Effect of dietary fibres present in traditional Japanese vegetables on carbohydrate-digesting enzyme activity

Introduction: Dietary fibre intake is widely recommended as a nutritional therapy to alleviate hyperglycaemic and diabetic conditions. Inulin is a heteropolysaccharide mainly found in burdock. It is known that inulin intake reduces postprandial glucose level; however, details regarding the mechanism of this action remain unclear. In this study, we investigated the effect of burdock and inulin on glucose metabolism. Methods: Burdock and inulin were added to a solution containing maltose and α-glucosidase. Glucose produced from digested maltose was measured using mutarotase-glucose oxidase assay as the activity of α-glucosidase. We employed Ostwald viscometer to measure viscosity of inulin solution. Results: Addition of 0.1-1.0 g raw burdock in 6 mL incubation solution significantly decreased activity of α-glucosidase in an amount-dependent manner. Boiled burdock and its supernatant liquid sample also significantly decreased activity of this enzyme, although degree of suppression in the boiled sample was less than in raw burdock. Addition of inulin, in the range of 0.67 to 6.70%, significantly decreased activity of α-glucosidase. Viscosity of inulin solution depended on its concentration from 0 to 6.70% and the relationship showed an almost direct proportion. α-glucosidase activity significantly decreased in the range of 1.03 to 1.40 in relative viscosity of inulin solution. Conclusion: These results suggested that inulin has a suppressive effect on α-glucosidase activity and that high viscosity of inulin solution may inhibit reaction of digestive enzyme.