Supplement · Protein & amino acids
L-Leucine
Also known as: Leucine
The key trigger amino acid for muscle protein synthesis — but not a standalone muscle builder.
- Evidence
- Silver· human data
- Sources cited
- 1
- Last reviewed
- 22 July 2026
Leucine is the branched-chain amino acid most directly responsible for switching on muscle protein synthesis; reaching a sufficient dose per meal (the 'leucine threshold') helps maximise the muscle-building signal, which is why protein quality matters.
Supplementing leucine on top of adequate, leucine-rich protein has not reliably added muscle in trials — the signal without the full set of building blocks fizzles. Its main practical relevance is choosing quality protein, or topping up lower-leucine plant meals, rather than as a standalone powder.