What Is Dietary Fiber?

Legumes such as soybeans contain dietary fibers.

Dietary fiber, sometimes called roughage, is the indigestible portion of plant foods that pushes food through the digestive system, absorbs water and eases defecation. It acts by changing the nature of the contents of the gastrointestinal tract, and by changing how other nutrients and chemicals are absorbed.

Dietary fiber can be soluble (able to dissolve in water) or insoluble. Fiber cannot be digested. However, soluble fiber is fermented by bacteria in the digestive tract, and absorbs water to become a gelatinous substance. Insoluble fiber has bulking action but is mostly not fermentated.

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. 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.