24 Gastrointestinal protein and amino acid metabolism in growing animals

The tissues of the gastrointestinal tract play a major role in the metabolism of protein and amino acids in growing animals. Intestinal tissue has a high rate of protein metabolism, which is directly linked to the high rates of proliferation, protein secretion, cell death and desquamation of various epithelial and lymphoid cells within the mucosa. Because the small intestine is the first tissue exposed to the diet, it has a key regulatory role in the digestion, absorption, metabolism and availability of dietary protein and amino acids for growth. The major oxidative fuels for the intestine are glutamate, glutamine, aspartate and glucose; however, some essential amino acids are also oxidized, such as lysine, leucine and phenylalanine. Among the essential amino acids, threonine utilization is particularly high, and may be critical for normal intestinal function. Intestinal protein and amino acid metabolism is primarily regulated by the modality (enteral versus parenteral) and quantity of nutrient intake, and to a lesser extent by the composition of nutrients.