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Leucine
L-Leucine is the most metabolically active branched-chain amino acid and the primary "switch" for muscle protein synthesis via mTORC1.
Aliases (4)
Overview
What is Leucine?
L-Leucine is the most metabolically active branched-chain amino acid and the primary "switch" for muscle protein synthesis via mTORC1. The 2-3 g per meal "leucine threshold" is the headline take from 20 years of research, with a 2023 systematic review (Wilkinson PMID 37537134) softening the model in young adults — leucine dose correlates with MPS magnitude in older adults but not consistently in young, well-fed lifters. For Dylan (20yo MMA, ~150-200 g protein/day from animal sources + whey in V4 stack), each high-protein meal already crosses the threshold 4-5x daily, so standalone leucine is redundant rather than wrong. Skip the pill. Where it does earn a place: plant-based meals (Lim 2024 — adding leucine to plant protein matches whey for MPS), aggressive weight cuts where total protein gets squeezed, older adults with anabolic resistance, or anyone auditing a borderline-leucine meal. Dose: 2-5 g free-form leucine per leucine-poor meal, 5-10 g pre/post-workout if isolated. Safety is excellent. Cost is negligible. The cancer-and-mTOR debate (proliferative mTORC1 signal vs. anti-cachexia benefit) is real but only practically relevant for active oncology contexts. HMB (β-hydroxy-β-methylbutyrate, the leucine metabolite) is a separate compound — see hmb.md — with stronger evidence in elderly + sarcopenia than in healthy young athletes.
Peptide Interactions
Whey already delivers 2.5-3 g leucine per 25 g scoop, so adding free leucine to a whey shake is generally redundant. The exception: very low-dose whey (10-15…
Full EAA powders (containing all 9 essential amino acids in physiologic ratios) cover both the leucine trigger and the sustained-MPS tail. Better than isolat…
Independent mechanism (PCr buffering, cell volumization, IGF-1 expression). Creatine + leucine + whey is the rare stack where each component has a non-overla…
Casein already supplies leucine over hours. Free leucine isn't a clean add — go with casein alone.
The leucine metabolite. Worth considering as a separate compound especially during energy restriction or anti-catabolic phases; see hmb.md for the full break…
Insulinogenic stack; carbs amplify leucine's modest insulin response and improve nutrient delivery to muscle.
when you're already taking standalone leucine — you're paying for the same leucine twice, plus iso/valine that may compete for transport.
if you're sensitive to leucine's competition for BBB LAT1 transport — large neutral amino acids compete; leucine pre-bed could (theoretically) blunt nocturna…
What to Expect
- Week 1Tolerability and dose-response.
- Week 2-4Early effect window.
- Week 4-8Peak benefit assessment.
- Week 8+Cycle decision point.
Side Effects & Safety 5
Side Effects
- 1Bitter taste — free-form leucine is notably bitter. Mix into a strongly flavored shake or use capsules. Not a safety concern.
- 2Mild GI bloat / loose stool at 5 g+ on empty stomach in some users. Take with food.
- 3Headache — occasional reports at higher doses (10 g+). Usually hydration-related.
- 4Nausea — rare at typical doses.
- 5Transient drowsiness post-dose in some users (possibly the mild insulin effect + tryptophan-BBB transport competition shifting brain neurotransmitter balance; speculative).
When to Stop
- Hyperammonemia at supra-physiologic chronic doses — Branched-chain amino acid catabolism produces ammonia via aminotransferase reactions. In healthy users with intact urea cycle, dietary-range and supplement-range leucine (up to ~10-15 g/day) does not produce clinically meaningful ammonia elevation. In liver disease (cirrhosis, hepatic encephalopathy) the calculus changes — and ironically BCAAs are sometimes used therapeutically in cirrhosis with ammonia monitoring. For healthy users, ammonia toxicity from leucine supplementation is not a realistic concern at typical doses.
- Maple Syrup Urine Disease (MSUD) — a rare autosomal recessive defect in BCKDH. Patients with MSUD cannot oxidatively decarboxylate KIC and accumulate leucine + KIC to neurotoxic levels. Strict contraindication. Diagnosis typically appears in infancy via newborn screening.
- Pancreatic cancer / HCC concern — Leucine drives mTORC1, which is pro-proliferative in tumor tissue. Animal models show leucine restriction can slow tumor growth in some cancers (breast, HCC, pancreatic) and leucine supplementation can accelerate proliferation in vitro. Clinical relevance in healthy adults is theoretical; the same molecule that builds your skeletal muscle also signals "grow" to any proliferating cells in your body. The 2024 "double-edge effects of leucine on cancer cells" review (MDPI Biomolecules) summarized current thinking: avoid chronic high-dose leucine supplementation in active oncology contexts; dietary-range leucine in healthy adults is not meaningfully implicated.
