Dietary fiber

Dietary fiber (also spelled fibre), or roughage, is the portion of plant-derived food that cannot be completely broken down by human digestive enzymes. Dietary fibers are diverse in chemical composition and can be grouped generally by their solubility, viscosity and fermentability, which affect how fibers are processed in the body. Dietary fiber has two main subtypes: soluble and insoluble. They are components of many foods from plants, such as legumes, whole grains, cereals, vegetables, fruits, nuts, and seeds. Regularly consuming substantial dietary fiber is generally associated with supporting health and lowering the risks of several diseases. Dietary fiber consists of non-starchpolysaccharides and other plant components, such as cellulose, resistant starch, resistant dextrins, inulins, lignins, chitins, pectins, beta-glucans, and oligosaccharides.

Food sources of dietary fiber have traditionally been divided according to proportion of soluble vs. insoluble fiber. The two types are defined according to their characteristics of viscosity and fermentability, with different advantages between the types. Plant foods contain both types in varying amounts. Bulking fibers, such as cellulose and hemicellulose (including psyllium), absorb and hold water, promoting bowel movement regularity. Viscous fibers, such as beta-glucan and psyllium, thicken the fecal mass. Fermentable fibers, such as resistant starch, xanthan gum, and inulin, feed the bacteria and microbiota of the large intestine and are metabolized to yield short-chain fatty acids, which have diverse roles in gastrointestinal health.

Soluble fiber (fermentable fiber, or prebiotic fiber) – which dissolves in water – is generally fermented in the colon into gases and physiologically active by-products, such as short-chain fatty acids, produced in the colon by gut bacteria. Examples are beta glucans (in oats, barley, and mushrooms) and raw guar gum. Psyllium – soluble, viscous, and non-fermented fiber – is a bulking fiber, retaining water as it moves through the digestive system, easing defecation. Soluble fiber is generally viscous and delays gastric emptying, which in humans can result in an extended feeling of fullness. Inulin (in chicory root), wheat dextrin, oligosaccharides, and resistant starches (in legumes and bananas) are soluble, non-viscous fibers. Regular intake of soluble fibers, such as beta-glucans, from oats or barley, have been shown to lower blood levels of LDLcholesterol. Soluble fiber supplements also significantly lower LDL cholesterol.

Insoluble fiber does not dissolve in water and is inert, unaffected by digestive enzymes in the upper gastrointestinal tract. Examples include wheat bran, cellulose, and lignin. Coarsely ground insoluble fiber triggers the secretion of mucus in the large intestine, providing bulking. However, finely ground insoluble fiber does not have this effect and can instead cause constipation. Some forms of insoluble fiber, such as resistant starches, can be fermented in the colon.