The presence or absence of a normal small bowel is evidently important for the digestion and absorption of nutrients in humans. The importance, however, of the large intestine as an organ with digestive potential and an ability to salvage energy is much less appreciated. Whereas the bacterial fermentation of plant polysaccharides, with the production and absorption of short-chain fatty acids (SCFAs), contributes 60-90% of all the energy requirements in plant-eating animals, the colonic fermentation in humans is of minor importance for nutrition, with only 5-10% of the energy requirements available from colonic digestion of starch, nonstarch polysaccharides, and protein not absorbed in the small bowel, if the intestine has a normal length and function. In contrast, the digestive resource of the large bowel might be important for people with reduced upper-gut function and with the malabsorption of large amounts of dietary nutrients to the cecum, e.g., in patients with short bowel. The substrates available for bacterial fermentation and for the maintenance of the colonic flora are largely starch and nonstarch polysaccharides (dietary fiber). Whereas nonstarch polysaccharides are undegradable by amylase in the small intestine, starch is hydrolyzed by amylase, but with very different rates and to very different degrees dependent on the origin and structure of the starches. The unequal susceptibility to amylase explains the different amount of starch that is not absorbed in the small intestine. The rate of colonic fermentation of starch has also been shown to relate to the rate of starch hydrolysis by amylase.(ABSTRACT TRUNCATED AT 250 WORDS)