🔑 Core Function

• Muscle protein synthesis: Activates mTOR pathway, signalling muscles to build protein.
• Exercise recovery: Supports repair, growth, and strength after training.
• ⚡ Energy source: Oxidised in muscle during exercise and fasting.
• Metabolic signalling: Helps regulate blood sugar and energy balance.

🧬 Family & Essentiality

🌿 Plain-Language Summary

Leucine is a key amino acid for building muscle. It not only provides raw material for protein but also acts as a “switch” that tells the body to start muscle repair and growth after exercise. It also helps regulate energy and blood sugar.

⚗️ Molecular & Chemical IDs

• Abbreviation: Leu / L
• Chemical formula: C₆H₁₃NO₂
• Structure: Branched-chain, hydrophobic side chain

⚡ Functions

• ️ Muscle builder: Triggers mTOR pathway → initiates protein synthesis.
• ⚡ Exercise fuel: Broken down in muscle to provide ATP.
• Glucose regulation: Aids glucose uptake and insulin signalling.
• Neurotransmitter precursor: Minor role in brain metabolism.

🍽️ Absorption & Bioavailability

🇦🇺 Australian NRVs (NHMRC)

• Adults (≥19 yrs) EAR: ~24 mg/kg/day
• RDI: ~39 mg/kg/day

Values based on NHMRC/FAO/WHO amino-acid requirement data, expressed per kilogram body weight.

🥗 Food Sources

• Meat, poultry, fish
• Dairy (milk, yoghurt, cheese, whey protein)
• Eggs
• Legumes, soy, nuts, whole grains

Whey protein is especially rich in leucine, often highlighted in sports nutrition.

🧪 Testing & Monitoring

• Plasma amino acid profiling (specialist contexts).
• ❌ Not a standard clinical test in Australia.

🔄 Interactions

• Must stay balanced with isoleucine & valine (BCAAs).
• ⚠️ Inborn error: Maple Syrup Urine Disease → impaired BCAA breakdown.
• Requires vitamin B6 (pyridoxal phosphate) as enzyme cofactor for metabolism.

🚨 Deficiency & Evidence

Deficiency

Rare outside severe protein–energy malnutrition or inborn metabolic disorders (e.g., Maple Syrup Urine Disease).

Evidence Snapshot

• ✅ Established: Strong activator of muscle protein synthesis; key in sports recovery.
• ⚖️ Mixed: Supplements may help athletes and older adults with low protein intake; less benefit if total protein intake is already adequate.