Essential amino acid (BCAA)
Leucine
Moderate evidenceThe amino acid that acts as the main "on switch" for muscle protein building.
What it is
Leucine is one of the three branched-chain amino acids. Beyond being a building block, it signals muscle cells to start building protein (through a pathway called mTOR).
Food sources
Whey and dairyMeat and poultryEggsSoyLentils
What the research shows
- Leucine content is a major reason some proteins (like whey) stimulate muscle building more than others.
- Extra leucine on top of an already adequate protein intake shows limited added benefit in most studies.
Read the published research on PubMed →
Food first: most people get what they need from a varied diet with enough total protein. Talk to a provider or registered dietitian before adding a supplement, especially if you take medication or have a health condition.