Performance/Weight/Mobility

L-Leucine

Essential branched-chain amino acid that can raise meal-triggered muscle protein synthesis, mainly for older adults or low-protein eaters.

score

evidence

Safe

risk

Our methodology: How we evaluate supplements and turn the underlying research into a single rating.

Read full methodology

Overview

Forms

Safety

Stack

Quick Take

Useful if your meals are low in protein or you are older; low payoff if you already eat enough high-quality protein.

L-leucine is an essential branched-chain amino acid found in whey, dairy, meat, eggs, and soy. It acts as a nutrient signal for mTORC1, helping switch on muscle protein synthesis, but it cannot sustain tissue building without the other essential amino acids. Best-supported uses are improving the muscle-building response of low-protein meals and modestly helping lean-mass retention in older adults or during low intake. It is most relevant for adults with low protein intake, anabolic resistance, or reduced appetite.

Potential benefits

Improves muscle protein synthesis

May preserve lean mass

May improve physical function

May reduce muscle loss in bed rest

May lower post-meal glucose

Protocol

Amount

2-3 g

Frequency

Once or twice daily with low-protein meals

When

With meals, especially breakfasts or other meals that do not already provide ~25-30 g of high-quality protein; leucine works better with protein than alone.

Onset Time

1-3 hours for muscle protein synthesis; 8-12 weeks for body-composition changes

Who Should Consider

Adults 50+ with low protein intake

People eating low-protein breakfasts or lunches

Vegetarians or vegans with lower-leucine meals

Adults in a calorie deficit trying to keep lean mass

People with reduced appetite who miss protein targets

Food Sources

Whey protein powder (~2.5-3 g per 25 g protein)
Chicken breast (~1.7 g per 100 g cooked)
Lean beef (~1.6-1.8 g per 100 g cooked)
Eggs (~0.5-0.6 g each)
Greek yogurt (~1.2-1.5 g per 200 g)
Firm tofu or tempeh (~1.2-1.6 g per serving)

How It Works

Leucine activates mTORC1 and related signaling in skeletal muscle, which increases the rate of muscle protein synthesis after a meal. It also modestly stimulates insulin release, but sustained net muscle gain still requires enough total protein and the other essential amino acids.

Updated 5/19/2026