Taking leucine, a building block found in protein, tells your muscles to start making more protein, helping them grow and repair.

We analyzed the available evidence and found that 66 studies or assertions support the idea that leucine, an amino acid found in protein, can signal muscles to increase protein production, which helps with muscle repair and growth. No studies or assertions in our review contradicted this. Leucine is one of the nine essential amino acids your body can’t make on its own, so you need to get it from food or supplements. When you consume leucine—especially after exercise—it appears to trigger a biological pathway that tells your muscle cells to start building more protein. This process is called muscle protein synthesis, and it’s how muscles recover and adapt after being stressed, like during weight training. The evidence we’ve reviewed so far leans toward leucine playing a clear role in this process. The number of supporting observations is high, and none have been found to challenge it. However, we note that these findings come from a range of study designs, including lab tests and human trials, and we haven’t reviewed how much leucine is needed, when it’s best taken, or how it compares to getting protein from whole foods. What we’ve found so far suggests that leucine may be an important signal for muscle growth, but we don’t yet know how much of an effect it has in real-world settings compared to eating protein-rich meals. If you’re trying to support muscle recovery, getting leucine through foods like eggs, chicken, dairy, or soy is a practical starting point. Supplements may help in specific cases, but whole foods offer other nutrients that also support muscle health.