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.

In this study, we aimed to investigate the properties of polyphenols in the husk of the "water chestnut", which is an aquatic plant, and effects on postprandial blood glucose levels in humans. We prepared a hot water extract of the husk of the Japanese water chestnut ( Trapa japonica ) , and investigated the content of polyphenols in the extract and their stability against heat. The result showed that the total polyphenol content in the hot water extract of water chestnut was approximately ２ ０ ０!/１ ０ ０!, and the total content of polyphenols (eugeniin, １, ２, ３, ６-tetra-O-galloyl-β -Dglucopyranose ( TGG ) and trapain ) exclusive to water chestnut was approximately ２ ０! /１ ０ ０! . Eugeniin and TGG were observed to be stable against heat ; however, trapain was sensitive to heat (８ ０℃ or higher) . Twenty-one healthy volunteers drank water or the hot water extract of water chestnut husk(２ ０ ０!) , and were subjected to a cooked rice load test. The group that consumed the hot water extract of water chestnut exhibited significant suppression in blood glucose elevation ２ ０ and ３ ０ minutes after meal. This was speculated to be due to the inhibitions of carbohydrase activity and glucose uptake into the small intestine by the polyphenols of water chestnut husk.Key words：water chestnut, polyphenols, suppressive effect on blood glucose elevation, inhibitory effect of carbohydrase, Caco-2 cells ヒシ，ポリフェノール，血糖値上昇抑制作用，糖質分解酵素の阻害活性，Caco-２細胞