NutriDex

The Supplement Research Compendium
💪

Leucine

L-Leucine

The amino-acid trigger for muscle protein synthesis — that rarely builds extra muscle on its own.

Mixed evidence ⚡Performance

✚ Updated · How we grade evidence

Evidence tier
Mixed
Research weight
Citations
7 verified / 7
Classification
Performance
❝
What the evidence says. Graded mixed: leucine reliably switches on muscle protein synthesis acutely via mTORC1, but RCTs show this rarely translates into extra lean mass or strength when dietary protein is already adequate — isolated leucine repeatedly fails to beat placebo on real-world outcomes. (Mixed evidence: Conflicting results across studies; benefit uncertain.)

What is Leucine?

Leucine (L-Leucine) is a performance supplement used for stimulates muscle protein synthesis. NutriDex grades the human evidence as Mixed. Leucine is one of three branched-chain amino acids and the strongest single trigger of muscle protein synthesis through the mTORC1 pathway. In short metabolic studies it consistently raises the muscle fractional synthetic rate, which fuels its reputation as a muscle-building supplement. However, that signal seldom translates into measurable results: meta-analyses in older adults found isolated leucine produced no change in lean body mass (WMD ~0.03 kg) and no clear strength gain, and controlled trials in young trainees eating enough protein show free leucine adds nothing over placebo for muscle size or strength. Benefits appear mainly when leucine is paired with adequate protein, resistance exercise, and sometimes vitamin D — and in sarcopenia, where modest gains in grip strength and gait speed are seen. BCAA mixtures (rich in leucine) modestly reduce post-exercise soreness. Its best-proven clinical role is BCAA therapy for hepatic encephalopathy, not athletic performance.

Purported Benefits

✓Stimulates muscle protein synthesis
✓May reduce post-exercise soreness
✓Possible aid in sarcopenia (with protein/vitamin D)
✓Supports recovery after muscle damage

Evidence by outcome

The same supplement can be well-proven for one use and unproven for another — here is the human evidence graded outcome by outcome.

OutcomeEvidenceEffectStudies
Muscle protein synthesis (acute)Consistently raises fractional synthetic rate in metabolic studies, but this signal rarely translates to mass. Moderate ↑ benefit · moderate 1
Lean mass / strength gainMeta-analyses and an RCT show isolated leucine adds no lean mass or strength when protein is adequate. Moderate — no effect · negligible 2
Sarcopenia (with protein/vitamin D)Sarcopenia meta-analysis showed modest grip-strength/gait gains; benefit largely requires vitamin D pairing. Preliminary ↑ benefit · small 2
Post-exercise soreness (BCAA-rich)BCAA meta-analysis cut soreness/CK, but a free-leucine RCT found no recovery benefit in untrained adults. Mixed ↔ mixed · small 2
Hepatic encephalopathy (BCAA therapy)Cochrane review (16 trials): BCAA improved encephalopathy (RR 0.73) though did not reduce mortality. Strong ↑ benefit · moderate 1

Dosing & Compounds

Typical Dose
Typically 2–5 g/day of free L-leucine, often taken with or after protein/resistance exercise; sarcopenia trials use ~3 g per meal.
Active Compounds
L-Leucineα-ketoisocaproate (KIC)HMB (β-hydroxy-β-methylbutyrate, metabolite)

Safety & Cautions

Leucine from food and typical supplement doses (up to ~5–10 g/day) is well tolerated; reported effects are mainly mild GI upset. Very high intakes compete with other amino acids and can lower valine, isoleucine and tryptophan availability, and large BCAA loads transiently raise blood ammonia, so caution applies in advanced liver disease, kidney impairment, and the rare metabolic disorder maple syrup urine disease (avoid). Because leucine can blunt the glucose-lowering AMPK pathway and stimulate insulin, people on antidiabetic drugs should monitor blood sugar; data on pregnancy and breastfeeding beyond dietary amounts are limited, so supplements are best avoided then. Educational only — always check with your doctor or pharmacist before combining Leucine with any medicine.

