🔑 Core Function

• Methylation donor: Provides methyl groups via S-adenosylmethionine (SAMe) for DNA, neurotransmitters, and phospholipids.
• ️ Antioxidant support: Precursor to cysteine → glutathione (the body’s master antioxidant).
• Collagen & connective tissue: Contributes sulphur for cartilage, skin, and nails.
• ⚡ Metabolic role: Initiator amino acid in protein synthesis (start codon = AUG).

🧬 Family & Essentiality

🌿 Plain-Language Summary

Methionine is like a molecular multitool. It starts every new protein your cells build, fuels key methylation reactions for DNA and neurotransmitters, and supplies sulphur to make glutathione, the body’s master antioxidant.

⚗️ Molecular & Chemical IDs

• Abbreviation: Met / M
• Chemical formula: C₅H₁₁NO₂S
• Structure: Sulphur-containing, non-polar side chain

⚡ Functions

• Methyl donor: Via SAMe, supports DNA methylation, neurotransmitters, phospholipids.
• ️ Antioxidant: Provides cysteine for glutathione production.
• Structural: Supplies sulphur for hair, skin, nails.
• Protein synthesis: Universal initiator amino acid in translation (start codon AUG).

🍽️ Absorption & Bioavailability

🇦🇺 Australian NRVs (NHMRC)

• Adults (≥19 yrs) EAR: ~10 mg/kg/day (Met + Cys)
• RDI: ~15 mg/kg/day (Met + Cys)

Values based on NHMRC/FAO/WHO amino-acid requirement data, expressed as total sulphur amino acids (methionine + cysteine).

🥗 Food Sources

• Meat, poultry, fish
• Dairy (milk, cheese, yoghurt)
• Eggs
• Nuts, seeds, whole grains, legumes (lower amounts than animal foods)

Tip: Combining legumes + grains improves Met/Cys adequacy in plant-based diets.

🧪 Testing & Monitoring

• Plasma amino acid profiling (specialist use).
• ❌ Not a standard GP pathology test in Australia.

🔄 Interactions

• ➕ Works with folate, vitamin B12, and vitamin B6 in methylation cycles (homocysteine metabolism).
• ⚠️ Low B-vitamin status → impaired methionine metabolism → homocysteine accumulation (vascular risk marker).
• ➕ Partnered with cysteine to meet sulphur amino acid requirements.

🚨 Deficiency & Evidence

Deficiency

Uncommon with adequate protein intake; low levels may impact methylation, glutathione production, and connective tissue health.

Evidence Snapshot

• ✅ Established: Crucial for protein initiation, methylation reactions, and glutathione production.
• ⚖️ Contextual: Excess intake without adequate B-vitamin support may raise homocysteine; balanced diets mitigate this.