L-Leucine
Silver· human dataAlso known as: Leucine
The key trigger amino acid for muscle protein synthesis — but not a standalone muscle builder.
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.
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Written and reviewed by Mathew Beale, MSc Biotechnology, University of Reading.
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