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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.

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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.

How to take it

TimingWith meals, especially the lower-protein ones where you fall short of the leucine threshold.
With food?With food; that is the whole point of the leucine threshold.
Worth knowingA 25-30 g whey shake already gives you 2.5-3 g, so buying leucine separately usually only makes sense for plant-based or low-protein meals.

Interactions

With other supplements

  • isoleucine and valine

    Leucine on its own competes with the other two BCAAs for the same transporter and can lower their plasma levels over time. prefer a balanced BCAA or EAA blend if you are dosing daily rather than per meal

  • L-tryptophan, 5-HTP and L-tyrosine

    Leucine competes with aromatic amino acids at the blood-brain barrier, reducing their uptake. separate by at least 2 hours

Well-established interactions only — this is not a complete list. Always tell your doctor and pharmacist what you take.

Frequently asked questions

Does leucine build muscle?

Leucine strongly triggers muscle protein synthesis in the short term, but in people already eating enough protein, adding leucine has generally not increased muscle or strength — that claim is limited.

How much leucine per meal?

Use 2–5 g of leucine per meal to reach the leucine threshold. A 25–30 g whey shake already provides about 2.5–3 g, so separate leucine mostly makes sense for plant-based or low-protein meals.

Does leucine help older adults keep muscle?

The evidence is moderate: meta-analyses found leucine raised muscle protein synthesis in older people, with modest gains in lean mass mainly in those with sarcopenia.

Does leucine prevent muscle loss during immobilisation?

No — the evidence is limited. A controlled 2020 trial found high-dose leucine during 7 days of limb immobilisation did not protect against muscle or strength loss.

Scientific references

Where to buy

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

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