The small intestine is not only responsible for terminal digestion and absorption of nutrients, but it also plays an important role in catabolism of arterial glutamine and dietary amino acids. Most of glutamine and almost all of glutamate and aspartate in the diet are catabolized by small intestinal mucosa, and CO2 accounts for 56-64% of their metabolized carbons. The small intestinal mucosa also plays an important role in degrading arginine, proline and branched-chain amino acids, and perhaps methionine, lysine, phenylalanine, threonine, glycine and serine in the diet, such that 30-50% of these dietary amino acids are not available to extraintestinal tissues. Dietary amino acids are major fuels for the small intestinal mucosa and are essential precursors for intestinal synthesis of glutathione, nitric oxide, polyamines, purine and pyrimidine nucleotides, and amino acids (alanine, citrulline and proline), and are obligatory for maintaining intestinal mucosal mass and integrity. Because intestinal amino acid catabolism plays an important role in modulating dietary amino acid availability to extraintestinal tissues, it has important implications for the utilization efficiency of dietary protein and amino acids in animals and humans.