Starch is usually eaten cooked. The heat of cooking gelatinizes the starch granules, increasing their susceptibility to enzymic (a-amylase) breakdown. However, a proportion of the starch, called resistant starch (RS), is undigestible even after prolonged incubation with the enzyme. In cereals, RS represents 0.4 to 2% of the dry matter; in potatoes, 1 to 3.5%; and in legumes, 3.5 to 5.7%. RS has been categorized as the sum of the starch and degradation products not absorbed in the small intestine of a healthy person (3a). There are three main categories: RS1, physically enclosed starch (partially milled grains and seeds); RS2, ungelatinized crystallite granules of the B-type x-ray pattern (as found in bananas and potatoes); and RS3, retrograded amylose (formed during the cooling of starch gelatinized by moist heating). Resistant starch escapes digestion in the small intestine, but it then enters the colon, where it can be fermented by the local resident bacteria (of which there are over 400 different types). In this respect, RS is somewhat similar to dietary fiber. Estimates of the RS and unabsorbed starch represent about 2 to 5% of the total starch ingested in the average Western diet. This approximates to less than 10 g carbohydrate/day ( 4). The end products of the fermentation of the RS in the colon are short-chain fatty acids (e.g., acetic, butyric, propionic), carbon dioxide, hydrogen, and methane (expelled as flatus).
Refractory starches stimulate bacterial growth in the colon. While the short-chain fatty acids stimulate crypt cell mitosis in animals, it is not known whether they do the same in the human colon (5). However, if the human colon is removed from the mainstream of foodstuffs flowing down the lumen of the gastrointestinal tract, then the colonocytes become defunctionalized and ionic absorption is reduced. Luminal short-chain fatty acids from bacterial fermentation are used by colonocytes as metabolic substrates and appear essential for normal colonic function (6).
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