Amino acid metabolism during exercise and following endurance training.

Exercise results in marked alterations in amino acid metabolism within the body. The branched-chain amino acids, especially leucine, are particularly important since they contribute as energy substrates and as nitrogen donors in the formation of alanine, glutamine and aspartate. Leucine oxidation increases during whole-body exercise. Nonetheless, leucine's contribution as a muscle energy substrate is amll, being 3 to 4% at rest, and even lower (1%) during exercise. Traditional energy substrates (carbohydrates, lipid) remain most important. These rates of leucine oxidation can be readily attributed to skeletal muscle. Following endurance training, whole-body leucine oxidation is increased at rest and during exercise. Since its oxidation by muscle is not augmented, this whole-body increase is not due to muscle. Thus, other tissues within the body (i.e. liver) must account for this. Comparisons of leucine oxidation in rats and humans indicate that species differences exist. Much larger increases in leucine oxidation are brought about by exercise in humans. Calculations based on steady-state rates of leucine oxidation at rest and during exercise indicate that the recommended dietary intake of leucine is inadequate, since it is lower than measured whole-body rates of leucine oxidation. This inadequacy is exacerbated in individuals who are physically active.