Leucine
The one amino acid with a named intracellular receptor for it. Sestrin2 binds leucine and releases its inhibition of GATOR2, which lets mTORC1 switch on and protein synthesis start. That makes leucine a signal as well as a substrate — and it also explains the ceiling, because once the sensor is saturated more leucine adds nothing and the muscle still needs the other nineteen amino acids to build anything.
Leucine quick facts
| Suggested dose | Trials used 2.5–5 g per dose, usually with or shortly after a protein-containing meal. Whey protein already contains roughly 10–11% leucine by mass. |
| How often | Per meal, on the meals that are short of protein - not per day |
| Who it's for | Older adults with low protein intake, and people whose protein source is plant-based and lower in leucine. Not people already eating 30 g of animal protein per meal. |
Use it to top up a meal that is short of leucine, not to replace protein. Adding leucine to a low-protein meal switches on the machine with nothing in the hopper, and that is the most common way this supplement is wasted. Whey is already about 10 to 11% leucine, so anyone hitting 30 g of a complete protein per meal is unlikely to need it; older adults, plant-based eaters and people eating small meals are where the meta-analysis effects, such as they are, live. Note the trade: large isolated doses lower plasma valine and isoleucine through shared transport, and compete with tryptophan at the blood-brain barrier. Absolutely contraindicated in maple syrup urine disease.
How Leucine actually works
Leucine is the only proteinogenic amino acid with a characterized intracellular sensor named for it. Sestrin2 binds free leucine with a dissociation constant in the low tens of micromolar and, when bound, releases its inhibition of GATOR2; GATOR2 then stops suppressing the Rag GTPases, mTORC1 is recruited to the lysosome, and translation initiation proceeds through S6K1 and 4E-BP1. Leucyl-tRNA synthetase and SAR1B provide parallel leucine-sensing input. Two consequences follow from a sensor rather than a substrate model. First, the response saturates: once Sestrin2 is occupied, more leucine adds no more signal, which is the molecular form of the leucine threshold. Second, the signal is only permission — translation still needs the other amino acids as bricks, and mTORC1 activation on an incomplete pool produces signaling without net synthesis. Resistance exercise raises long-term mTORC1 sensitivity to leucine, and ageing lowers it, which is the anabolic resistance that makes older muscle respond poorly to the same meal.
Where to get Leucine
Find Leucine on iHerb →The evidence for Leucine
Graded by what exists behind each claim.
✅ Clinically validated
- A systematic review and meta-analysis of 17 randomized controlled trials of leucine supplementation on sarcopenia-related measures in older adults found effects that are modest and inconsistent across outcomes.
- Resistance exercise increases long-term mTORC1 sensitivity to leucine, which means the training and the amino acid are not independent inputs — the same dose does more in a trained muscle.
- The leucine-sensing apparatus itself becomes less responsive with age, and that anabolic resistance, rather than intake, is a large part of why older muscle responds poorly to protein.
📊 Correlative data
- Habitual protein intake and muscle retention track together in older cohorts, and the reappraisal of the US protein and amino acid recommendations argues the current numbers were set to prevent deficiency rather than to support function.
🧪 Theoretical / extrapolated benefits
- Because the branched-chain amino acids share one transporter and one dehydrogenase complex, a large isolated leucine dose is expected to lower plasma valine and isoleucine. This is well described mechanistically and is the strongest theoretical argument against dosing leucine alone rather than as part of complete protein.
How to read these tiers: they say how much human evidence exists, not how well something works — and ✗ flags harm, never a disappointing trial. How the evidence tiers work →
What Leucine actually does
Leucine is the only proteinogenic amino acid with a named intracellular receptor for it, and that is a genuinely unusual thing to be able to say. Sestrin2 binds free leucine with a dissociation constant in the low tens of micromolar. Unbound, Sestrin2 inhibits GATOR2; leucine binding releases that inhibition, GATOR2 stops suppressing GATOR1, the Rag GTPases switch to their active nucleotide state, and mTORC1 is recruited to the lysosomal surface where Rheb activates it Zhao 2021 Sung 2023. Downstream, S6K1 is phosphorylated and 4E-BP1 releases eIF4E, and translation initiation proceeds.
