What is Dietary Fiber?

Dietary fiber, dietary fibre, or sometimes roughage and ruffage is the indigestible portion of plant foods having two main components:
Soluble fiber dissolves in water. It is readily fermented in the colon into gases and physiologically active byproducts, and can be prebiotic and/or viscous. Soluble fibers tend to slow the movement of food through the system.
Insoluble fiber does not dissolve in water. It can be metabolically inert and provide bulking or prebiotic, metabolically fermenting in the large intestine. Bulking fibers absorb water as they move through the digestive system, easing defecation. Fermentable insoluble fibers mildly promote stool regularity, although not to the extent that bulking fibers do, but they can be readily fermented in the colon into gases and physiologically active byproducts. Insoluble fibers tend to accelerate the movement of food through the system.

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 are 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.

Fiber Intake Recommendations

On average, North Americans consume less than 50% of the dietary fiber levels recommended for good health. In the preferred food choices of today's youth, this value may be as low as 20%, a factor considered by experts as contributing to the obesity levels seen in many developed countries.

The actual fiber intake gaps of different age groups of Americans are shown in the graph from USDA:

Recognizing the growing scientific evidence for physiological benefits of increased fiber intake, regulatory agencies such as the Food and Drug Administration (FDA) of the United States have given approvals to food products making health claims for fiber.

In clinical trials to date, these fiber sources were shown to significantly reduce blood cholesterol levels, an important factor for general cardiovascular health, and to lower risk of onset for some types of cancer.

Soluble (fermentable) fiber sources gaining FDA approval are:
Psyllium seed husk (7 grams per day)
Beta-glucan from oat bran, whole oats, oatrim, or rolled oats (3 grams per day)
Beta-glucan from whole grain or dry-milled barley (3 grams per day)

Other examples of fermentable fiber sources (from plant foods or biotechnology) used in functional foods and supplements include inulin, resistant dextrins, fructans, xanthan gum, cellulose, guar gum, fructooligosaccharides (FOS), and oligo- or polysaccharides.

Consistent intake of fermentable fiber through foods like berries and other fresh fruit, vegetables, whole grains, seeds, and nuts is now known to reduce risk of some of the world’s most prevalent diseases—obesity, diabetes, high blood cholesterol, cardiovascular disease, and numerous gastrointestinal disorders. In this last category are constipation, inflammatory bowel disease, ulcerative colitis, hemorrhoids, Crohn's disease, diverticulitis, and colon cancer—all disorders of the intestinal tract where fermentable fiber can provide healthful benefits.

Insufficient fiber in the diet can complicate defecation. Low-fiber feces are dehydrated and hardened, making them difficult to evacuate—defining constipation and possibly leading to development of hemorrhoids or anal fissures.

This definition may contain information from Wikipedia.