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Leucine

Leucine is the primary BCAA, and is the BCAA where most benefit is given to. Supplementing Leucine on its own is still beneficial and may be cheaper than BCAA mixes; they all still taste bitter, however.

Our evidence-based analysis on leucine features 94 unique references to scientific papers.

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Summary of Leucine

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

Leucine is one of the three branched chain amino acids and sometimes referred to as the 'main' amino acid due to the most popular benefit of BCAAs (muscle building) being mostly due to leucine. Leucine is an activator of the protein known as mTOR, which then induces muscle protein synthesis via S6K; the other two BCAAs may also activate mTOR, but are much weaker than leucine in doing so (and as such, 5g of leucine will be more effective than 5g mixed BCAAs). The leucine metabolite, HMB, is also weaker than leucine at inducing muscle protein synthesis despite being more effective at preserving lean mass from breakdown.

Leucine is a tad different from the other two BCAAs isoleucine and valine as leucine seems to have a fair bit of testing on the amino acid in isolation rather than in a BCAA mixture, whereas the other two BCAAs are not as well studied.

The studies assessing leucine mostly look at muscle protein synthesis when additional leucine is added to the diet or to a test meal, and it appears that leucine is able to reliably increase muscle protein synthesis after test meals. Whether this results in more lean mass over a period of time is somewhat less reliable though, and leucine appears to be more effective at promoting gains in muscle in people with lower dietary protein intake and in the elderly (who tend to have impaired muscle protein synthesis in response to the diet).

The interactions of leucine on glucose are not clear, to be honest. Leucine possesses both blood sugar reducing properties (can release insulin from the pancreas, can directly stimulate glucose uptake into a cell without insulin) but also the opposite (via stimulating S6K, it can inhibit insulin-stimulated glucose uptake). In a cell culture, leucine stimulates glucose uptake for up to 45 minutes and then hinders itself while in living systems acute doses of leucine do not appear to do anything remarkable (some limited evidence that leucine can be rehabilitative in diabetes, but this is preliminary). Isoleucine is a more potent hypoglycemic agent, but to less inhibition of its own actions.

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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

Unlocked for Examine members

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.

Full details are available to Examine members.
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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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 94 references.