Dual effects of quercetin on protein digestion and absorption in the digestive tract.

Quercetin is a well-studied natural product with multiple pharmacological properties. In this study, we demonstrated that quercetin suppressed protein digestion in the intestinal fluid by inhibiting trypsin, a key digestive enzyme. However, we also observed a previously unknown property of quercetin: promoting the intestinal absorption of proteins. In addition, the promoted protein absorption was mediated by internalization of digested oligopeptides in the intestinal epithelia rather than increasing the intestinal paracellular permeability. Notably, four other flavonoids also achieved such enhanced intestinal absorption, suggesting that this effect was associated with the aglycone flavonol backbone, but not related to their inhibitory potencies against trypsin. This study demonstrates that quercetin exhibits dual effects on protein digestion and absorption: 1) suppressing protein digestion by inhibiting trypsin in the intestinal fluid; 2) promoting the intestinal absorption of oligopeptides in the intestinal villi cells.