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.

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

Proven Benefits

01Improves muscle protein synthesis

02May preserve lean mass

03May improve physical function

04May reduce muscle loss in bed rest

05May lower post-meal glucose

Chemical Forms

Recommended

L-Leucine (free-form powder or capsules)

Avoid

Proprietary BCAA blends with <2 g leucine per serving (often underdose the active trigger)

Expert Note

Leucine itself is the active amino acid, so form matters less than dose transparency. Free-form L-leucine makes it easier to reach the studied ~2-3 g per meal, while many BCAA blends split the dose across leucine, isoleucine, and valine and miss the leucine threshold.

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.

Condition-Based Dosing

Adults 50+ with low-protein meals

2.5-3 g with breakfast and/or lunch— Most useful when a meal does not already contain enough high-quality protein to reach the leucine threshold.

Calorie deficit or reduced appetite

2-3 g with the lowest-protein meal of the day— Use as a top-up, not a replacement for adequate daily protein intake.

Plant-based meals lower in leucine density

2-3 g with mixed plant-protein meals— Most relevant when meals rely on smaller portions of grains, legumes, or soy foods.

Safety & Limits

Upper Safe Limit

No official UL established; 5 g/day is a practical ceiling for unsupervised use because long-term safety above that is not well defined.

Cycling

Safe for continuous use

Contraindications

Maple syrup urine disease — impaired branched-chain amino acid metabolism

Advanced kidney disease or medically prescribed low-protein diet — extra amino acid load may be inappropriate

Levodopa — large neutral amino acids can reduce absorption when taken together

Pregnancy or breastfeeding — high-dose supplemental safety beyond normal dietary intake is not well established

Diabetes medications or insulin — leucine can modestly affect post-meal insulin and glucose responses

Synergies

Whey protein

Leucine turns on mTOR, while whey supplies the rest of the essential amino acids needed to sustain net muscle protein synthesis.

Essential amino acids

Leucine is the trigger signal, but the other essential amino acids provide the building blocks for actual tissue repair and growth.

Avoid Combining With

✕Taking it without enough total protein (briefly raises the signal, but building blocks run short)
✕Heavy alcohol intake around meals (suppresses muscle protein synthesis)
✕Under-dosed BCAA blends (may provide too little leucine to reach an effective meal threshold)

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