🔑 Core Function

• Antioxidant defence: Provides sulphur for glutathione synthesis, the body’s “master antioxidant.”
• Structural protein: Key in keratin → supports hair, skin, and nail strength.
• Detoxification: Important in liver Phase II conjugation of toxins.
• Protein stability: Forms disulphide bonds (–S–S–) that stabilise enzymes, antibodies, and structural proteins.

🧬 Family & Essentiality

🌿 Plain-Language Summary

Cysteine is the “sulphur specialist”. It helps the body build strong hair and skin, detoxify harmful compounds, and make glutathione — a critical antioxidant for cellular health. Normally, the body makes enough, but in stress or illness, extra from food may be needed.

⚗️ Molecular & Chemical IDs

• Abbreviation: Cys / C
• Chemical formula: C₃H₇NO₂S
• Structure: Polar, sulphur-containing side chain → forms disulphide bonds

⚡ Functions

• Glutathione synthesis: Combines with glycine and glutamate to form glutathione.
• Keratin formation: Provides strength to hair, skin, and nails.
• Detox support: Needed in liver Phase II conjugation pathways.
• Protein stability: Disulphide bonds maintain shape of enzymes and structural proteins.

🍽️ Absorption & Bioavailability

🇦🇺 Australian NRVs (NHMRC)

• EAR: ~10 mg/kg/day (methionine + cysteine combined)
• RDI: ~15 mg/kg/day (methionine + cysteine combined)
• Source: NHMRC / FAO / WHO requirement values

🥗 Food Sources

• Meat, poultry, fish
• Dairy products
• Eggs (especially yolk)
• Legumes, nuts, seeds
• Garlic, onions, cruciferous vegetables

Sulphur-rich plant foods (garlic, onions, broccoli) boost cysteine status.

🧪 Testing & Monitoring

• Plasma amino acid profiling (specialist/research labs).
• ❌ Not part of routine GP pathology testing in Australia.

🔄 Interactions

• ➕ Works with selenium and riboflavin in glutathione recycling.
• ➕ Vitamin C maintains cysteine and glutathione in reduced form.
• ⚠️ NAC supplements may interact with medicines (e.g., nitroglycerin).

📊 Evidence Snapshot

• ✅ Established: Critical for glutathione, detoxification, keratin, and protein stability.
• ⚖️ Contextual: NAC is clinically used in respiratory illness & paracetamol overdose; general wellness supplementation evidence remains mixed.