OBJECTIVE: Cereal fiber intake is linked to reduced risk of type 2 diabetes in epidemiological observations. The pathogenic background of this phenomenon is unknown. Based on recent findings, we hypothesized that intake of purified insoluble oat fiber may improve whole-body insulin sensitivity. RESEARCH DESIGN AND METHODS: A randomized, controlled, single-blind, cross-over study was performed, and 17 overweight or obese subjects with normal glucose metabolism were analyzed. After consumption of nine macronutrient-matched portions of fiber-enriched bread (white bread enriched with 31.2 g insoluble fiber/day) or control (white bread) over a time period of 72 h, whole-body insulin sensitivity was assessed by euglycemic-hyperinsulinemic clamp. Energy intake was individually adjusted by providing standardized liquid meals. Hydrogen breath tests were performed to control for dietary adherence. RESULTS: When analyzing the entire cohort, whole-body glucose disposal was improved after fiber consumption (M value 6.56 +/- 0.32 vs. 6.07 +/- 0.27 mg . min(-1) . kg(-1); P = 0.043). Thirteen subjects had increased hydrogen breath test concentrations after fiber consumption, indicating probable dietary adherence. Restricting analysis to these subjects, improvements in M value (6.85 +/- 0.34 vs. 6.06 +/- 0.32 mg . min(-1) . kg(-1); P = 0.003) and insulin sensitivity, expressed as M/I ratio (M value divided by mean serum insulin at steady state: 3.73 +/- 0.23 vs. 3.21 +/- 0.27; P = 0.02), after fiber consumption were more pronounced. Plasma lipids, serum magnesium, ghrelin, and adiponectin concentrations, as well as substrate utilization and body weight, were not significantly changed by fiber intake (P > 0.15). CONCLUSIONS: Increased insoluble dietary fiber intake for 3 days significantly improved whole-body insulin sensitivity. These data suggest a potential mechanism linking cereal fiber intake and reduced risk of type 2 diabetes.