L-Leucine
Summary
Leucine is an essential branched-chain amino acid that signals muscle to build protein; adults need about 42 mg per kg daily. A 2022 meta-analysis of 17 trials in older adults found leucine alone did not increase lean mass or strength (grade C), and a 6-month trial at 7.5 g/day found no muscle or blood sugar change.
Key facts
- Estimated average requirement 34 mg/kg/day; RDA 42 mg/kg/day (56 mg/kg/day in pregnancy)[1]
- Leucine alone did not raise lean mass, grip or leg-press strength across 17 trials (1,418 people in total)[2]
- Leucine raised the measured rate of muscle protein synthesis in older people without changing lean mass (9-study meta-analysis)[3]
- About 500 mg/kg/day (~35 g/day at 70 kg) proposed from acute studies in men; a cautious 351 mg/kg/day for older men. No official UL[7]
- Oral leucine (100 mg/kg) reversed levodopa's effect in people with Parkinson's disease[8]
- 4 graded human outcomes on this page
- October 13, 2026
How it works
EstablishedLeucine is a building block of protein and also acts as a signal that switches on muscle protein synthesis after meals; in older adults this raised the measured synthesis rate without measurably increasing lean mass over months. Tube-fed leucine raised insulin and lowered post-meal glucose in healthy volunteers. As a large neutral amino acid it competes with levodopa for transport into the brain.[3][6][8]
What the human research says
Human trials only, one row per outcome. The form column shows what each trial actually tested. Grades follow our evidence-grading policy.
At a glance: strongest evidence per outcome
| Outcome | Grade | Effect | Strongest evidence | Rows |
|---|---|---|---|---|
| Muscle | Grade C | No effect | Meta-analysis · n = 1,418 · 2022 | 3 rows |
| Blood sugar | Grade D | Improved | Crossover RCT · n = 12 · 2016 | 1 row |
Every graded row
| Outcome | Effect | Grade | Best evidence | PMID |
|---|---|---|---|---|
| Lean mass, handgrip strength and leg press in older adults | No effect | Grade C | Meta-analysis · n = 1,418 · 2022 Older adults in 17 randomized trials (2009–2022) Form: Leucine alone, or leucine combined with other nutrients such as vitamin D · Dose: Varied by trial · Duration: Varied by trial Leucine on its own did not change total lean mass, handgrip strength or leg press. Combined products containing leucine plus vitamin D improved handgrip strength and gait speed slightly. | 35845777 (opens PubMed)Source [2] |
| Lean mass, strength and blood sugar control in older men with type 2 diabetes | No effect | Grade D | RCT · n = 60 · 2011 Older men with type 2 diabetes, habitual protein intake adequate Form: L-leucine · Dose: 2.5 g with each main meal (7.5 g/day) · Duration: 6 months Lean tissue mass, body fat, strength, muscle fibre characteristics, HbA1c and insulin sensitivity did not change or differ from placebo. | 21525248 (opens PubMed)Source [4] |
| Sarcopenia measures (walking time, lean mass index) in nursing-home residents | Improved | Grade D | RCT · n = 50 · 2020 Institutionalised adults aged 65 and over Form: L-leucine · Dose: 6 g/day (placebo: 6 g lactose) · Duration: 13 weeks Walking time and lean mass index improved, and maximum expiratory force improved versus placebo; function scales, cognition and inflammatory markers did not change. | 32230954 (opens PubMed)Source [5] |
| Blood glucose and insulin after a mixed-nutrient drink | Improved | Grade D | Crossover RCT · n = 12 · 2016 Healthy lean volunteers Form: L-leucine · Dose: 5 g or 10 g by intragastric infusion 15 minutes before the drink · Duration: Single dose per visit 10 g lowered the blood glucose area under the curve and raised insulin; 5 g had no significant effect. Neither dose changed later food intake. | 27655440 (opens PubMed)Source [6] |
Forms
| Form | Elemental % | Absorption (human data) | GI tolerance | Studied for |
|---|---|---|---|---|
| Free-form L-leucine (powder or capsules) | Varies[10][11] | No head-to-head human data on capsule versus powder absorption.[3] | 7.5 g/day for 6 months and 6 g/day for 13 weeks were well tolerated in older adults.[4][5] | Muscle mass and function in older adults; glucose laboratory studies |
| BCAA blends (leucine, isoleucine, valine) | Varies[2] | No head-to-head human data on leucine absorption from blends versus free leucine.[2] | Not assessed in the leucine studies cited here.[2] | Muscle and exercise outcomes (see BCAAs page) |
| Leucine-enriched protein or multinutrient products | Varies[2] | No head-to-head human data.[2] | Not summarised in the meta-analysis abstract.[2] | Handgrip strength and gait speed in older adults (combinations with vitamin D) |
Studied doses
Doses studied in human trials ranged from 6 g/day for 13 weeks in nursing-home residents to 7.5 g/day (2.5 g with each meal) for 6 months in older men with type 2 diabetes. Single 5–10 g doses were used in glucose laboratory studies.[4][5][6]
Interactions
Published interactions only: each row cites a source. No row means none was found in our sources, not that a combination is safe.
