Fiber mixture-specific effect on distal colonic fermentation and metabolic health

Infusions of the short-chain fatty acid (SCFA) acetate in the distal colon improved metabolic parameters in men. Here, we hypothesized that combining rapidly and slowly fermentable fibers will enhance distal colonic acetate production and improve metabolic health. METHODS: In vitro cultivation studies in a validated model of the colon were used to identify fiber mixtures that yielded high distal colonic acetate production. Subsequently, in two randomized rossover studies, lean and prediabetic overweight/obese men were included. In one study, participants received supplements of either long-chain inulin+resistant starch (INU+RS), INU or maltodextrin (PLA) the day prior to a clinical investigation day (CID). The second trial studied beta glucan+RS (BG+RS) versus BG and PLA. During each CID, breath hydrogen, indirect calorimetry, plasma metabolites/hormones were assessed during fasting and postprandial conditions. Additionally, fecal microbiota composition and SCFA were determined. RESULTS: In prediabetic men, INU+RS increased plasma acetate compared to INU or PLA (P<0.05), but did not affect metabolic parameters. In lean men, INU+RS increased breath hydrogen and fasting plasma butyrate, which was accompanied by increased energy expenditure, carbohydrate oxidation and PYY and decreased postprandial glucose concentrations (all P<0.05) compared to PLA. BG+RS increased plasma butyrate compared to PLA (P<0.05) in prediabetic individuals, but did not affect other fermentation/metabolic markers in both phenotypes. Fiber-induced shifts in fecal microbiota were individual-specific and more pronounced with INU+RS versus BG+RS. CONCLUSION: Administration of INU+RS the day prior to investigation promoted microbial fermentation and increased (postprandial) energy expenditure, carbohydrate oxidation and insulin sensitivity (as indicated by decreased postprandial glucose but no changes in insulin concentrations) in lean individuals. Despite an increase in circulating acetate, no effects on substrate and energy metabolism were observed in prediabetic individuals. With BG+RS no effects on fermentation markers and metabolic responses were shown, illustrating the acute metabolic effects were fiber-specific. Further research has to elucidate whether in the prediabetic phenotype a longer-term supplementation period is required to demonstrate beneficial effects on metabolic health.