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The aim of this study was to compare the effects of two nondigested, nonabsorbed dietary components on objective and subjective measures of gastrointestinal (GI) function.A placebo-controlled parallel study compared the effects of wheat bran (20 g/day or 40 g/day in cereal), a well-known dietary fiber, with those of olestra (20 g or 40 g/day in potato chips), a nonabsorbed fat, on stool output, stool apparent viscosity (log peak force for extrusion [PF]), stool water content, and GI symptoms. Sixty subjects resided on a metabolic ward for 9 days: 3 days baseline and 6 days treatment.Compared with placebo, consumption of 20 g/day wheat bran for 6 days resulted in a rapid (within 38 h) increase in mean (±SE) stool output (placebo, 150 ± 29 g/day; bran, 246 ± 35 g/day, p < 0.05), a directional increase in mean stool water content (placebo, 81.2 ± 0.8%; bran, 83.9 ± 0.8%), stool water output (placebo, 159 ± 54 g/day; bran, 238 ± 30 g/day), and bowel movement frequency (BM/day) (placebo, 2.2 ± 0.4; bran, 2.6 ± 0.4), and no stool-softening effect (placebo log PF, 2.9 ± 0.1 g; bran log PF, 2.9 ± 0.1 g). Wheat bran 40 g/day results were not significantly different from wheat bran 20 g/day. Compared with placebo, consumption of olestra 20 g/day and 40 g/day for 6 days showed no significant difference in mean stool output (151 ± 18 g/day and 204 ± 28 g/day, respectively), mean BM frequency (1.8 ± 0.2 BM/day and 2.1 ± 0.3 BM/day, respectively), and stool water output (138 ± 13 g/day and 184 ± 31 g/day, respectively), a significant (p < 0.05) decrease in stool water content (75.5 ± 1.7% and 72.6 ± 2.2%, respectively), and either no effect on stool apparent viscosity (olestra 20 g/day, mean log PF, 3.0 ± 0.1 g) or a gradual stool-softening effect beginning study day 6 (olestra 40 g/day, log PF, 2.7 ± 0.1 g). None of the treatment groups showed a significant increase in GI symptoms compared with placebo.Consumption of wheat bran in excess of levels in a typical Western diet significantly increased stool output, but did not soften normal-viscosity stool nor result in an increase in common GI symptoms. The observed plateau effect for wheat bran at 40 g/day suggests a maximal mechanical stimulatory effect. Consumption of olestra in excess of usual snacking conditions did not result in a significant increase in stool output or common GI symptoms. At the highest level tested, olestra resulted in a gradual stool-softening effect after several days of consumption.