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Leucine

The single amino acid that flips the muscle-building switch — and why a threshold matters more than total grams.

Reviewed by Bodil Isaksson, Licensed Biomedical Scientist (Sweden)Last reviewed July 2026Supplement guide

SupplementsAmino acidsMuscleProtein

Leucine is the most anabolic of the amino acids: it directly activates the signalling that starts muscle protein synthesis. But it is a trigger, not a whole meal.

Key takeaways

  • Leucine activates mTOR, the master switch for muscle protein synthesis.
  • There's a leucine threshold — roughly 2.5–3 g per meal — to maximally “switch on” synthesis.1
  • Useful to fortify low-leucine meals (many plant proteins) so they hit the threshold.
  • It is not a standalone muscle-builder — the other EAAs are still needed as building blocks.

What it is

Leucine is one of the three BCAAs and the strongest single trigger of protein synthesis. Once the switch is flipped, the body still needs the full set of amino acids to actually build tissue.

The evidence

Reaching the leucine threshold per meal maximises the anabolic response; adding leucine to a sub-threshold (e.g. plant-based) meal can raise its muscle-building effect.1 Adding leucine above threshold to an already complete meal adds little.

Dose & how to use

2–3 g with a meal that is otherwise low in leucine. Whole-food sources include dairy, eggs, meat and soy.

Safety & who benefits

Safe. Most relevant for plant-based eaters and older adults topping up individual meals to the threshold.

Bottom line: the ignition for muscle growth — helpful to top up low-leucine meals, but useless without the other amino acids to build with.

References

Adult research summary for education, not a prescription. These are food supplements, not treatments for disease.

  1. Jäger R, et al. ISSN Position Stand: protein and exercise. J Int Soc Sports Nutr 2017. pmc.ncbi.nlm.nih.gov

This article summarises nutrition and exercise science for education. It is not a substitute for individual medical advice; consult a professional before starting a supplement, in pregnancy, or alongside medication.