L-Leucine

Supplements · Amino Acids · Supplement, Typed Supps

A building-block amino acid your body can’t make on its own, and the specific one that tells your muscles “start building” after you eat protein.

Stage by stage

Intake (Capsule or powder): Taken by mouth, often with a meal or post-workout.
Small intestine (Absorbed): Absorbed via neutral amino acid transporters alongside other amino acids.
Bloodstream (Circulates): Carried through the blood to muscle tissue throughout the body.
Skeletal muscle (mTORC1 activation): Triggers the mTORC1 pathway, the primary acute driver of muscle-protein synthesis.

What the evidence supports

Stimulating muscle-protein synthesis as the primary trigger among the amino acids — strong evidence. Leucine independently activates the mTORC1 pathway in skeletal muscle; as little as roughly 1g can raise muscle-protein synthesis after exercise, and its removal from an amino acid mixture blunts the anabolic response far more than removing any other single amino acid.
Improving muscle mass, strength, or physical function in older adults with sarcopenia when added to protein — moderate evidence. Systematic reviews of leucine-enriched protein supplementation in sarcopenic older adults show improved muscle strength and some lean-mass measures, though effect sizes are modest and usually studied alongside resistance training or extra total protein, making it hard to isolate leucine's standalone contribution.
Providing extra benefit over adequate whole dietary protein in healthy, already protein-replete adults — insufficient evidence. There is no consistent evidence that isolated leucine supplementation outperforms eating enough complete protein in people who already meet their protein needs — a normal high-protein meal already supplies more leucine than most single supplemental doses.

Typical dose

2–5g per dose, often 1–3 times daily around meals or training

Upper limit: No formal UL; dosing studies identified rising blood-ammonia levels above roughly 500mg/kg/day (about 35g/day for a 75kg adult)

Timing

With or after a protein-containing meal or workout, when the anabolic signal matters most.

Time to effect

1–2 hours: Muscle-protein-synthesis signalling rises acutely, per lab studies.
Weeks: Measurable strength or lean-mass changes require weeks of consistent resistance training and adequate total protein.

Common myth

“Leucine supplements build more muscle than eating protein.” In reality: Leucine amplifies the anabolic signal already present in a complete protein meal — it does not replace the need for adequate total dietary protein, and most of its measurable benefit shows up in people who are under-eating protein, not those already meeting their needs.