Effects of carbohydrase inhibiting polyphenols on glycaemic response in vivo

Background and Objective: Diabetes is a global problem and high postprandial blood glucose is one of the risk factors for developing type 2 diabetes. In vitro studies have shown that polyphenols have inhibitory effects on carbohydrate-digesting enzymes and glucose transporters which could lead to reduced postprandial glucose in vivo. This study investigated the effects of a mixture of polyphenols (for maximum effect) capable of inhibiting the different stages of carbohydrate digestion on glycaemic response. Additionally, a single source of polyphenols in two different matrices was also examined. Methods: In vitro inhibition assays were used to determine the inhibition potential of polyphenols from the polyphenol and fibre-rich food (PFRF) consisting of freeze-dried apple, blackberry, blackcurrant, strawberry and green tea as well as from a single source (pomegranate). Three randomised, crossover studies were conducted on healthy volunteers (n=16 for each study) to determine the effects of polyphenols on glycaemic response in vivo using PFRF, pomegranate capsules and juice as sources of polyphenols. Results: Polyphenols found in PFRF dose-dependently inhibited α-amylase and α- glucosidase in vitro and gave rise to a decrease in postprandial glucose area under the curve by (IAUC) =-27.4±7.5 % (mean ± SD) p<0.001 and IAUC=-49.0±15.3 %, p<0.001) for the single and double dose respectively. Insulin IAUC was also attenuated by -46.9±13.4% (mean ± SD; p<0.01) for the double dose. Pomegranate polyphenols in juice dose-dependently inhibited α-amylase and α-glucosidase in vitro and gave rise to a decrease (-33.1±18.1 %, p <0.01) in postprandial glucose IAUC but did not show any effect when administered in the form of capsules. ii Conclusions: Certain polyphenol-rich foods have the potential to be used in the risk prevention and management of type 2 diabetes since they inhibit carbohydrate digestive enzymes in vitro and reduce postprandial glycaemic response in healthy volunteers, but only in certain food matrices.