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-galloyl-β-d-glucopyranose,
and trapain, were predominant with dry-weight contents of 2.3 ± 0.0, 2.7 ± 0.1, and
1.2 ± 0.1g/100g, respectively. These WCPs exhibited inhibitory activity against α-amylase
and α-glucosidase. Whereas (-)-epigallocatechin gallate does not inhibit α-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.