Dietary starch delivery to the colon and excretion in stools and the ability of unabsorbed carbohydrates to promote hydrogen and methane release in breath were evaluated in 6 volunteers during two 8-day periods on starch diets of 100 and 300 g, respectively. Significantly less starch was recovered from the terminal ileum by aspiration per 24 h during the low-starch period (4.1 +/- 0.3 vs. 9.5 +/- 1.1 g, mean +/- SEM, p less than 0.01). Unabsorbed glucose tended to rise during the high-starch period (2.7 +/- 0.8 vs. 1.1 +/- 0.3 g). Fecal outputs of starch, glucose, volatile fatty acids, and lactic acid were not significantly different during the two periods. Daily breath hydrogen excretion was unchanged (181.2 +/- 22.7 vs. 193.7 +/- 19.8 ml for the low- and high-starch periods, respectively), whereas breath methane excretion increased markedly in the three methane producers during the high-starch period (217.2 +/- 80.9 vs. 32.4 +/- 7.3 ml). Starch malabsorption in the healthy small intestine was moderate even with a high-starch diet and less than that previously estimated by indirect methods. Unabsorbed starch catabolism by the colonic flora does not seem to explain most of the breath hydrogen excretion.