Role of the gut in the amino acid economy of the host.

The intestinal tissues, especially the small intestinal mucosa, have a high rate of protein synthesis and energy expenditure. Because a substantial proportion of mucosal protein synthesis is probably devoted to secretion into the lumen, the intestine is in a permanent state of positive protein balance. In addition to the impact that it has on whole body protein turnover, the gut may also play a crucial role in amino acid metabolism. It seems likely that glutamate, glutamine, and aspartate are the dominating sources of carbon for mucosal energy generation, and it appears that dietary amino acids, as opposed to systemic amino acids, may be the more important in this respect. Moreover, there is increasing evidence that the small intestinal mucosa is a nutritionally significant site of dietary essential amino acid catabolism, at least in young mammals. Finally, the small intestinal mucosa is a key site of amino acid biosynthesis and other metabolic transformations. In this chapter, we discuss aspects of all these functions, starting with the quantification of systemic amino acid bioavailability, and passing to the relative importance of intestinal protein turnover, secretion and amino acid metabolism to the amino acid requirements of the host.