Some starch and protein, as well as fiber, remains unabsorbed in the small intestine and is degraded by anaerobic bacteria to short-chain fatty acids, hydrogen, methane, and carbon dioxide in the large intestine. The production of butyrate from starch has received the most attention, because butyrate seems to possess several important functions in the large bowel, including antineoplastic properties. In 16.6% fecal homogenates, starch polysaccharides, whether digestible or resistant to in vitro hydrolysis by amylase, pectin, and glucose, were all completely degraded to equal amounts of short-chain fatty acids (mean 60 wt/wt%; range 49-67 wt/wt%). However, starch that was resistant to hydrolysis by amylase was much more slowly fermented with the production of proportionally less butyrate and propionate than digestible starch (butyrate, 15 and 33%, respectively; propionate, 3 and 20%, respectively). The daily intake of 35 g resistant starch (100 g amylomaize starch) by 7 ileostomy subjects increased ileal dry-matter effluent by 38 +/- 2 g/day, due exclusively to increased excretion of carbohydrates of nonfiber origin (starch-polysaccharides and oligo- and monosaccharides) from 14 +/- 1 to 51 +/- 2 g/day, with no change in excreted nonstarch polysaccharides, nitrogen, and ileal volume. The ileal excreted resistant starch increased the formation of total short-chain fatty acids by 50% in fecal homogenates incubated with ileal dry matter from the amylomaize starch period, with comparatively little effect on the ratio of produced butyrate.(ABSTRACT TRUNCATED AT 250 WORDS)