Leucine is the most well known, and well researched, of the three Branched Chain Amino Acids (BCAAs). In fact, it's typically considered the "primary" muscle building amino acid as it directly influences both anabolism and protein synthesis. The body cannot produce leucine on its own and therefore must be obtained via food or supplementation, making it an essential amino acid.
Leucine drives protein synthesis by stimulating the mTOR pathway -- greater mTOR activity leads to increased protein construction in the body, ultimately yielding more muscle growth and bigger, better gains! If you’re an aspiring bodybuilder or physique athlete, keeping your body in an anabolic state is absolutely crucial, and Leucine is the only amino acid that can directly stimulate protein synthesis.
It’s also worth mentioning that Leucine exerts a powerful anti-catabolic effect as well, meaning Leucine supplementation helps prevent muscle loss during periods of prolonged dieting (a.k.a. “cutting” season).
To adequately stimulate the mTOr pathway, aim to consume 2.5 - 3g of Leucine as part of a BCAA blend (which also includes Isoleucine and Valine) in your pre, intra, or post workout supplements.