Water chestnut is an annual aquatic plant that grows in Asia and Europe. Although water chestnut has been used as food and herbal medicine, its physiological functions and active ingredients are unknown. Here, we extracted polyphenols from the husk of the Japanese water chestnut (Trapa japonica) and assessed their effects on blood glucose levels. Three hydrolysable polyphenolics (WCPs), eugeniin, 1,2,3,6-tetra-O-galloy1-8-beta-D-glucopyranose, and trapain, were predominant with dry-weight contents of 2.3 +/- 0.0, 2.7 +/- 0.1, and 1.2 +/- 0.1 g/100 g, respectively. These WCPs exhibited inhibitory activity against alpha-amylase and alpha-glucosidase. Whereas (-)-epigallocatechin gallate does not inhibit alpha-amylase, WCPs exhibited high inhibitory activity (>80% at 0.15 mg/mL). In mice, administration of WCPs (40 mg/kg) significantly reduced blood glucose and serum insulin levels as assessed by the carbohydrate tolerance test. (C) 2014 Elsevier Ltd. All rights reserved.
