Know2eat

L-leucine

Amino Acids & Proteins

What l-leucine may help with

Ranked by how strong the human evidence is — best supported first.

Best supportedMuscle protein synthesisLimited evidence· 2 sources

Then, in order

About L-leucine

An essential branched-chain amino acid and the principal amino acid trigger for muscle protein synthesis. Present at roughly 8–11% of the amino acids in most complete proteins.

Leucine is the signal molecule of muscle protein synthesis, and the threshold concept built around it — roughly 2–3 g of leucine per meal to maximally trigger the response — is genuinely useful for planning protein distribution. What has not been demonstrated is that adding free leucine on top of adequate protein does anything. Trials adding leucine to meals in older adults have produced mixed results, and adding it to an already sufficient protein dose does not increase the response further, because the signal was already saturated and the limiting factor becomes the other amino acids.

Sold as
Powder, Capsule, Component of BCAA and EAA blends
Standardised to
Grams of free L-leucine — a product without this marker may not resemble what was studied.
Also known as
Leucine, L-leucine

Regulation: Sold as a dietary supplement with no pre-market efficacy review. Not prohibited in sport.

What the research used

The amounts, forms and durations published trials of L-leucine actually administered.

These are the amounts published trials administered — not a recommendation, and not a dose to copy. Trial participants were screened, supervised and given a characterised preparation. What is in a retail bottle is frequently not what was studied, and the amount that suits one person is not general advice.

  • Studied for The leucine threshold per meal

    2–3 g

    Moderate

    Leucine from food or supplement, per protein feeding

    The amount associated with maximal stimulation of muscle protein synthesis from a single feeding, and higher in older adults who show anabolic resistance. It is usually reached by eating 25–30 g of a complete protein rather than by supplementing.

  • Studied for Supplementation on top of meals in older adults

    2–7.5 g

    Limited

    Free leucine added to meals daily · 3–6 months

    Trials adding leucine to the diet of older adults report inconsistent effects on lean mass and strength, with several finding nothing when protein intake was already adequate.

This is general information about food, not medical advice. It cannot diagnose, treat or replace guidance from a clinician who knows your history.

When it was taken

Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.

Each timing below was actually compared against an alternative in a trial, which is unusual.

  • Split through the day

    Moderate

    The threshold concept is inherently about distribution: reaching it at three or four feedings produces a greater daily synthetic response than reaching it once. That distribution effect has been measured with tracer methods, though it is a finding about protein rather than about free leucine.

    Timing was tested. A trial compared this window against another and reported a difference.

The evidence in depth

Best supported: Limited

What each association actually rests on, and the papers behind it. A rating describes the state of the research, not how promising the supplement sounds. How we rate evidence.

Muscle protein synthesis

Muscle & Physical Performance
Limited evidence

Leucine is the amino acid that triggers muscle protein synthesis, and reaching a threshold amount per meal matters. Adding free leucine on top of a protein intake that already crosses that threshold has not been shown to increase muscle mass or strength in trials.

What may be going on: Leucine activates mTORC1 through sestrin2 and related sensors, initiating translation. The signal saturates, which is why more leucine beyond the threshold adds nothing.

Limited evidence. Early human work, or mostly lab and animal research.

View the 2 sources

Deep research mode adds each paper’s own conclusion and stated limitations.

Exercise recovery

Muscle & Physical Performance
Not enough evidence

There is not enough evidence that free leucine improves recovery beyond the effect of adequate protein. Most supporting data come from blends where leucine cannot be isolated.

Not enough evidence. Not enough human research to say anything useful yet.

No source is attached to this association yet, so it is rated not enough evidenceby default. It is listed because people commonly ask about it — not because there is evidence for it.

Things to know

Interactions, contraindications and dose limits, most serious first. Concentrated extracts interact with medication in ways the whole plant does not, so this section carries more weight here than it does on a food page.

  • Worth knowing

    Adds nothing on top of adequate protein

    The synthetic signal saturates. Once a meal supplies 2–3 g of leucine from food, adding free leucine has no further measured effect, and the limiting factor becomes the other essential amino acids.

  • Worth knowing

    Amino acid competition and ammonia

    Large free leucine doses lower plasma isoleucine and valine and increase ammonia production. Very high intakes have been reported to cause nausea and, in tolerance studies, an upper limit for chronic intake has been proposed.

  • Avoid

    Maple syrup urine disease

    Contraindicated in this inherited disorder of branched-chain amino acid metabolism.

Evidence ratings on this page: 1 limited evidence, 1 not enough evidence.

Supplements are sold in the US without pre-market review of efficacy or safety. Nothing on this page is a recommendation to take L-leucine, or a dose to follow.