About Cystine
Cystine is the oxidized dimer of cysteine and a rate-limiting precursor for glutathione synthesis; it is present in dietary protein. Most clinical data are for a fixed cystine+theanine combination (700 mg cystine + 280 mg theanine/day), where small Japanese RCTs reported fewer common colds and blunted exercise-induced immune suppression, plausibly via enhanced glutathione and antibody responses. Evidence for cystine ALONE is minimal, and the combo trials are small and mostly single-group. Distinct from N-acetylcysteine (NAC) and free cysteine, which have their own entries. Grade low.
What Cystine supports
- Precursor for glutathione, the body's main antioxidant (mechanistic)
- Cystine+theanine combo linked to fewer colds in small trials
How much Cystine to take
The RDA prevents deficiency. The effective range is what clinical trials used to actually move the outcome.
Effective
250–700
mg
No established standalone therapeutic dose; nominal range for a blend component. Cystine+theanine immune trials used 700 mg/day L-cystine (with 280 mg theanine).
Clinical evidence
Limited clinical evidence. Glutathione precursor; modest immune data only for cystine+theanine combo, not cystine alone.
NIH Fact Sheet