L-leucine dosage and evidence
Written by Aaron CuhaReviewed Sep 2026
The short answer
L-leucine did not help mice live longer, in either sex. That was in the same test where rapamycin plus acarbose did better than either drug alone. In 30 healthy older men, 7.5 g a day for three months did nothing for muscle size or strength.
Evidence grade for L-leucine: Limited. The rule: people have taken it in a published study, but fewer than 100 of them. The sum on this page: 1 completed trial, 30 people given it. A grade counts how much has been done to people, not whether it works.
- People given it
- 30
- Human studies
- 1
- Found nothing
- 1
Dose
No human dose established
Amounts that were given in studies sit in Human evidence on this page. None of them is a personal dose.
1 completed trial, 30 people given it
3 effects reported, 2 unwanted, 1 wanted, by people using it, not a trial
Goal pages: Fat loss and metabolism, Muscle and strength, Ageing and longevity
In plain English
In plain English, from Aaron
This is how I would explain it to a friend. The evidence, with the numbers, is further down the page.
Leucine is an amino acid found in all protein. It is the switch that tells muscle to start building. It is sold on its own as a supplement.
What people take it for
- Building or keeping muscle.
- Stopping muscle loss with age.
- Living longer, as part of an ageing stack.
What studies in people showed
A review pooled 16 studies with 999 older people. Leucine added about 1 kg of lean tissue, mostly in people already losing muscle. It did not make anyone stronger. In 30 healthy older men, 7.5 g a day for three months did nothing for muscle size or strength. Now the mice. A US program fed it to mice for life. They did not live longer or shorter.
What people report
Reports, not study results
The good
- Older people who were already losing muscle gained about 1 kg of lean tissue in the review.
The bad
- The review found no gain in grip or leg strength.
- Healthy older men saw no change in muscle at all after three months.
- We have not collected user reports on this one yet.
Where these come from: The studies on this page. We have no forum reports on leucine taken alone.
My bottom line
Leucine is not magic. Eat enough protein first. If you are older, eat little and are losing muscle, it may help a bit. If you already eat well, it adds nothing.
An opinion, not a finding. I am a coach, not a doctor.
What it has been measured to do
Measured in people
Goals L-leucine has been tested for in people, biggest studies first. We count only what the studies on this page say, so the real numbers may be higher.
- Keeping or building muscle30 people · 1 study · 1 found nothing
- Getting stronger30 people · 1 study · 1 found nothing
- Blood sugar and insulin30 people · 1 study · 1 found nothing
Animal and lab work, not yet tested in people
Real results, but from animals or cells, not people. What works in a mouse often does not work the same way in a person. Nobody can patent this, so nobody would ever earn back the cost of a study in people. That is usually why there is none. The record on this one is set out below.
- Living longer1 animal or lab study
31 of the 36 indexed goals have no study of any kind behind L-leucine
No study in people, animals or cells on this page reports a result for these. That does not prove L-leucine does nothing for them. It does mean anything you have heard about L-leucine and one of these did not come from a study cited here.
Show full evidence
What is L-leucine?
L-leucine did not help mice live longer, in either sex. That was in the same test where rapamycin plus acarbose did better than either drug alone. In 30 healthy older men, 7.5 g a day for three months did nothing for muscle size or strength.
Leucine is the amino acid that switches on muscle building. That is not a sales claim. It flips on mTORC1, the cell's main growth switch, through a sensor called Sestrin2. How much leucine a meal has predicts the muscle-building response better than its total protein does.
That same effect is how it ended up in a lifespan study, from an awkward angle. Turning down mTOR is the most repeatable way to make mice live longer. Rapamycin does it with a drug. Leucine does the opposite. The Interventions Testing Program tested it anyway, in its 2017 run. The thinking was that keeping muscle in old age might help mice live longer, even if the switch went the wrong way.
It did not. In that run, rapamycin plus acarbose, started at 9 months, made male mice live longer than either drug alone had in past runs. Captopril gave female mice a small benefit. Leucine was one of four compounds that gave no lifespan benefit at all.
The human evidence is more interesting than the mouse result. A review pooled 16 studies in 999 older people. Leucine added 0.99 kg of lean tissue, such as muscle, and 1.02 kg of body weight compared with controls. The effect was mostly in people who were already losing muscle with age. It did not make anyone stronger. And in 30 healthy older men given 7.5 g a day for three months, there was no change in muscle size or strength at all.
The pattern holds together. Leucine helps when the problem is too little protein signal. It does nothing when that is not the problem.
How does L-leucine work?
Leucine is one of three branched-chain amino acids. It is both a building block and a signal. It binds a sensor inside the cell called Sestrin2. That releases a brake and lets the growth switch mTORC1 turn on, which starts muscle protein building. That is why a meal rich in leucine gives a bigger burst of muscle building than a meal with the same protein but less leucine. People call it a trigger, not a supply. Here is the problem for long life. Turning mTORC1 down long-term with rapamycin is the best-repeated way to make mice live longer. Leucine pushes the same switch the other way.
