L-Leucine
Also known as: Leucine
Essential BCAA that activates mTORC1 via Sestrin2 sensing, driving phosphorylation of S6K1 and 4E-BP1, initiating cap-dependent translation and muscle protein synthesis.
Last reviewed: 2026-06-23 · 1 indexed study
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Overview
L-Leucine is the branched-chain amino acid that uniquely activates mTORC1 to trigger muscle protein synthesis; the threshold ~3 g per meal drives the 'leucine trigger' MPS response.
Mechanism & Function
Mechanism of Action
Initiates muscle protein synthesis above the leucine threshold, supplements suboptimal protein meals, and partially preserves MPS in caloric restriction.
Physiological Role
Most studied in protein-pulse strategies for older athletes, restricted-protein meals, and as the active component of EAA/BCAA mixes.
Primary Pathways
Performance / Health Relevance
Robust RCT and meta-analysis evidence that leucine fortification of suboptimal-protein meals raises MPS to maximal response.
Evidence Review
Practical Use & Dosage
- Minimum Effective Dose
- 2.5 g per meal
- Optimal Range
- 3–5 g per meal alongside complete protein
- Upper / Caution Level
- ~10 g/day total supplemental
- Timing
- With meals containing <25 g protein
- Best Format
- Powder, capsule
- Bioavailability
- ~95% absorption.
- Absorption Considerations
- Active intestinal transport; rapid plasma rise within 30 min.
Safety & Interactions
Side Effects
- •Bitter taste
- •GI upset at very high doses
Contraindications
- Maple syrup urine disease
Drug Interactions
- •Levodopa (competes for transport)
Who Should Avoid
- •Inherited BCAA disorders
Monitoring
None routine.
Frequently Asked Questions
Auto-generated from indexed evidence — dosage, safety, interactions, and more.
When is L-Leucine typically used or indicated?
L-Leucine is most commonly used for: Muscle protein synthesis, Recovery, Anti-catabolic. Most studied in protein-pulse strategies for older athletes, restricted-protein meals, and as the active component of EAA/BCAA mixes.
How does L-Leucine work?
Initiates muscle protein synthesis above the leucine threshold, supplements suboptimal protein meals, and partially preserves MPS in caloric restriction.
What is the recommended dose of L-Leucine?
The optimal range is 3–5 g per meal alongside complete protein. with a minimum effective dose of 2.5 g per meal. Best taken: With meals containing <25 g protein. Upper / caution level: ~10 g/day total supplemental.
What are the side effects of L-Leucine?
Reported side effects include: Bitter taste; and GI upset at very high doses. Monitoring: None routine.
Does L-Leucine interact with medications?
Known or theoretical interactions: Levodopa (competes for transport). Always check with a clinician before combining with prescription medication.
Who should avoid L-Leucine?
L-Leucine is generally not recommended for: Inherited BCAA disorders; and Maple syrup urine disease.
How strong is the evidence for L-Leucine?
Evidence level: Strong (overall score 82/100), based on 1 indexed study on SuppIndex.
Research & References (1)
Leucine supplementation and muscle protein synthesis: meta-analysis
Leucine fortification of suboptimal protein meals significantly raised MPS rates, especially in older adults.
Related Ingredients
Other top-rated ingredients in Amino Acids.
Beta-Alanine
78/100A rate-limiting precursor to carnosine (β-alanyl-L-histidine) synthesis, enhancing intramuscular pH buffering capacity during high-intensity anaerobic exercise.
EAA Blend
78/100Free-form blend of 9 EAAs (Leu, Ile, Val, Lys, Met, Phe, Thr, Trp, His) at evidence-based ratios; bypasses gastric digestion for rapid plasma aminoacidaemia.
BCAA (Branched-Chain Amino Acids)
76/100Meta‑analyses of randomized trials report that BCAA supplementation reduces blood creatine kinase after damaging exercise and can reduce delayed onset muscle soreness in the short term. Individual trials in older or clinical populations using leucine‑enriched BCAA formulas have shown increases in lean mass over 12 weeks; however, functional benefits are not consistently demonstrated and some positive trials are part of multifactorial interventions.
Research Overview
Strong EvidenceCite this ingredient
One-click citation export in APA, MLA, Chicago, BibTeX, or HTML. The URL below is stable and safe to link.
Stable URL
https://thesuppindex.com/ingredients/l-leucine
SuppIndex. (2026). L-Leucine: Evidence summary, dosage and clinical studies. Retrieved October 16, 2026, from https://thesuppindex.com/ingredients/l-leucine
Based on 1 indexed studies · Evidence level: Strong.
Goal Ratings
Stack Compatibility
Best as part of complete EAA/whey rather than alone — solo leucine without other EAAs can deplete other BCAAs.