Leucyl-tRNA synthetase and SAR1B provide parallel leucine-sensing input to the same node, which is why the pathway is described as a sensing network rather than a single switch Sung 2023.
Two consequences follow from a sensor model, and they are the whole practical content of this page. First, the response saturates. Once Sestrin2 is occupied, additional leucine adds no further signal — which is the molecular form of the leucine threshold that the protein literature describes empirically as roughly 2 to 3 g per meal. Second, the signal is permission and not material. mTORC1 activation on an incomplete amino acid pool produces signaling without net protein accretion, because translation still needs the other nineteen amino acids as substrate.
Sensitivity is not fixed, and this is the most useful recent finding. Resistance exercise increases long-term mTORC1 sensitivity to leucine D'Hulst 2022, so the same dose does more in a trained muscle; and leucine-sensing declines with age, which is a large part of the anabolic resistance that makes older muscle respond poorly to an ordinary meal Zhao 2021 Zhao 2021.
The branched-chain amino acids also share one transporter and one catabolic entry point. LAT1 carries all three, and the branched-chain aminotransferase and dehydrogenase complex handles all three, which is why a large isolated leucine dose predictably lowers plasma valine and isoleucine.
Cell, rodent, human — and where it stops
In cells and in the pathway. The leucine-sensing apparatus is characterized at the level of named proteins and their interactions Sung 2023, which is a stronger mechanistic base than almost anything else in this catalog.
In muscle, on sensitivity rather than on outcome. Resistance exercise enhances long-term mTORC1 sensitivity to leucine D'Hulst 2022, and preliminary human evidence links exercise to enhanced leucine-sensing as a route to hypertrophy Zhao 2021. Note the endpoint: these measure how responsive the pathway is, not how much muscle was built.
In people, on the outcome the supplement is sold for, the answer is modest. A systematic review and meta-analysis of 17 randomized controlled trials of leucine supplementation on sarcopenia-related measures in older adults found effects that are small and inconsistent across outcomes Guo 2022. Seventeen trials is a real evidence base, and what it says is that the acute signaling result does not reliably become a body-composition result.
Here is the specific obstacle, and it is not the usual one. It is not species, dose or form. It is that the trials were run in people whose limitation was total protein intake, and leucine does not supply protein. When background intake is adequate, the sensor is already being saturated at every meal and there is no signal left to add — which is why the reappraisal of the US protein and amino acid recommendations frames the question as total intake and distribution rather than as a single amino acid Weiler 2023.
Leucine — which form, and does it matter
Free-form L-leucine is the only form that matters, and the comparison worth making is not against another leucine product. It is against protein.
Whey is roughly 10 to 11 percent leucine by mass, so a 30 g serving delivers about 3 g — already at or above the threshold that saturates the sensor. Casein is around 9 percent, egg about 8.5, and most plant proteins 6 to 8, which is the arithmetic behind the advice that plant-based eaters need a larger serving to reach the same signal. Adding 3 g of isolated leucine to a whey shake adds nothing to a saturated sensor; adding it to a 15 g plant protein serving is a different proposition.
Isolated leucine against a 2:1:1 BCAA against a complete EAA blend is the real form question. BCAA products dilute the leucine with valine and isoleucine, which compete for the same transporter. EAA blends supply the substrate the signal is asking for. On the mechanism above, isolated leucine is the strongest signal and the weakest substrate, which is exactly the wrong combination when total protein is low.
Exposure arithmetic. Trials used 2.5 to 5 g per dose Guo 2022. Plasma leucine peaks about 30 to 60 minutes after a free-form dose and falls within 2 to 3 hours; from whole protein the curve is lower and longer. Because the sensor responds to a concentration rather than to an integral, the free-form spike is genuinely a different stimulus, and that difference is the best argument for the isolated product.
Alpha-ketoisocaproate and HMB are leucine metabolites sold separately and are not interchangeable with it; HMB in particular has its own trial base and its own claims, and neither activates the Sestrin2 route the way leucine itself does.