Medicines
- LevodopaPK studyLarge neutral amino acids compete with levodopa for transport into the brain: oral leucine, isoleucine or phenylalanine (100 mg/kg) reversed the effect of infused levodopa in people with Parkinson's disease without lowering blood levodopa.[8]
Who should be careful
- People taking levodopa
- Oral leucine at 100 mg/kg reversed levodopa's effect in a 1984 study of people with Parkinson's disease.[8]
- Maple syrup urine disease
- This inherited disorder requires lifelong restriction of branched-chain amino acids, leucine above all; leucine supplements are unsuitable.[9]
- Very high intakes
- Blood ammonia rose above normal at leucine intakes above about 550 mg/kg/day in acute studies in young and older men.[7]
- Pregnancy and breastfeeding
- Requirements are higher (RDA 56 mg/kg/day in pregnancy, 62 mg/kg/day in lactation), but no trial has tested leucine supplements in these groups.[1]
Talk to your pharmacist or clinician before starting a supplement, especially if you take prescription medicines.
How to read the label
- Per serving, not per capsuleCapsule products list 500 mg per 1-capsule serving; powders list 5,000 mg (about 2 teaspoons) per serving. Check the serving size before comparing.[10][11]
- Label dose vs studied doseOne 500 mg capsule is a twelfth of the 6 g/day used in the positive nursing-home trial; a 5,000 mg powder serving is close to it.[10][11][5]
- Label dose vs requirementA 70 kg adult's RDA is about 2.9 g of leucine a day, most of which normally comes from protein foods.[1]
Cost per studied dose, without the sales pitch
Compare any product you already have by the amount it delivers, not by capsule count or front-of-pack weight.
Cost per studied dose
Enter the price, servings and the amount per serving from your label.
Frequently asked questions
How much leucine do adults need?
The Institute of Medicine RDA is 42 mg per kg of body weight per day for adults, about 2.9 g for a 70 kg person, mostly supplied by protein foods.[1]
Does leucine build muscle in older adults?
Not on its own, according to a 2022 meta-analysis of 17 trials. One small 13-week trial in nursing-home residents found better walking time and lean mass index with 6 g/day.[2][5]
Why does leucine get so much attention?
It switches on muscle protein synthesis after a meal, and leucine raised the measured synthesis rate in older people. That short-term signal has not translated into more lean mass in trials.[3][2]
Is there a safe upper limit?
There is no official UL. Researchers proposed about 500 mg/kg/day for men from acute studies, with blood ammonia rising above that range; a cautious estimate for older men is 351 mg/kg/day.[7]
Does leucine affect blood sugar?
10 g given by tube raised insulin and lowered post-meal glucose in healthy volunteers. In a 6-month trial in older men with type 2 diabetes, 7.5 g/day did not change HbA1c.[6][4]
Can leucine interfere with medication?
Yes for levodopa: oral leucine at 100 mg/kg blocked levodopa's effect in people with Parkinson's disease in a 1984 study.[8]
Sources
- [1]Dietary Reference Intakes: The Essential Guide to Nutrient Requirements. Appendix E: DRI values for indispensable amino acids by life stage and gender group. Institute of Medicine, National Academies Press, 2006. guideline
- [2]The Effect of Leucine Supplementation on Sarcopenia-Related Measures in Older Adults: A Systematic Review and Meta-Analysis of 17 Randomized Controlled Trials.. Front Nutr, 2022. meta-analysis
- [3]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. meta-analysis
- [4]Prolonged leucine supplementation does not augment muscle mass or affect glycemic control in elderly type 2 diabetic men.. J Nutr, 2011. RCT
- [5]Effects of Leucine Administration in Sarcopenia: A Randomized and Placebo-controlled Clinical Trial.. Nutrients, 2020. RCT
- [6]Intragastric administration of leucine or isoleucine lowers the blood glucose response to a mixed-nutrient drink by different mechanisms in healthy, lean volunteers.. Am J Clin Nutr, 2016. crossover-RCT
- [7]Safety and Tolerability of Leucine Supplementation in Elderly Men.. J Nutr, 2016. review
- [8]The "on-off" phenomenon in Parkinson's disease. Relation to levodopa absorption and transport.. N Engl J Med, 1984. PK-study
- [9]Simplifying supplementation in MSUD: tolerance and acceptability of liquid valine and isoleucine supplements in maple syrup urine disease.. Orphanet J Rare Dis, 2026. open-label
- [10]L-Leucine 500 mg Capsules (BulkSupplements.com), DSLD label 312918: 500 mg L-leucine per 1-capsule serving, once daily. NIH Office of Dietary Supplements, Dietary Supplement Label Database, 2024. label
- [11]L-Leucine Powder 5000 mg (BulkSupplements.com), DSLD label 331168: 5,000 mg L-leucine per serving (about 2 teaspoons), once daily. NIH Office of Dietary Supplements, Dietary Supplement Label Database, 2025. label
What changed
- Strict adversarial pass: PMIDs re-checked; DSLD doses verified per serving (500 mg per capsule; 5,000 mg per powder serving). Muscle grade held at C/no gain for leucine alone; UL stated as a research proposal, not an official limit.
- First draft rebuilt from scratch after the earlier draft files were lost. Every PMID checked against NCBI esummary and abstracts read for each row. Requirement values from the Institute of Medicine DRI Essential Guide, Appendix E. Label doses re-read from the DSLD API as amounts per serving: 500 mg per 1-capsule serving (label 312918) and 5,000 mg per powder serving (label 331168).