Inhibition of human starch digesting enzymes and intestinal glucose transport by walnut polyphenols

One approach to controlling type 2 diabetes (T2D) is to lower postprandialglucose spikesby slowing down the digestion of carbohydrates and the absorption of glucose in the small intestine. The consumption of walnuts is associated with a reduced risk of chronic diseases such as T2D, suggested to be partly due to the high content of (poly)phenols. This study evaluated, for the first time, the inhibitory effect of a (poly)phenol-rich walnut extract on human carbohydrate digesting enzymes (salivary and pancreatic alpha-amylases, brush border sucrase-isomaltase) and on glucose transport across fully differentiated human intestinal Caco-2/TC7 monolayers. The walnut extract was rich in multiple (poly)phenols (70 % w/w) as analysed by Folin-Ciocalteau and by LCMS. It exhibited potent inhibition of both human salivary (IC(50): 32.2 +/- 2.5 microg walnut (poly)phenols (WP)/mL) and pancreatic (IC(50): 56.7 +/- 1.7 microg WP/mL) alpha-amylases, with weaker effects on human sucrase (IC(50): 990 +/- 20 microg WP/mL), maltase (IC(50): 1300 +/- 80 microg WP/mL), and isomaltase (IC(25): 830 +/- 60 microg WP/mL) activities. Selected individual walnut (poly)phenols inhibited human salivary alpha-amylase in the order: 1,3,4,6-tetragalloylglucose > ellagic acid pentoside > 1,2,6-tri-O-galloyl-beta-D-glucopyranose, with no inhibition by ellagic acid, gallic acid and 4-O-methylgallic acid. The (poly)phenol-rich walnut extract also attenuated (up to 59 %) the transfer of 2-deoxy-D-glucose across differentiated Caco-2/TC7 cell monolayers. This is the first report on the effect of (poly)phenol-rich extracts from any commonly-consumed nut kernel on any human starch-digesting enzyme, and suggests a mechanism through which walnut consumption may lower postprandial glucose spikes and contribute to their proposed health benefits.