Dietary essential amino acids are extracted in the first-pass metabolism by the small intestine.Besides incorporation into mucosal protein,they are also the fuels for the utilization by small intestinal mucosal cells and the precursors for intestinal synthesis of new amino acids and enormous biological important molecules such as glutathione,polyamines,etc.Thus,their gut metabolism is not only crucial for the maintenance of the integrity and function of intestinal mucosa,but also important for the regulation of whole body metabolism.