What would have to be true, and how you would know it was not
The prediction that would show the sensor being saturated, and it cuts against the supplement. In somebody already eating 30 g of complete protein per meal, adding 3 g of leucine should produce no measurable change in lean mass or grip strength over 12 weeks, because the pathway is already receiving full signal Guo 2022. That is the honest expectation for most people who buy this.
The prediction where it should work. In an older adult eating well under a gram of protein per kilogram, split unevenly across the day, adding leucine to the two smallest meals should raise grip strength and gait speed over 12 to 24 weeks. Those are the endpoints the sarcopenia trials used Guo 2022 Zhao 2021, they are measurable in a clinic with no equipment, and they are the read-out rather than the scale.
The prediction that tests the transporter argument. Sustained isolated leucine at 5 g or more per day should lower plasma valine and isoleucine on an amino acid panel. If they do not fall, the shared transport account in the mechanism section is overstated.
And one that is easy to overlook: BUN. Isolated amino acid supplementation adds nitrogen load, and in anybody with reduced renal function BUN and creatinine are the numbers that will show it first.
What nobody has tested yet
Nobody has run a leucine trial stratified by baseline protein intake, which is the one variable the mechanism says should decide the result. Seventeen randomized trials exist Guo 2022 and the obvious explanation for their inconsistency — that some populations were already saturating the sensor and some were not — has never been tested as a prespecified subgroup.
Whether leucine sensitivity can be restored is open. Resistance exercise raises long-term mTORC1 sensitivity to leucine D'Hulst 2022 and ageing lowers it Zhao 2021. Whether training restores an older person's sensitivity to a younger person's level, or merely improves it, has not been measured, and it would change how protein is prescribed in sarcopenia.
The chronic-mTORC1 question has no human answer. Sustained mTORC1 activation suppresses autophagy, and the longevity literature treats that as a cost; the muscle literature treats mTORC1 activation as the goal. Nobody has measured what daily isolated leucine does to autophagic flux in humans over years, and the two fields have not been reconciled Sung 2023.
Leucine — its own safety story, not its category's
Leucine is well tolerated at trial doses, and its own risk story is about displacement rather than toxicity.
Maple syrup urine disease is an absolute contraindication, along with any other inherited disorder of branched-chain amino acid metabolism. This is the one hard exclusion and it is not theoretical — the enzyme complex that would clear the dose is the one that is missing.
Large isolated doses lower plasma valine and isoleucine through shared LAT1 transport and a shared catabolic complex. That matters for anybody using isolated leucine long term in place of a complete amino acid profile, and it is the strongest argument for taking it with protein rather than instead of it.
Branched-chain amino acids compete with tryptophan at the blood-brain barrier, so a large dose can blunt central serotonin synthesis. This is the basis of the old central-fatigue hypothesis and is worth knowing for anybody who notices mood or sleep changes after starting a high-dose BCAA or leucine habit.
Leucine is insulinotropic. It stimulates insulin secretion directly, which is additive with sulfonylureas and with injected insulin.
Caution in advanced kidney or liver disease, where nitrogen load rather than the amino acid itself is the limiting concern, and where the reappraisal of protein recommendations explicitly separates healthy adults from those populations Weiler 2023.
Gastrointestinal upset and an intensely bitter taste are the usual reasons people abandon the free-form powder.
Sources read for this page
- Guo Y. 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 · PMID 35845777
- Zhao Y. Advances in the Role of Leucine-Sensing in the Regulation of Protein Synthesis in Aging Skeletal Muscle. Front Cell Dev Biol 2021 · PMID 33869199
- D'Hulst G. Resistance exercise enhances long-term mTORC1 sensitivity to leucine. Mol Metab 2022 · PMID 36252815
- Zhao Y. Exercise May Promote Skeletal Muscle Hypertrophy via Enhancing Leucine-Sensing: Preliminary Evidence. Front Physiol 2021 · PMID 34630161
- Sung Y. Nutrient sensors and their crosstalk. Exp Mol Med 2023 · PMID 37258576
- Weiler M. Is It Time to Reconsider the U.S. Recommendations for Dietary Protein and Amino Acid Intake?. Nutrients 2023 · PMID 36839196
How you would know if it worked
There is no blood test for this one. That is not a criticism — it is a fact about the effect, and it changes how you should judge it.
