Leucine is a branched-chain amino acid your body needs but cannot make on its own. You must get it from food or supplements. It acts as a direct signal to trigger muscle protein synthesis.
Athletes and active people take leucine before or after training to support muscle repair and growth. Others add it to meals to help hold on to lean body mass. You can buy it as a standalone powder or capsule, or find it inside BCAA blends and protein formulas.
Large doses sometimes upset the stomach. If you take medication that lowers blood sugar, check with a healthcare provider before using leucine.