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Leucine

Leucine is a branced chain amino acid (BCAA) and potent nutrient- based signal to activate protein synthesis.

Our evidence-based analysis on leucine features 106 unique references to scientific papers.

Research analysis led by and reviewed by the Examine team.
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Summary of Leucine

Primary information, health benefits, side effects, usage, and other important details

Leucine, along with isoleucine and valine, is one of the three branched chain amino acids (BCAAs). Out of all of the other amino acids, leucine is the most potent activator of protein synthesis. Cells are able to sense leucine levels, and in response turn on protein synthesis via the enzyme protein mammalian target of rapamycin (mTOR), a master-regulator of protein synthesis.

Given the well-established protein-synthesis activating properties of leucine, a number of human trials have been conducted to determine whether adding supplemental leucine to various protein sources can augment muscle protein synthesis (MPS).

Results in elderly individuals have been consistently positive. Anabolic resistance is a well-established phenomenon during aging, with cells becoming more resistant to turning on protein synthesis. This can be partly ameliorated with extra leucine, as research indicates that older adults need twice as much leucine compared to younger adults for similar activation of muscle protein synthesis (MPS).[1]

This increased leucine requirement for MPS in the elderly can be partially explained by increased retention of orally ingested leucine in the gut, which is retained twice as much compared to young adults,[2] limiting the amount of leucine that makes it into the blood stream. Resistance to leucine-stimulated protein synthesis also occurs at the cellular level in aging cells.

Studies on leucine supplementation in younger adults have been positive, with some caveats. In a study that compared a low (6 g) dose of whey protein alongside 3 or 5 g of extra leucine to 25 g whey protein alone, the 6 g whey plus 5 g leucine meal stimulated MPS to a similar, but lesser extent than 25 g whey protein.[3][4][5][6] Another study comparing the same 25 g dose of whey protein to 6 g whey with an amount of leucine equivalent to 25 g of whey found that only the 25 g whey protein group had elevated MPS 3 to 5 hours post-exercise in young adults. Taken together this works suggests that supplemental leucine can augment MPS in the context of limited total protein, but a 25 dose of whey alone is likely better, at least in non-elderly adults.[7]

Research suggests that supplemental leucine in older adults can augment muscle protein synthesis. For non-elderly adults, leucine can increase MPS when overall protein intake is limited. Additional leucine will not likely increase MPS in non-elderly adults beyond that obtained with moderate amounts (20+ g ) of whey protein.

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How to Take

Recommended dosage, active amounts, other details

Leucine tends to be supplemented in the 2,000-5,000mg range for acute usage.

It tends to be taken either in a fasted state or alongside meals with an inhernetly low protein content (or protein sources that are low in leucine).

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Human Effect Matrix

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The Human Effect Matrix summarizes human studies to tell you what effects Leucine has on your body, how much evidence there is, and how strong these effects are.

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Grade Level of Evidence
Robust research conducted with repeated double-blind clinical trials
Multiple studies where at least two are double-blind and placebo controlled
Single double-blind study or multiple cohort studies
Uncontrolled or observational studies only
Level of Evidence
? The amount of high quality evidence. The more evidence, the more we can trust the results.
Outcome Magnitude of effect
? The direction and size of the supplement's impact on each outcome. Some supplements can have an increasing effect, others have a decreasing effect, and others have no effect.
Consistency of research results
? Scientific research does not always agree. HIGH or VERY HIGH means that most of the scientific research agrees.
Notes
grade-c - - See study
grade-c - - See study
grade-c - - See study
grade-c - - See study
grade-c - - See study

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Study Deep Dives

Things to Note

Primary Function:

Also Known As

L-Leucine

Do Not Confuse With

BCAAs, Leucic acid (a metabolite)

Goes Well With

  • Leucine is non-stimulatory

  • Leucine has a bitter taste to the powder, which can be attenuated by either reducing the temperature or by adding sour flavoring

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Click here to see all 106 references.