L-Leucine

Leucine

Snapshot

Indications	Contraindications
Sarcopenia with combined nutrients¹ Appendicular muscle mass² Gait speed with vitamin D³ Lean mass in older adults⁴ Muscle strength and endurance⁵	Maple syrup urine disease⁵

Leucine is an essential branched-chain amino acid that the body cannot make, obtained from dietary protein and sold as a stand-alone supplement⁵. It drives muscle protein synthesis through the mechanistic target of rapamycin (mTOR) pathway⁵. Supplementation follows from that effect: age-related muscle loss (sarcopenia) in older adults, muscle strength and endurance in athletes, and glycemic control, where a trial in older men with type 2 diabetes found no improvement⁵.

Evidence for stand-alone leucine is weak.³ A meta-analysis of randomized trials in older adults found no effect on lean mass or handgrip, while leucine combined with other nutrients including vitamin D improved handgrip and gait speed³. A systematic review in sarcopenia reached the same split¹, and another meta-analysis rated the evidence low quality⁶. A 2026 randomized proof-of-concept trial found isolated leucine did not improve muscle mass or function in head and neck cancer, though older adults without cancer gained modestly⁷. Products listing L-leucine number about 11,800, mostly powders and capsules; see Form comparison.

Maple syrup urine disease is the firm exclusion, because leucine accumulates and impairs nerve cells⁵. Health Canada labels ask for practitioner advice before use in pregnancy or breastfeeding⁸. US product labels ask for practitioner consultation in kidney or liver disease⁹.

Last updated October 16, 2026. Read how we make monographs.

In the full monograph

59 sources, 45 studies

When to useThe situations practitioners use it for, each with the reason and the study behind it.
SafetyWho should avoid it, who needs extra care, and the side effects reported in studies.
InteractionsThe medicines and supplements it interacts with, the effect of each, and what labeling says to watch.
Protocol notesThe doses used in studies, how and when they were taken, and how the forms on the market compare.
How it worksWhat it is and how it works in the body, in plain terms.
Published studiesThe studies behind each use, popular claims checked against them, and trials still under way.
Word of mouthWhat patients and practitioners report online, kept separate from the clinical evidence.
Clinical perspectiveA closing read of where the evidence is solid, where it is thin, and what has not been studied.

Snapshot

In the full monograph

Sources