Key Studies

Meta-analysis Huang 2025 ✓ PubMed
Sarcopenia meta-analysis (11 studies, ~1,133): leucine modestly improved grip strength (+1.76 kg), muscle mass and gait speed (+0.05 m/s); low-quality evidence.
Meta-analysis Salem 2024 ✓ PubMed
Meta-analysis (18 RCTs): BCAA (leucine-rich) cut muscle soreness at 24–72 h (g up to −1.82) and creatine kinase at 72 h post-exercise.
Meta-analysis Guo 2022 ✓ PubMed
Meta-analysis of 17 RCTs: isolated leucine gave no change in lean mass (WMD 0.03 kg) or grip strength; benefit only when combined with vitamin D.
Meta-analysis Xu 2015 ✓ Full text
Meta-analysis (9 studies, older adults): leucine raised muscle fractional synthetic rate but produced no gain in lean body mass or leg lean mass.
Meta-analysis Gluud 2017 (Cochrane) ✓ PubMed
16 trials, 827 patients: BCAA improved hepatic encephalopathy (RR 0.73, 95% CI 0.61–0.88) but did not reduce mortality (RR 0.88).
RCT Lim 2021 ✓ PubMed
Crossover RCT: 6 g/day free leucine did not improve muscle recovery (strength, soreness, CK) after lower-limb resistance exercise in untrained adults.
RCT Aguiar 2017 ✓ PubMed
RCT in young men: 3 g/day free leucine during 8-wk resistance training did not increase strength or muscle cross-sectional area over placebo.
7 sources cited · 7 verified Search all studies on PubMed ↗

Common questions about Leucine

What is Leucine used for?

Leucine is most often taken for Stimulates muscle protein synthesis, May reduce post-exercise soreness, Possible aid in sarcopenia (with protein/vitamin D), Supports recovery after muscle damage. The amino-acid trigger for muscle protein synthesis — that rarely builds extra muscle on its own.

Does Leucine work — what does the evidence say?

Mixed evidence. Conflicting results across studies; benefit uncertain. Leucine is one of three branched-chain amino acids and the strongest single trigger of muscle protein synthesis through the mTORC1 pathway. In short metabolic studies it consistently raises the muscle fractional synthetic rate, which fuels its reputation as a muscle-building supplement. However, that signal seldom translates into measurable results: meta-analyses in older adults found isolated leucine produced no change in lean body mass (WMD ~0.03 kg) and no clear strength gain, and controlled trials in young trainees eating enough protein show free leucine adds nothing over placebo for muscle size or strength. Benefits appear mainly when leucine is paired with adequate protein, resistance exercise, and sometimes vitamin D — and in sarcopenia, where modest gains in grip strength and gait speed are seen. BCAA mixtures (rich in leucine) modestly reduce post-exercise soreness. Its best-proven clinical role is BCAA therapy for hepatic encephalopathy, not athletic performance.

What is the typical dose of Leucine?

Typically 2–5 g/day of free L-leucine, often taken with or after protein/resistance exercise; sarcopenia trials use ~3 g per meal.

Is Leucine safe? Any cautions or side effects?

Leucine from food and typical supplement doses (up to ~5–10 g/day) is well tolerated; reported effects are mainly mild GI upset. Very high intakes compete with other amino acids and can lower valine, isoleucine and tryptophan availability, and large BCAA loads transiently raise blood ammonia, so caution applies in advanced liver disease, kidney impairment, and the rare metabolic disorder maple syrup urine disease (avoid). Because leucine can blunt the glucose-lowering AMPK pathway and stimulate insulin, people on antidiabetic drugs should monitor blood sugar; data on pregnancy and breastfeeding beyond dietary amounts are limited, so supplements are best avoided then.

How many studies support Leucine?

NutriDex cites 7 sources for Leucine, graded "Mixed".

Cite this page
APA

Peh, D. (2026). Leucine (L-Leucine): Benefits, Dosage, Side Effects & Evidence. NutriDex — The Supplement Research Compendium. Retrieved 26 Jun 2026, from https://nutridex.info/s/leucine

BibTeX
@misc{nutridex_leucine,
  author       = {Peh, Daryl},
  title        = {Leucine (L-Leucine): Benefits, Dosage, Side Effects \& Evidence},
  year         = {2026},
  howpublished = {NutriDex --- The Supplement Research Compendium},
  url          = {https://nutridex.info/s/leucine},
  note         = {Reviewed by Dr Daryl Peh, MBBS Singapore, MMed FM. Accessed 2026-06-26}
}

For medical claims, citing the underlying primary studies linked above is preferred. NutriDex is an educational reference, not medical advice.

← Back to the full dex · All substances