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Leucine Drives Protein Synthesis

It is probably not news that simply consuming protein induces an increase in muscle protein synthesis, which is why protein supplements are popular to consume after exercise. Researchers have begun to refine their knowledge on what it is about protein that stimulates anabolism in muscle. For example, scientists have shown that it is the level of amino acids in the blood that directly boosts protein synthesis in muscles. Specifically it is the amino acid leucine that is most highly related to protein synthesis. In a series of elegant experiments, researchers directly examined muscle protein synthesis after feeding animals various formulations of amino acids and compared them to glucose ingestion. When a complete protein was consumed (one that contains all the amino acids), protein synthesis was increased. When just essential amino acids were provided and the nonessential ones left out, the same increase was noted indicating nonessential amino acids are not required to stimulate protein synthesis. When just the BCAA were given, again there was the same increase in protein synthesis. Finally when just leucine was consumed, yet again protein synthesis increased to the same magnitude. These findings provided strong evidence that Leucine was the driving force behind the ability of dietary protein to stimulate protein synthesis. Cutting edge work in the last few years has revealed how leucine directly activates a critical compound in muscle cells called the mammalian target of rapamycin (mTOR). It turns out mTOR is like a molecular switch that turns on the protein synthetic machinery in muscle and leucine is one of the major activators of mTOR. Thus, leucine not only provides the building blocks for protein synthesis, it also has a critical role in up-regulating the process. The application of all this work is that adding additional leucine to typical protein beverages is an effective strategy to maximize muscle anabolism after resistance exercise.

Garlick PJ, Grant I. Amino acid infusion increases the sensitivity of muscle protein synthesis in vivo to insulin. Effect of branched-chain amino acids. Biochem. J. 1988 Sep 1;254(2):579-84.