal tract, and by changing how other nutrients and chemicals are absorbed. although a major dietary insoluble fiber source - lignans - may alter the fate and metabolism of soluble fibers. Chemically, dietary fiber consists of non-starch polysaccharides such as cellulose and many other plant components such as dextrins, inulin, lignin, waxes, chitins, pectins, beta-glucans and oligosaccharides. A novel position has been adopted by the US Department of Agriculture to include functional fibers as isolated fiber sources that may be included in the diet. The term "fiber" is somewhat of a misnomer, since many types of so-called dietary fiber are not fibers at all. Food sources of dietary fiber are often divided according to whether they provide (predominantly) soluble or insoluble fiber. Plant foods contain both types of fiber in varying degrees according to the plant’s characteristics. Advantages of consuming fiber are the production of salubrious compounds during the fermentation of soluble fiber, and insoluble fiber's ability (via its passive hydrophilic properties) to increase bulk, soften stool and shorten transit time through the intestinal tract.

[Last contributor : Guy M , Content under LGPL licence]