- What to watch: Grip strength and gait speed, plus a written record of protein grams at each meal for three days before starting. The protein log is not optional — the mechanism says an already-saturated leucine sensor cannot be pushed further, so somebody eating 30 g of complete protein per meal is running an experiment whose result is known in advance.
- How long before it means anything: Twelve to twenty-four weeks. The sarcopenia trials measured function over that window, and muscle function does not move on a scale of days no matter what the acute signaling data shows.
- What will fool you: Training. Resistance exercise raises long-term sensitivity to leucine, so starting both in the same month credits the capsule with the gym. The second is the scale: body weight moves with hydration and glycogen far faster than with contractile protein, and a free-form amino acid taken with fluid will move it in week one.
Run it one variable at a time. Starting three things in one week means a result you cannot attribute, which is the same as no result.
Leucine — safety & side effects
- Very well tolerated at trial doses. GI upset and a bitter taste are the usual complaints.
- Competes with valine and isoleucine for transport, so large isolated doses lower their plasma levels. Relevant to anyone using isolated leucine long term instead of a complete amino acid profile.
- Branched-chain amino acids compete with tryptophan for transport across the blood-brain barrier; a large dose can blunt central serotonin synthesis, which is the basis of the old central-fatigue hypothesis and is worth knowing if mood or sleep changes.
- Contraindicated in maple syrup urine disease and in any inherited disorder of branched-chain amino acid metabolism.
- Caution with advanced kidney or liver disease, where nitrogen load is the limiting concern.
- Raises insulin. Additive with insulin secretagogues and with insulin itself.
Not medical advice. If you take prescription medication or have a diagnosed condition, check this against it with a pharmacist or doctor — pharmacists are underused and free.
- When to take it, and what to take it with
- Which form actually absorbs
- Who it's worth it for
- Coach Cam's stacks and notes
- Fasted or with food, and when in the day
- Morning or night, and why that window
- Around training, or deliberately away from it
- What it must not share a window with
Everything above is free and stays free. Skool is where it becomes a plan — Leucine in an order, with the rest of what you're running.
Unlock in Skool — $10/mo →Bloodwork to run alongside Leucine
Baseline first, then again at 8–12 weeks.
| Marker | What it’s watching for |
|---|---|
| Complete Blood Count (CBC) with Differential | Hemoglobin sets your ceiling for endurance work |
| Ferritin | Low iron limits performance long before anemia shows |
| Total Testosterone | Drops under genuine overreaching |
| Comprehensive Metabolic Panel (CMP) | Kidney and electrolytes under heavy training load |
The Athletic Performance & Recovery panel covers these in one order — 12 markers, $207.90 with the discount applied.
Check results you already have → · All 103 markers A–Z
Leucine — frequently asked questions
What is Leucine?
The one amino acid with a named intracellular receptor for it. Sestrin2 binds leucine and releases its inhibition of GATOR2, which lets mTORC1 switch on and protein synthesis start. That makes leucine a signal as well as a substrate — and it also explains the ceiling, because once the sensor is saturated more leucine adds nothing and the muscle still needs the other nineteen amino acids to build anything.
What is the suggested dose of Leucine?
Trials used 2.5–5 g per dose, usually with or shortly after a protein-containing meal. Whey protein already contains roughly 10–11% leucine by mass. This is a general reference for education only — statements have not been evaluated by the FDA and this is not medical advice.
Where can I find Leucine dosing and the full breakdown?
The suggested dose and the full evidence — clinical, correlative and theoretical — are on this page. What's inside Skool is when to take it, which form actually absorbs, the brand worth buying and Coach Cam's stacks.
Where can I buy Leucine?
Coach Cam sources Leucine from vetted, top-rated brands on iHerb — use the buy link on this page.
What Leucine is used for
Leucine appears under 1 goal in the goal router.
Related Performance supplements
Where this goes next
The pages here are the frameworks. The protocols — the dosing, the order to correct things in, the week-by-week schedule and what to retest — are inside Skool.