Dietary fiber, dietary fibre, or sometimes roughage and ruffage is the indigestible portion of food derived from plants and waste of animals that eat dietary fiber.

There are 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 which 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.

Effects of fiber intake[edit]

Research has shown that fiber may benefit health in several different ways. Lignin and probably related materials that are resistant to enzymatic degradation, diminish the nutritional value of foods.[59]

Table legend

Color coding of table entries:

Both Applies to both soluble and insoluble fiberSoluble Applies to soluble fiber onlyInsoluble Applies to insoluble fiber only

Effects

Increases food volume without increasing caloric content, providing satiety which may reduce appetite.

Attracts water and forms a viscous gel during digestion, slowing the emptying of the stomach and intestinal transit, shielding carbohydrates from enzymes, and delaying absorption of glucose, which lowers variance in blood sugar levels

Lowers total and LDL cholesterol, which may reduce the risk of cardiovascular disease

Regulates blood sugar, which may reduce glucose and insulin levels in diabetic patients and may lower risk of diabetes

Speeds the passage of foods through the digestive system, which facilitates regular defecation

Adds bulk to the stool, which alleviates constipation

Balances intestinal pHand stimulates intestinal fermentation production of short-chain fatty acids, which may reduce risk of colorectal cancer

Fiber does not bind to minerals and vitamins and therefore does not restrict their absorption, but rather evidence exists that fermentable fiber sources improve absorption of minerals, especially calcium. Some plant foods can reduce the absorption of minerals and vitamins like calcium, zinc, vitamin C, and magnesium, but this is caused by the presence of phytate (which is also thought to have important health benefits), not by fiber.

An experiment designed with a large sample and conducted by NIH-AARP Diet and Health Study studied the correlation between fiber intake and colorectal cancer. The analytic cohort consisted of 291 988 men and 197 623 women aged 50–71 y. Diet was assessed with a self-administered food-frequency qustionnaire at baseline in 1995-1996; 2974 incident colorectal cancer cases were identified during 5 y of follow-up. The result was that total fiber intake was not associated with colorectal cancer. But on the other hand, the analyses of fiber from different food sources showed that fiber from grains was associated with a lower risk of colorectal cancer.

Although many researchers believe that dietary fiber intake reduces risk of colon cancer, one study conducted by researchers at the Harvard School of Medicine of over 88,000 women did not show a statistically significant relationship between higher fiber consumption and lower rates of colorectal cancer or adenomas. Similarly, a 2010 study of 58,279 men found no relationship between dietary fiber and colorectal cancer.