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Fiber is part of plant that is resistant to hydrolysis (A chemical decomposition in which a substance is split into simpler compounds by addition or taking up of elements of water) by human digestive enzymes and, with exception of lignin, fibers are complex carbohydrates. These include pectin, gums, mucilages, hemicellulose, polysaccharides cellulose, and nonpolysaccharide lignins. Fibers are water-soluble except cellulose, hemicellulose, and lignin, all of which form part of cell walls. Soluble fibers are sticky and combine with water to form gel-like substances. Pectin is a water-soluble fiber found in soft fruits and vegetables. Gums that are common food additives are also water-soluble, found in stems and seeds of some tropical plants. In general, fruits are higher in pectin and vegetables are higher in cellulose. Although cellulose and hemicellulose are not hydrolyzed, intestinal bacterial can digest some fiber to produce lipid fragments known as short-chain fatty acids. These fatty acids are absorbed in colon and yield energy when metabolized.

Water Insoluble fibers pass through gastrointestinal track unchanged, absorbing up to 15 times their weight, important since they provide digestive tract with ‘bulk’ that helps facilitate food through intestines to be evacuated as solid waste; hence, fiber often is called "nature’s natural laxative". Cellulose’s ability to absorb water produces softer stools and regular bowel movements. Also, insoluble fiber may prevent colon and rectal cancer and help to control diverticulosis (A sac or pouch in walls of a canal or organ [e.g., GI tract] that becomes inflamed and causes pain and stagnation of feces. Source: The American Dietetic Association's Complete Food & Nutrition Guide, p. 145). Water-soluble fiber, as found in beans, fruit, and oat bran lowers cholesterol by binding to cholesterol found in liver bile, to help control diabetes.

Overall, dietary fiber does not provide much nourishment to human diet because of inability to break down these carb sources for energy, yet reduces available kcalories by providing a sense of satiety and by absorbing some nutrients including fat. However, fiber also absorbs and eliminates essential fatty acids, food substances that are essential for good health and energy metabolism.