- BCAA-insulin-resistance axis — Chronic supra-physiologic BCAA intake in obese / insulin-resistant subjects may worsen IR via S6K1-mediated IRS-1 serine phosphorylation. Lean trained athletes are not the population where this signal matters clinically.
- Pregnancy: As part of normal dietary protein intake, leucine is essential. Supplement form has no specific safety data in pregnancy; default to whole-protein dietary sources during pregnancy.
- Liver disease (cirrhosis, hepatic encephalopathy): Different ballgame — BCAAs are sometimes therapeutically beneficial here. Don't self-supplement; specialist input required.
- MSUD diagnosis: Hard contraindication.
References
Norton & Layman 2006 — Leucine regulates translation initiation of protein synthesis in skeletal muscle after exercise (PMID 16424142)
foundational leucine-threshold paper.
View StudyNorton 2012 — Leucine content of dietary proteins is a determinant of postprandial skeletal muscle protein synthesis (PMID 22818257)
protein quality + leucine rescue of low-leucine meals in adult rats.
View StudyWolfson 2016 — Sestrin2 is a leucine sensor for the mTORC1 pathway (PMID 26449471)
molecular mechanism of leucine sensing.
View StudyDevries 2018 (J Nutr) — Leucine, not total protein, content of a supplement is the primary determinant of MPS in older women (PMID 29901760)
leucine-enriched lower-protein dose matches conventional dose in 65-75yo women.
View StudyDevries 2018 (AJCN) — Protein leucine content is a determinant of shorter- and longer-term MPS responses in older women (PMID 29529146)
companion paper with longer-term MPS measurements.
View StudyWilkinson 2023 — Association of postprandial postexercise MPS rates with dietary leucine: systematic review (PMID 37537134)
softened threshold model; older vs. younger adult differences.
View StudyFerrando 2023 — ISSN position stand: Effects of EAA supplementation on exercise and performance (PMID 37800468)
current consensus on EAA vs. BCAA vs. leucine alone.
View StudySalem 2024 — BCAA supplementation and post-exercise recovery: overview of systematic reviews (PMID 38241335)
11 SRs pooled; small DOMS / CK effect, no performance effect.
View StudyLim 2024 — MPS in response to plant-based protein isolates with and without added leucine vs whey (PMID 38846451)
plant + leucine matches whey for MPS in healthy young adults.
View StudyFu 2024 — BCAAs and the risks of dementia, Alzheimer's disease, and Parkinson's disease (PMID 38659706)
UK Biobank Cox analysis on BCAA-neurodegeneration associations.
View StudyBrosnan & Brosnan 2006 — Branched-chain amino acids: enzyme and substrate regulation (PMID 16365084)
BCAA metabolism reference review.
View StudyAreta 2013 — Timing and distribution of protein ingestion during prolonged recovery from resistance exercise (PMID 23459753)
pulse vs. intermediate vs. bolus protein dosing on MPS.
View StudyISSN combat-sports nutrition position stand 2025 — Coswig et al.
protein and weight-cut recommendations for MMA athletes.
View StudyBideshki 2025 — Umbrella review of HMB on body composition and muscle strength
separate-but-adjacent HMB meta-meta-analysis.
View StudyDouble-Edge Effects of Leucine on Cancer Cells 2024 — Biomolecules MDPI
cancer + mTOR signaling review.
View StudyLatest research
- rctMuscle protein synthesis in response to plant-based protein isolates with and without added leucine versus whey protein in young men and womenPlant-protein + added leucine matches whey for MPS in healthy young adults — validates leucine-fortification rescue protocol for plant-based eaters.
- observationalBranched-chain amino acids and the risks of dementia, Alzheimer's disease, and Parkinson's diseaseUK Biobank Cox analysis: higher plasma BCAA (incl. leucine) associated with reduced dementia/AD risk but increased Parkinson's risk. BCAAs may function as neurodegenerative biomarkers; mechanism unclear.
- metaBranched-chain amino acids supplementation and post-exercise recovery: an overview of systematic reviewsOverview of 11 systematic reviews: BCAA ingestion attenuates CK levels (medium effect) and muscle soreness (small effect) post-exercise; no effect on LDH, myoglobin, or performance recovery.
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