High cereal fibre intake is associated with reduced risk for type 2 diabetes, but wheat fibre had little or no effect on glycaemic control or oral glucose tolerance in clinical trials lasting 4-12 weeks. To explain this discrepancy, we hypothesised that colonic adaptation to increased wheat fibre intake takes many months but eventually results in increased SCFA production and glucagon-like peptide-1 (GLP-1) secretion. Thus, the primary objective was to determine the time-course of the effects of increased wheat fibre intake on plasma acetate, butyrate and GLP-1 concentrations in hyperinsulinaemic human subjects over 1 year. Subjects with fasting plasma insulin >or= 40 pmol/l were randomly assigned by computer to receive either a high-wheat fibre cereal (fibre group; 24 g fibre/d; twenty assigned; six dropped out, fourteen included) or a low-fibre cereal (control group; twenty assigned; six dropped-out, fourteen included) daily for 1 year. Acetate, butyrate and GLP-1 were measured during 8 h metabolic profiles performed every 3 months. There were no differences in body weight in the fibre group compared with the control group. After 9 months baseline-adjusted mean 8 h acetate and butyrate concentrations were higher on the high-fibre than the control cereal (P < 0.05). After 12 months on the high-fibre cereal, baseline-adjusted mean plasma GLP-1 was 1.3 (95 % CI 0.4, 2.2) pmol/l (P < 0.05) higher than at baseline (about 25 % increase) and 1.4 (95 % CI 0.1, 2.7) pmol/l (P < 0.05) higher than after 12 months on control. It is concluded that wheat fibre increased SCFA production and GLP-1 secretion in hyperinsulinaemic humans, but these effects took 9-12 months to develop. Since GLP-1 may increase insulin sensitivity and secretion, these results may provide a mechanism for the epidemiological association between high cereal fibre intake and reduced risk for diabetes.
Trial registration: ClinicalTrials.gov NCT00247455.