From eHealthMD:

What Is Fiber?

"Fiber is a virtually indigestible substance that is found mainly in the outer layers of plants. Fiber is a special type of carbohydrate that passes through the human digestive system virtually unchanged, without being broken down into nutrients. Carbohydrates constitute the main source of energy for all body functions."

From Wikipedia, the free encyclopedia:

"Dietary fiber, sometimes called "roughage", is the indigestible portion of plant foods that pushes food through the digestive system, absorbing water and easing defecation.

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.

Dietary fiber can be soluble (able to dissolve in water) or insoluble (not able to dissolve in water). Soluble fiber, like all fiber, cannot be digested. But it does change as it passes through the digestive tract, being transformed (fermented) by bacteria there. Soluble fiber also absorbs water to become a gelatinous substance that passes through the body. Insoluble fiber, however, passes through the body largely unchanged."
Source(s):
http://www.ehealthmd.com/library/fiber/fib_whatis.html
http://en.wikipedia.org/wiki/Dietary_fiber

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