Leucine is one of the ten essential amino acids. This means that the body is not capable of manufacturing leucine and must obtain it, or a precursor to leucine, from the diet. Of course, as an amino acid, leucine is used as a building block with other amino acids to make proteins, but as a branched chain amino acid (BCAA) leucine has several other effects as well. Leucine has been shown to stimulate skeletal muscle protein synthesis by increasing insulin sensitivity. Leucine has also been shown to have a direct effect on muscle protein synthesis by stimulating mTOR, a downstream target of IGF-1. This amino acid also has an anticatabolic effect, that is, it reduces the breakdown of skeletal muscle proteins. Leucine has also been shown to have a stimulatory effect on the secretion of insulin from the pancreas. So, not only does leucine increase the responsiveness of skeletal muscle to the anabolic effects of insulin but it also increases the amount of insulin in the body as well. This means that you get a potent anabolic punch from leucine and the increased insulin with an anticatabolic response producing a potent increase in skeletal muscle mass.
Amino Acids: Leucine