Supplement analysis backed by scientific evidence · PT

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Moderate evidencePhysical performance

L-Leucine

Leucine is the amino acid that switches on the mTOR pathway, and adding it to low-protein meals demonstrably increases muscle protein synthesis. But acute signalling has repeatedly failed to translate into extra muscle or strength in people already eating enough protein.

What the science says

  • Increasing muscle protein synthesis acutely Strong evidence

    Leucine is the primary trigger of the mTORC1 anabolic signal, and adding roughly 3-5 g to a suboptimal protein dose reliably raises myofibrillar protein synthesis rates in both young and older adults. This mechanistic effect is one of the best-replicated findings in nutrition physiology.

  • Increasing lean mass in older adults Moderate evidence

    Xu's 2015 meta-analysis found leucine supplementation significantly increased muscle protein synthesis in older people but produced no significant gain in overall lean body mass, with benefits concentrated in sarcopenic participants. A 2025 meta-analysis in older adults with sarcopenia likewise reported modest improvements, so the effect appears real but confined to people who are deficient or already losing muscle.

  • Preventing muscle loss during disuse Limited evidence

    A well-controlled 2020 RCT gave healthy young men high-dose leucine during 7 days of limb immobilisation and found no protection against muscle atrophy or strength loss versus placebo. Leucine's anabolic signal does not override disuse-driven catabolism.

  • Extra hypertrophy in trained lifters eating enough protein Limited evidence

    Training studies adding leucine on top of adequate protein intake in healthy young adults have generally shown no additional gains in lean mass or strength. If your protein intake already delivers ~2.5-3 g leucine per meal, the threshold is met and more does not help.

Dosage & safety

Studied dose2-5 g per meal is the range used to reach the leucine threshold; chronic trials in older adults typically use 4-8 g/day in divided doses. About 25-30 g of whey or 100-120 g of chicken breast already provides roughly 2.5-3 g.
SafetyGenerally safe at typical doses. High single doses (>10 g) can cause nausea and transiently lower plasma serotonin precursors and other BCAA levels by competing for the same transporters, and can raise blood ammonia. Contraindicated in maple syrup urine disease. Caution with advanced kidney or liver disease. Not a banned substance.

Scientific references

Where to buy

NOW Foods, Sports, L-Leucine Powder, 9 oz (255 g)

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