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L-Leucine is the primary branched chain amino acid known for the ability to support protein synthesis and help slow the catabolic process. Leucine is probably the most well researched amino acid to date for increasing protein synthesis. For example, leucine, but not isoleucine or valine, can stimulate an increase in muscle protein synthesis through activation of the mTOR pathway in animals. mTOR is such a powerful activator of muscle growth; scientists have reported that if you block mTOR with the drug rapamycin, muscle growth is completely inhibited despite muscle overload.

Leucine is like the gas pedal for stimulating protein synthesis. So now that you know that mTOR is needed for muscle building, and that leucine acts as a stimulator of mTOR, here are a few things that inhibit mTOR:

• Protein restriction
• Calorie restriction/ Intermittent Calorie Restriction
• Cortisol/Glucocorticoids

The good thing about leucine is that you don’t need a big dose to get an anabolic effect. In a previous study, researchers wanted to examine how adding leucine to a protein drink would affect muscle protein synthesis. Researchers assessed the effect on muscle protein synthesis at rest and after resistance exercise. The men completed leg extensions before the ingestion of various intakes of whey protein, BCAAs, and leucine: