a1 Department of Nutritional Sciences, University of Toronto, Toronto, Ontario, Canada M5S 3E2
a2 Li Ka Shing Knowledge Institute and Division of Endocrinology and Metabolism, Department of Medicine, St Michael's Hospital, Toronto, Ontario, Canada M5B 1W8
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 ≥ 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.
(Received November 19 2008)
(Revised April 29 2009)
(Accepted June 01 2009)
(Online publication August 07 2009)
Abbreviations: GLP-1, glucagon-like peptide-1