Inhibitory effect of black tea and its combination with acarbose on small intestinal α-glucosidase activity

Ethnopharmacological relevance: It is said that black tea is effective against type 2 diabetes mellitus because it can help modulate postprandial hyperglycemia. However, the mechanism underlying its therapeutic and preventive effects on type 2 diabetes mellitus is unclear. In this study, we focused on the effect of black tea on the carbohydrate digestion and absorption process in the gastrointestinal tract. We examined whether black tea can modulate postprandial hyperglycemia. Materials and methods: The freeze-dried powder of the aqueous extract of black tea leaves GAT) was used for in vitro studies of alpha-amylase activity, alpha-glucosidase activity, and glucose uptake by glucose transporters in Caco-2 cells; ex vivo studies of small intestinal a-glucosidase activity; and in vivo studies of oral sugar tolerance in GK rats, an animal model of nonobese type 2 diabetes mellitus. Results: Half maximal inhibitory concentration values indicated that JAT significantly reduced alpha-glucosidase activity, but weakly reduced alpha-amylase activity. Kinetic studies of rat small intestinal a-glucosidase activity revealed that the combination of JAT and the alpha-glucosidase inhibitor, acarbose, showed a mixed-type inhibition. JAT had no effect on the uptake of 2'-deoxy-D-glucose by glucose transporter 2 (GLUT2) and the uptake of alpha-methyl-D-glucose by sodium-dependent glucose transporter 1 (SGLT1). In the oral sucrose tolerance test in GK rats, JAT reduced plasma glucose levels in a dose-dependent manner compared with the control group. The hypoglycemic action of JAT was also confirmed: JAT, in combination with acarbose, produced a synergistic inhibitory effect on plasma glucose levels in vivo. In contrast to the oral sucrose tolerance test, JAT showed no effect in the oral glucose tolerance test. Conclusions: JAT was demonstrated to inhibit the degradation of disaccharides into monosaccharides by alpha-glucosidase in the small intestine. Thereby indirectly preventing the absorption of the dietary source of glucose mediated by SGLT1 and GLUT2 transporters localized at the apical side of enterocytes in the small intestine. The results indicate that black tea could be useful as a functional food in the dietary therapy for borderline type 2 diabetes mellitus that could modulate postprandial hyperglycemia. (C) 2014 Elsevier Ireland Ltd. All rights reserved.