Dietary fiber (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.
Soluble fiber absorbs water to become a gelatinous, viscous substance and is fermented by bacteria in the digestive tract. Insoluble fiber has bulking action and is not fermented, although a major dietary insoluble fiber source, lignin, may alter the fate and metabolism of soluble fibers.
Chemically, dietary fiber consists of non-starch polysaccharides such as arabinoxylans, cellulose and many other plant components such as resistant dextrins, inulin, lignin, waxes, chitins, pectins, beta-glucans and oligosaccharides.
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.
The term dietary fibre also includes a type of starch known as resistant starch (found in pulses, partly-milled seeds and grains, some breakfast cereals) because it resists digestion in the small intestine and reaches the large intestine unchanged.
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.