Leucine: Muscle Protein Synthesis, mTOR and What Trials Show for Added Leucine
Leucine switches on a muscle-building signal called mTOR in the lab. This page separates that mechanism from what trials of added leucine show for muscle mass and strength, with study amounts labelled.
Content last updated . This page has not been medically reviewed.
Terms
- mTORC1 (often "mTOR"): a cell-growth signal. When amino acids are plentiful, mTORC1 activity drives protein synthesis and growth in cells. [3]
- Muscle protein synthesis: how fast muscle builds new protein. A faster rate in a short experiment is a mechanism result.
- Lean body mass, strength, physical performance: outcomes people can notice. These are what the trials below try to measure.
Leucine is one of the branched-chain amino acids; see the BCAA guide and the essential amino acids overview.
Where leucine comes from
Leucine is part of protein in milk, meat, eggs and other protein-rich foods. [4] Health Canada's food-protein scoring pattern (people aged 1 year and over) lists leucine at 55 mg per gram of protein, the largest figure of the nine; this is a yardstick for judging food protein, not a daily target. [5] A verified individual daily requirement for leucine from NIH ODS, EFSA, the NHS, MedlinePlus or Health Canada was not found, so none is quoted. See protein quality.What research found
Evidence labels: established nutritional role, evidence in adequately nourished people, promising but limited, mixed, insufficient evidence, preclinical.| Question | What the source found | Label |
|---|---|---|
| Does leucine activate mTORC1? | A 2014 critical review states that, of all the amino acids, leucine is the most potent stimulator of mTORC1 and protein synthesis in vitro and in vivo. [3] | Mechanism; established in cells and short experiments |
| Older adults, muscle protein synthesis | A 2015 meta-analysis (nine randomized trials, searched to 2013) found the muscle protein fractional synthetic rate increased with leucine versus control, but no difference in lean body mass or leg lean mass. [6] | Promising mechanism result; no clinical gain in that analysis |
| Older adults, body composition and strength | A 2015 meta-analysis of 16 studies enrolling 999 people found leucine increased lean body mass by about 1 kg versus controls (more in people with sarcopenia) but not muscle strength; the authors noted heterogeneity between trials. [7] | Mixed |
| Long-term trials overall | A 2014 critical review concluded that long-term, placebo-controlled, iso-caloric studies in humans consistently show no beneficial effect of leucine on skeletal muscle mass or function, and that leucine as a stand-alone intervention is not effective in preventing muscle wasting. [3] | Mixed to no good evidence of benefit (as a stand-alone) |
| Sarcopenia, leucine or leucine-enriched protein | A systematic review of 23 articles (13 randomized, placebo-controlled) found improved lean muscle mass in elderly people with sarcopenia, with most protocols also giving vitamin D; strength results were mixed and physical performance rarely studied. [8] | Promising but limited |
| A leucine-enriched whey protein plus vitamin D product, older adults with sarcopenia (13 weeks, 380 people) | Handgrip strength and a physical performance score improved in both groups with no significant between-group difference; the active group did better on a chair-stand test and gained 0.17 kg more appendicular muscle mass. [9] | Promising but limited; it tested a combined product, so leucine's own share is unknown |
| Healthy younger people who already eat enough protein | A narrative review describes BCAA and leucine efficacy for strength and muscle gain as an ongoing source of controversy. [2] | Mixed |
Why the mechanism and the outcomes can disagree: the critical review argues that a short spike in mTORC1 signalling does not necessarily mean more muscle over time, and that inflammation can blunt amino acid sensing in muscle wasting conditions. [3] Trials also differ in age, health, total protein eaten, exercise and the form given (leucine alone, BCAAs, or leucine-enriched protein). Do not carry a result from one form to another. See why protein and amino acid trials disagree.
