The relationships between fermentation in the gut and colonic function were studied by using data derived from a human metabolic study. Five healthy men were fed diets that were fiber-free or contained cellulose, xylan, pectin or corn bran. Fermentation was assessed by measuring the excretion of flatus gas, volatile fatty acids (VFA) in feces and fecal pH. Colonic function was assessed by measuring transit time, fecal frequency, fecal output and fecal composition. Fibers that were only marginally fermented included cellulose and corn bran, and these fibers caused large fecal output, frequent defecations and prolonged the residence time of digesta in the gut; feces contained high levels of neutral detergent fiber (NDF); neither diet influenced fecal pH nor flatus gas excretion; and only corn bran increased VFA excretion in feces. Fibers that were mostly fermented in the gut included xylan and pectin, and these fibers did not influence fecal frequency or fecal output, but they did decrease transit time; both diets caused high levels of flatus gas to be excreted; pectin caused higher VFA excretion in feces and lower fecal pH. The excretion of VFA in feces was highly correlated with total fecal output, and high levels of VFA were associated with low flatus gas excretion. Mutagenic activity in feces was negligible for all subjects on all diets as measured by the fluctuation test.