Control of the metabolic fate of amino acids in ruminants: a review.

In general, ruminants convert ingested feed protein (N) to body tissues with low efficiency (0 to 35%). Although some of this inefficiency is due to the peculiarities of ruminal action and digestion, a large proportion is associated with metabolic events in the tissues. In the fasted condition, amino acid catabolism is greater than in the maintenance-fed animal, and perhaps 40% of this loss is due to provision of carbon sources for gluconeogenesis. The contributions of other pathways to these basal losses are poorly quantified. Below maintenance intake, insulin seems to be a major determinant of the rate of protein loss, primarily through reduction of protein degradation (especially in muscle tissue) with an accompanied decrease in the rate of branched-chain amino acid (BCAA) oxidation. At intakes above maintenance, protein anabolism and amino acid catabolism are more probably regulated by the growth hormone/insulin-like growth factor I (GH/IGF-I) axis, with the major control via alterations in protein synthesis. The actions of insulin and GH/IGF-I may provide overlapping regulatory mechanisms, which would explain the biphasic alterations in protein dynamics and amino acid catabolism observed for the ruminant between the fasted and ad libitum intake conditions. The BCAA may assume a key regulatory role in integrating the metabolism of peripheral tissues with the metabolic and oxidative functions of the liver. This integration seems well-coupled in the ruminant, for which the relationship between the extent of BCAA catabolism in peripheral and hepatic metabolism remains fairly constant under a range of nutritional and physiological conditions.