Leucine is one of the three branched-chain amino acids (BCAAs), alongside isoleucine and valine. As an essential amino acid, it cannot be produced by the body and must be obtained through diet or supplements. It plays a critical role in various physiological processes, most notably acting as a powerful stimulator of muscle protein synthesis (MPS). Leucine primarily activates the mTOR (mammalian target of rapamycin) pathway, which is a key regulator of cell growth, proliferation, and protein synthesis. This activation is fundamental for muscle repair, growth, and recovery after exercise. Beyond muscle building, leucine is also involved in regulating blood sugar levels and energy production. Its significant role in muscle metabolism makes leucine a popular supplement among athletes and individuals looking to maintain or increase muscle mass. It is often included in BCAA or complete amino acid formulations to support athletic performance, aid in recovery, and prevent muscle breakdown, especially during periods of calorie restriction or intense training.