The importance of the level of dietary fiber intake on bowel function was measured in 16 healthy young males consuming self-selected and liquid enteral diets. Subjects consumed liquid Ensure as their sole nutrient source plus 0 (diet a), 30 (diet c), and 60 (died d) g/day soy fiber, a fiber source high in hemicellulose that was added to the Ensure. Further, to examine whether heat processing affects the physiologic action of fiber, subjects consumed Enrich, an enteral formula similar to Ensure, which contains 30 g of the same soy fiber (diet b). The four diets were consumed in randomized order each for 10 days. Daily wet stool weight averaged 144.6 g on the self-selected diet and decreased significantly to 67.3 g when Ensure was consumed alone. Average stool weights on diets (b), (c), and (d) were 114.6, 100.2, and 150.3 g/day, respectively. Average fecal dry weights for the four diets were 19.1, 28.7, 25.0, and 30.3 g/day for (a-d), respectively. Gastrointestinal transit time, as measured with radiopaque pellets, was longest on the Ensure diet, 72.4 hr, and approximately 2 days on the fiber-supplemented diets and the self-selected diet. Thus, soy fiber's physiologic effect on laxation was not changed by heat processing. Also, although stool weights were larger on higher fiber intakes, transit rates were similar on all the fiber-containing diets.