Green Tea Polyphenols Reduce Protein Digestibility and Suppress Cecal Fermentation in Rats

Abstract

Positive effects of green tea polyphenols (GP) on human health have been well documented. In vitro studies have also shown that plant polyphenols generally bind to proteins. We examined the effect of dietary addition of GP (0.2% and 0.4%) on protein utilization in male SD rats. A dietary level of 0.4% GP corresponds to about 11 cups of green tea for a Japanese on an average diet. GP significantly lowered the apparent digestibility of casein, and concurrent addition of 1% glycine to the GP-containing diet could not prevent the adverse effects of GP. GP did not appear to greatly affect endogenous nitrogen metabolism. Proteins that escape digestion may alter the cecal fermentation profile. Therefore, we examined how cecal fermentation was modulated when rats were fed a diet containing high-amylose cornstarch with or without GP. The results revealed unexpectedly that the concentration of cecal short-chain fatty acids was significantly decreased in rats fed diets with GP, suggesting that GP has an inhibitory action on cecal fermentation, in particular, n-butyrate production: GP lowers both protein digestion in the small intestine and the activity of microflora in the large intestine. Thus, excessive intake of GP does not appear to be beneficial for human health.