by fermentation into short-chain fatty acids. Aim of this study was to investigate acute metabolic effects of replacing digestible carbohydrates by the fermentable carbohydrate inulin in a high-fat mixed meal in overweight males. Methods: In this randomized, double-blind, crossover study 14 healthy, overweight males (age 34±3 y, BMI 30.4±0.7 kg/m2) underwent two test days: one day ingesting a high-fat milkshake (2.3MJ, 46.2E% fat, 42.1E% carbohydrates) containing 24 g inulin, of which 0.5 g was U-13C-labeled, and the other day the same milkshake containing 24 g maltodextrin as placebo. Fat oxidation was measured via an open-circuit ventilated hood. Breath samples for CO2, blood samples, and visual analogue scale (VAS) scores for hunger and satiety were collected for 7 h after ingestion. Repeated measures ANOVA and incremental area under the curve were used to detect differences between interventions for the early (0 3 h after ingestion) and late postprandial phase (4 7 h after ingestion). Results: CO2 enrichments in exhaled breath increased significantly from 3 hours after ingestion of the inulin. Fat oxidation increased in the early postprandial phase after inulin ingestion (P < 0.05) when compared with placebo. Plasma free fatty acids (FFA) were higher (P < 0.05) whilst plasma glucose and insulin levels were lower after inulin ingestion compared with placebo (P < 0.05). There were no effects on plasma triglycerides, free glycerol, and hunger and satiety. Conclusion: Replacing digestible carbohydrates by inulin in a high-fat mixed meal acutely increased fat oxidation in overweight males. This was accompanied by an increase in FFA and increased breath CO2 enrichment, suggesting an increased fermentation of inulin-derived microbial products. These effects may be positive in view of body weight control and insulin sensitivity.