- Various flavonoid-rich plant extracts have shown efficacy in the treatment of diabetes. Although it is often assumed that dietary flavonoids exert biological antioxidant effects, it is possible that the observed effects work in carbohydrate metabolism. In this study we evaluated the effect of catechins and catechin containing extracts on α-amylase and α-glucosidase activity, enzymes of great importance in carbohydrate metabolism and diabetes mellitus. Grape seed extract, white tea and green tea were tested, as well as their constituent catechins, or flavon-3-ols: catechin, epicatechin, epigallocatechin, epigallocatechin gallate, gallocatechin gallate, and epicatechin gallate. In all tests, acarbose, a pharmacological inhibitor of α-glucosidases was included as a positive control. For each compound, the concentration required for 50% inhibition of enzyme activity was determined to compare the potency. For α-amylase, grape seed extract demonstrated a similar α-amylase inhibitory activity to acarbose and white and green tea extracts showed a weaker inhibition. In α-glucosidase experiments, grape seed, green and white tea extracts inhibited enzymatic activity much more strongly than acarbose. The inhibitory activities of individual catechins are also reported. With this information, it is possible that catechins and certain plant extracts play a role in the prevention or treatment of type II diabetes mellitus.