Amounts
Study amounts are not recommendations. A sarcopenia review noted that the protocols it reviewed used a range of 1.2 to 6 g of leucine per day (alone or as leucine-enriched protein) and judged them well tolerated. [8]No national health agency has set an upper intake level for amino acids, according to a 2023 review; researchers working under a nonprofit amino acid science council proposed 35 g a day of leucine for young adults and 30 g a day for older adults, from clinical studies in relatively healthy people. [10] These are research proposals, not official limits, and they do not apply to people with kidney or liver disease or metabolic disorders.
For muscle and aging, start with the bigger picture
Strength training and enough total protein are covered on strength training and protein for older adults, sarcopenia and muscle loss and protein and sarcopenia: what trials show.Frequently asked questions
Is leucine essential?
Yes. It is one of the nine essential amino acids. [1]
Does leucine build muscle?
In short experiments it raises muscle protein synthesis, [3] but long-term placebo-controlled studies in a 2014 critical review found no consistent benefit of leucine alone on muscle mass or function. [3]
Does leucine help older adults keep muscle?
Results are mixed: one meta-analysis found about 1 kg more lean mass but no strength gain, [7] and another found no difference in lean mass. [6]
Is there a safe upper limit for leucine?
No official limit exists. A research proposal suggested 35 g a day for young and 30 g a day for older healthy adults. [10]
Who should avoid leucine supplements?
People with maple syrup urine disease. [4] Everyone else with kidney or liver disease, pregnancy or medicines should ask a clinician or pharmacist.
More details: evidence, limits and cautions
The details behind this page: cautions, limits of the evidence and where sources disagree. Open any note to read it.
Safety and who should ask first
- Maple syrup urine disease: leucine, isoleucine and valine cannot be broken down normally and build up to levels toxic to the brain and other organs. [4] Leucine products are not for people with this condition unless their metabolic team says so.
- Older adults, kidney or liver disease, pregnancy, breastfeeding, children: the sources read give no leucine-specific guidance for these groups, so ask a clinician, pharmacist or dietitian first (Proper evidence not found; searched PubMed for leucine safety reviews). See amino acid supplements in pregnancy, children and older adults and protein and kidney disease.
- Medicines: see medication interactions.
- Before surgery: tell the team about every supplement (supplements before surgery).
- Product quality: quality testing and certifications.
References
- MedlinePlus. Amino acids. National Library of Medicine, US. Review date 1/21/2025. Read 2026-10-22.
- Plotkin DL et al. Isolated leucine and branched-chain amino acid supplementation for enhancing muscular strength and hypertrophy: a narrative review. Int J Sport Nutr Exerc Metab. 2021. Read 2026-10-22.
- Ham DJ et al. Leucine as a treatment for muscle wasting: a critical review. Clin Nutr. 2014. Read 2026-10-22.
- MedlinePlus Genetics. Maple syrup urine disease. National Library of Medicine, US. Read 2026-10-22.
- Health Canada. Dietary reference intakes tables: Reference values for macronutrients. Government of Canada, page dated 2025-11-19. Read 2026-10-22 through a web-fetch tool that summarised the page; rows as reported by that tool.
- Xu ZR et al. The effectiveness of leucine on muscle protein synthesis, lean body mass and leg lean mass accretion in older people: a systematic review and meta-analysis. Br J Nutr. 2015. Read 2026-10-22.
- Komar B et al. Effects of leucine-rich protein supplements on anthropometric parameter and muscle strength in the elderly: a systematic review and meta-analysis. J Nutr Health Aging. 2015. Read 2026-10-22.
- Martínez-Arnau FM et al. Beneficial effects of leucine supplementation on criteria for sarcopenia: a systematic review. Nutrients. 2019. Read 2026-10-22.
- Bauer JM et al. Effects of a vitamin D and leucine-enriched whey protein nutritional supplement on measures of sarcopenia in older adults, the PROVIDE study: a randomized, double-blind, placebo-controlled trial. J Am Med Dir Assoc. 2015. Read 2026-10-22.
- Elango R. Tolerable upper intake level for individual amino acids in humans: a narrative review of recent clinical studies. Adv Nutr. 2023. Read 2026-10-22.