About Cysteine
L-Cysteine is a semi-essential amino acid and the rate-limiting substrate for glutathione synthesis. It is distinct from NAC (N-Acetyl-L-Cysteine), which is the acetylated, more stable form preferred in supplements and clinical studies. Free-form L-Cysteine is less commonly supplemented due to rapid oxidation and instability, but appears in some amino acid blends and hair/nail formulas. Most clinical evidence for cysteine benefits comes from NAC studies rather than free-form cysteine. Contains a thiol group critical for keratin cross-linking in hair and nails.
What Cysteine supports
- Precursor to glutathione, the body's master antioxidant
- Provides sulfur for keratin in hair and nails
How much Cysteine to take
The RDA prevents deficiency. The effective range is what clinical trials used to actually move the outcome.
Effective
600–1800
mg
Limited RCT data for free-form L-Cysteine specifically. Doses extrapolated from NAC studies (600–1800 mg/day) where cysteine is the active moiety.
Clinical evidence
Moderate clinical evidence. Most clinical evidence comes from NAC studies rather than free-form cysteine; limited standalone data
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