What have studies in people found about L-leucine?
A fair amount of human research on body make-up in older adults. It helps lean tissue a little, never helps strength, and says nothing about lifespan or deaths.
- A review that pooled 16 studies with 999 older adults: lean tissue was 0.99 kg higher than in controls, mostly in people already losing muscle with age.
- The same review found no real effect on grip strength or leg strength.
- In 30 healthy older men, 7.5 g a day for three months changed neither muscle size nor strength.
- Short studies keep showing that leucine raises muscle building after a meal. That is a lab measure taken over hours, not a real-life result.
- No human study has looked at lifespan, deaths or healthy ageing.
What this does not tell you: The gap between the short-term signal and the long-term result is the whole story. Leucine reliably boosts muscle building measured over hours. Over months, in healthy older men already eating enough protein, it did nothing. Where it does help, the gain is about one kilogram of lean tissue in people who were already losing muscle. It did not turn into measurable strength.
The state of the research
When this program finds no lifespan effect, that is a real, costly, careful result. It is also narrower than it sounds. It tested one amount in the food, started at one age, in one kind of mouse, and it counted deaths. A no-effect result tells you that this dose, in these mice, did not change how long they lived. It does not tell you the compound does nothing. It does not tell you a different dose would fail, that the idea behind it is wrong, or that the reason people actually take it has been disproved. Those are separate questions, and most were never asked. This site tries to avoid mistakes in both directions. A mouse lifespan win is not proof a supplement works. A mouse lifespan miss is not proof it does nothing.
Leucine also raises a tension worth saying out loud. Most supplement content picks one side and pretends the other does not exist.
The most repeatable way to make mice live longer is to turn down mTOR. Rapamycin does it, and so does eating less protein or less of certain amino acids. Leucine is the strongest food switch for mTORC1 there is. If mTOR is the lever, leucine pulls it the wrong way.
On the other side, muscle in older adults predicts how well they function and whether they stay independent. Losing muscle with age is a real medical problem, and switching mTOR on helps fix it. Both things can be true. What the mouse result adds is that leucine, at the dose tested, did not shorten mouse lives either. It showed no lifespan effect in either direction. That is less dramatic than either camp would like.
The Interventions Testing Program is paid for by the US National Institute on Aging. It runs each test at three separate labs at the same time, in Bar Harbor, Ann Arbor and San Antonio. It uses a mixed breed of mice, a cross of four strains, so no result can come from one odd strain. It tests males and females. Each group is big enough to spot about a 10 percent change in lifespan. Each compound is mixed into the food from a set age. And the program promises in advance to publish results that show nothing, alongside the ones that work. That promise is why this page can exist. Almost nothing else in ageing research reliably tells you what did not work.
The evidence, in charts
Fig. 1a · how many people
30people, across 1 study in people cited here
| Study | People | Share |
|---|---|---|
| American Journal of Clinical Nutrition 2009 | 30 | 100% |
The whole total is one study of 30. Counts are the number of people each study on this page says it signed up. That includes studies that only watched people and gave nobody the compound. Studies still signing people up are left out, and only studies cited on this page are counted.
Fig. 1b · evidence mix
1 of 3 sources here is a study in people. The rest are not.
- Study in people · done in people133%
- Animal study · done in animals133%
- Review of studies · sums up other studies133%
Counted from the list of sources on this page. It counts only what we cite, so there may be more studies out there. A review counts once, however many studies it sums up.
Fig. 2 · evidence over time
The studies cover 3 different years, 2009 to 2022, counted from the list of sources on this page.
Fig. 3 · legal status at a glance
US
Not approved
UK
Not approved
AU
Not approved
CA
Not approved
Not approved in any of the four countries shown. This is what the law says, not a verdict on the science. See Legal status below for the full text and any notes.
Fig. 4 · how much does what
Nobody has tested different amounts in people
This chart would show how the effect changes as the amount goes up. For this compound, nobody has measured that in people. So we leave it empty. We will not borrow an animal chart and pretend it works the same in people.
Fig. 5 · what the molecule is
- Kind of compound
- Supplement
- Molecule weight
- 131.2 Da. An amino acid.
- Half-life
- About 1.5 hoursShort, 1 to 6 hours
- Chain length
- None on file
Half-life as stated on file: Roughly 1 to 2 hours in plasma after an oral dose, which is why leucine is described as producing a pulse of signalling rather than sustained exposure.
No amino acid sequence is on file for L-leucine, which is expected: a supplement is not built from amino acids.
The studies we cite
- Study in people2009Long-term leucine supplementation does not increase muscle mass or strength in healthy elderly men
Three months of leucine did nothing at all for muscle in healthy older men. 30 men averaging 71 took 2.5 g of leucine with each main meal, 7.5 g a day, or a dummy, decided by chance. Scans of muscle size showed no change in either group. Strength on a one-lift maximum did not change either. Insulin sensitivity, long-run blood sugar and blood fats were all unmoved. These men were healthy to begin with, not frail and not already losing muscle.
