Leucine (L-Leucine) is an essential amino acid and belongs to the group of branched-chain amino acids (BCAA), along with isoleucine and valine. The human body cannot synthesize it endogenously, therefore its intake is necessary through diet or supplements. Leucine is considered the most powerful amino acid in stimulating muscle protein synthesis, with a central role in regulating muscle growth and recovery. At the biochemical level, leucine acts mainly as a regulatory nutrient signal and not only as a structural component. It directly activates the mTOR (mammalian Target of Rapamycin) pathway, which is the main cellular mechanism for initiating muscle protein synthesis. Activation of mTOR leads to increased muscle protein synthesis, supporting muscle repair and the maintenance of muscle tissue after mechanical or metabolic stress. At the same time, leucine exhibits a strong anti-catabolic effect, as it reduces muscle proteolysis and contributes to maintaining a positive nitrogen balance. In situations of caloric deficit, fasting or intense training, adequate leucine availability helps to protect lean muscle mass. In addition, leucine can be used as an energy substrate by skeletal muscle, enhancing metabolic efficiency during prolonged exercise. From an ergophysiological perspective, leucine supplementation has been associated with increased muscle protein synthesis, improved recovery and increased muscle strength, especially when combined with resistance training. However, for the maximum efficiency of the protein synthesis process, the presence of all essential amino acids is required, which makes leucine more functional when taken in the context of a complete protein or EAA supplement.