Dietary fiber

Dietary fiber or roughage is the indigestible portion of food derived from plants. It has two main components:

• Soluble fiber, which dissolves in water, is readily fermented in the colon into gases and physiologically active byproducts, and can be prebiotic and viscous.
• Insoluble fiber, which does not dissolve in water, is metabolically inert and provides bulking, or it can be prebiotic and metabolically ferment in the large intestine. Bulking fibers absorb water as they move through the digestive system, easing defecation.^[1]

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.^[2] 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.^[3] Lignin, a major dietary insoluble fiber source, may alter the rate and metabolism of soluble fibers.^[1] Other types of insoluble fiber, notably resistant starch, are fully fermented.^[4] Some but not all soluble plant fibers block intestinal mucosal adherence and translocation of potentially pathogenic bacteria and may therefore modulate intestinal inflammation, an effect that has been termed contrabiotic.^[5][6]

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, chitins, pectins, beta-glucans, and oligosaccharides.^[1] 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.^[1] 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. A disadvantage of a diet high in fiber is the potential for significant intestinal gas production and bloating.

Contents

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• 1Definition
• 2Types and sources of dietary fiber
  • 2.1Fiber contents in food
  • 2.2Plant sources of fiber
  • 2.3Fiber supplements
    • 2.3.1Inulins
    • 2.3.2Vegetable gums
• 3Mechanisms of action
  • 3.1Physicochemical properties
  • 3.2Dietary fiber and the upper gastrointestinal tract
  • 3.3Fiber in the colon
  • 3.4Dietary fiber and cholesterol metabolism
  • 3.5Dietary fiber and fecal weight
• 4Effects of fiber intake
  • 4.1Table legend
• 5Dietary fiber and obesity
• 6Guidelines on fiber intake
  • 6.1Fiber recommendations
    • 6.1.1United States
    • 6.1.2United Kingdom
• 7Fiber and calories
• 8Fiber and fermentation
• 9Short-chain fatty acids
• 10U.S. FDA-approved health claims
• 11Research
• 12See also
• 13Footnotes
• 14External links