30 healthy men, mean age 71, randomised to 2.5 g leucine or placebo with each main meal, 7.5 g daily for three months. No change in skeletal muscle mass measured by computed tomography and DXA, and no change in one-repetition-maximum strength, in either group. No improvement in whole-body insulin sensitivity, HbA1c or plasma lipids.
- Review of studies2015Effects of leucine-rich protein supplements on anthropometric parameter and muscle strength in the elderly: a systematic review and meta-analysis
A review that pooled 16 studies with 999 older people. Leucine added about a kilogram of body weight and of lean tissue, such as muscle, compared with people who did not take it. The gain was bigger in people already losing muscle with age. Grip strength, leg strength and blood sugar did not change in any clear way.
16 studies, 999 older participants. Against controls, leucine supplementation increased body weight by 1.02 kg (95 percent CI 0.19 to 1.85), lean body mass by 0.99 kg (95 percent CI 0.43 to 1.55) and BMI by 0.33 kg per square metre. Effects on weight and lean mass were larger in participants already affected by sarcopenia. Hand grip strength, knee extension strength and glucose metabolism markers were not significantly affected.
- Animal study2022Lifespan benefits for the combination of rapamycin plus acarbose and for captopril in genetically heterogeneous mice
In mice. This 2017 test run tried several treatments. Rapamycin plus acarbose, started at 9 months, made male mice live longer than either drug alone had before. Captopril made female mice live 4 to 5 percent longer. The authors say the other four, including leucine, gave no lifespan benefit.
Interventions Testing Program C2017 cohort, testing (R/S)-1,3-butanediol, captopril, leucine, the Nrf2-activating botanical mixture PB125, sulindac, syringaresinol, and rapamycin plus acarbose started at 9 or 16 months. The rapamycin and acarbose combination started at 9 months produced longer male lifespan than either drug alone in prior cohorts. Captopril gave a small 4 to 5 percent female increase. The authors state that none of the other four tested agents, which includes leucine, led to any lifespan benefit.
Where to buy L-leucine
1 seller lists L-leucine in our latest read. They are ranked by trust score, then by archive rating, then by certificate tier, then by the lowest price per milligram. Archive evidence is counted before source-backed. Each path is out of 4, and the two are not added. A higher price does not raise the score. The letter is the certificate chip. 0 of them have a published certificate that names L-leucine.
Some seller links may pay this site when you buy through them. A payment never changes a seller's tier or rank, and no seller has a say in any page.
| Seller | Trust score | Vial sizes | From, per mg | Certificate for L-leucine | Seller link |
|---|---|---|---|---|---|
| Boston Molecules | 28 InsufficientSource-backed 1 of 4. Archive evidence 1 of 4.C | 10 mg | $81.0 | None found for L-leucine | View |
- LC Lab-confirmed: the issuing laboratory echoed this certificate back
- A Current certificate, independent laboratory named, laboratory not yet checked
- B Certificate published, but no laboratory link, older than a year, or the laboratory did not recognise the published lookup
- C No certificate could be read
Every listing and vial size for L-leucine · How the trust score is counted
Sellers named on the price pages are ranked by certificate tier, then price, and any affiliate income is disclosed there. Naming a seller is not a recommendation to buy or use anything.
Common questions
Has L-leucine been tested in people?
Yes. 30 people given it across 1 completed trial cited here, 1 of which found nothing. L-leucine did not help mice live longer, in either sex. That was in the same test where rapamycin plus acarbose did better than either drug alone. In 30 healthy older men, 7.5 g a day for three months did nothing for muscle size or strength.
What is the L-leucine dosage?
No human dose established. Amounts that were given in studies sit in Human evidence on this page. None of them is a personal dose. 1 completed trial, 30 people given it
Is L-leucine FDA-approved?
A food nutrient the body needs but cannot make, found in all protein in food. Not an approved drug and not approved to treat anything.
Does leucine build muscle?
It reliably switches on muscle building, measured over hours. Over months, a review of 16 studies in 999 older adults found about 1 kg more lean tissue than in controls. That was mostly in people already losing muscle with age, and grip and leg strength did not improve. In 30 healthy older men, 7.5 g a day for three months did nothing.
Does leucine affect lifespan?
In mice, no. The National Institute on Aging tested it in its 2017 run and found no lifespan benefit in either sex. There is no human lifespan data.
Does leucine work against rapamycin?
In theory they push the same switch in opposite directions. Rapamycin turns mTORC1 down and makes mice live longer. Leucine is the strongest food switch that turns mTORC1 up. Nobody has run the test that would show whether that matters in real life. The mouse run that tested leucine found no lifespan effect either way.
Is leucine better than eating more protein?
Not shown. In studies, leucine's edge is that it raises the leucine in a meal without adding calories. That matters for older adults with a poor appetite. For someone already eating enough protein, the one long study in healthy older men found no benefit over three months.