Leucine seems to be the central amino acid in muscle protein synthesis.
There’s plenty of interest in gaining muscle while cutting fat. Here’s an interesting item from a paper on that subject (Donald K. Layman, Jamie I. Baum. Dietary Protein Impact on Glycemic Control during Weight Loss. The Journal of Nutrition, Volume 134, Issue 4, 1 April 2004, Pages 968S–973S):
During catabolic periods such as energy restriction, supplementation with leucine or a complete mixture of the 3 BCAAs, leucine, isoleucine, and valine, stimulates muscle protein synthesis (35-37).
The three references (35-37) are:
35. Li, J. B. & Jefferson, L. S. (1978) Influence of amino acid availability on protein turnover in perfused skeletal muscle. Biochim. Biophys. Acta 544:351–359.
36. Buse, M. G. & Reid, S. S. (1975) Leucine. A possible regulator of protein turnover in muscle. J. Clin. Invest. 56:1250–1261.
37. Hong, S. C. & Layman, D. K. (1984) Effects of leucine on in vitro protein synthesis and degradation in rat skeletal muscle. J. Nutr. 114:1204–1212.
All three were rat studies. From 36.
The data presented indicate that leucine may act as a regulator of the turnover of protein in muscle cells. They are compatible with the hypothesis that leucine inhibits protein degradation and promotes protein synthesis in muscle.
Lundholm K, Edström S, Ekman L, Karlberg I, Walker P, Scherstén T. Protein Degradation in Human Skeletal Muscle Tissue: The Effect of Insulin, Leucine, Amino Acids. Clin Sci (Lond). 1981 Mar;60(3):319-26.
Satoshi Fujita,et.al. Effect of insulin on human skeletal muscle protein synthesis is modulated by insulin-induced changes in muscle blood flow and amino acid availability Am J Physiol Endocrinol Metab. 2006 Oct; 291(4): E745–E754.
Changes in muscle protein synthesis were strongly associated with changes in muscle blood flow and phenylalanine delivery and availability. In conclusion, physiological hyperinsulinemia promotes muscle protein synthesis as long as it concomitantly increases muscle blood flow, amino acid delivery and availability.