Dietary Fiber

Dietary fiber, dietary fibre, or sometimes roughage and ruffage is the indigestible portion of plant foods having two main components:

• soluble (may be prebiotic and/or viscous) fiber that is readily fermented in the colon into gases and physiologically active byproducts, and
• insoluble fiber (may be metabolically inert and provide bulking or metabolically fermented in the large intestine as a prebiotic fiber). Bulking fibers absorb water as they move through the digestive system, easing defecation. Fermentable insoluble fibers mildly promote regularity, although not to the exent that bulking fibers do, but they can be readily fermented in the colon into gases and physiologically active byproducts.

Dietary fibers can act by changing the nature of the contents of the gastrointestinal tract and by changing how other nutrients and chemicals are absorbed. Some types of soluble fiber absorb water to become a gelatinous, viscous substance and is fermented by bacteria in the digestive tract. Some types of insoluble fiber have bulking action and is not fermented. Lignin, a major dietary insoluble fiber source, may alter the rate and metabolism of soluble fibers. Other types of insoluble fiber, notably resistant starch, are fully fermented.

Chemically, dietary fiber consists of non-starch polysaccharides such as arabinoxylans, cellulose, and many other plant components such as resistant starch, resistant 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 something of a misnomer, since many types of so-called dietary fiber are not actually fibrous.

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 healthful compounds during the fermentation of soluble fiber, and insoluble fiber's ability (via its passive hygroscopic properties) to increase bulk, soften stool, and shorten transit time through the intestinal tract.

Disadvantages of a diet high in fiber is the potential for significant intestinal gas production and bloating. Constipation can occur if insufficient fluid is consumed with a high-fiber diet.