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L-Leucine

Amino acidmuscleModerate dose evidence

Also known as: leucine, free-form leucine

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The branched-chain amino acid that most directly triggers muscle protein synthesis through mTOR signaling. Free leucine is added to protein powders and meal replacements to reach the roughly 2–3 g leucine threshold that maximizes the anabolic response of a meal, and it is studied in older adults for lean mass retention. On its own it does not build muscle without the other essential amino acids present.

Research summary

Activates mTORC1 signaling to stimulate muscle protein synthesis when other essential amino acids are available.

StrengthMuscle recoveryLongevity

Dosage

Daily amounts, of compound.

Lowest studiedTypical rangeHighest studiedUpper limit
2 g2.5–5 g7.5 g—

2.5–3 g with a meal or protein serving is the leucine threshold most cited; trials in older adults have used 2.5 g with each of three daily meals (7.5 g/day). Leucine added to a low-protein meal raises its anabolic effect, but the benefit is small when the meal already contains 25–40 g of quality protein.

Source material: fermentation-derived

Format suitability

  • Capsule
  • Powder
  • Tablet
  • Softgel
  • Gummy
  • Liquid

Poorly soluble and floats on water; bitter. Best embedded in a flavored protein or EAA powder rather than sold alone. 2.5 g is about 3–4 size-00 capsules.

Formulation notes

Add to plant proteins (pea, rice) to bring their leucine content up to whey levels. Count leucine from BCAA, EAA, and protein sources toward one total.

Sources

  1. The effectiveness of leucine on muscle protein synthesis, lean body mass and leg lean mass accretion in older people: a systematic review and meta-analysis (Xu 2015)PubMed
  2. Leucine — ExamineExamine

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