L-Leucine

●●●Strong Evidence

The most anabolically active amino acid. A 'leucine threshold' (~2g per meal) must be reached to trigger maximal MPS — useful for boosting low-protein meals.

Effective Dose

2–5g / serving

per clinical evidence

Evidence Level

Strong

Protein & Amino Acids

Mechanism

mTORC1 activation — primary MPS trigger

primary action

Best For

MPS enhancement

Protein quality augmentation, Plant protein users

This profile is for informational purposes only and does not constitute medical advice. Consult a qualified healthcare professional before starting any supplementation, especially if you have pre-existing conditions or take medications.

What Is L-Leucine?

Leucine is one of the three BCAAs and the primary trigger for mTORC1-mediated muscle protein synthesis. Every protein source is evaluated partly by its leucine content. The leucine threshold concept — that approximately 2–3g of leucine per meal is needed for maximum MPS stimulation — has practical implications for meal planning and supplementation.

How It Works: The Science

Leucine is sensed by the Sestrin2 protein, which gates the Ragulator–Rag GTPase complex that recruits mTORC1 to the lysosomal surface. There it is activated by Rheb-GTP, phosphorylating S6K1 and 4E-BP1 to initiate ribosomal mRNA translation for muscle proteins. This is a post-meal signal — leucine must be accompanied by other EAAs for sustained MPS.

Primary Mechanism

mTORC1 activation — primary MPS trigger

Evidence-Based Benefits

Maximises post-meal MPS when added to a protein-light meal

strong

Norton & Layman (2006) — leucine addition to a suboptimal meal restores MPS rates

Enhances MPS stimulation of plant proteins

moderate

Adding leucine to soy or wheat protein brings MPS rates closer to whey

Dosage Guide

Effective Dose

2–5g / serving

2–5g per serving, taken with meals that are otherwise low in high-quality protein. Standalone leucine is less useful if meals already exceed the leucine threshold (~3g leucine per meal from food). Usually provides maximum benefit added to plant-based meals.

Safety Profile & Side Effects

Safe at recommended doses. Very high doses (>20g/day) may impair absorption of other BCAAs and raise blood glucose. Not typically relevant at supplement doses.

Who Should (and Shouldn't) Take It

Best for

MPS enhancementProtein quality augmentationPlant protein users

Who it's for

Plant-based athletes, those eating frequent small meals with modest protein, elderly individuals with reduced anabolic sensitivity, and anyone trying to boost the quality of lower-protein meals.

Frequently Asked Questions

What is the recommended dose of L-Leucine?

Is L-Leucine safe?

Safe at recommended doses. Very high doses (>20g/day) may impair absorption of other BCAAs and raise blood glucose. Not typically relevant at supplement doses.

How does L-Leucine work?

Who should take L-Leucine?

Plant-based athletes, those eating frequent small meals with modest protein, elderly individuals with reduced anabolic sensitivity, and anyone trying to boost the quality of lower-protein meals.

Related Ingredients

EAAs (Essential Amino Acids)BCAAs (Leucine / Isoleucine / Valine)Whey Protein Isolate HMB (β-Hydroxy β-Methylbutyrate)

Medical Disclaimer

Ingredient profiles are for informational purposes only and do not constitute medical advice. Consult a qualified healthcare professional before starting any supplementation, especially if you have pre-existing conditions or take medications. Full disclaimer →