L-cysteine, a sulfur-containing amino acid in protein-rich foods, is sold as an inexpensive supplement to build the body's main antioxidant and bind a cancer-linked by-product of alcohol and tobacco smoke. It appears well tolerated at studied doses, but human evidence is thin, often maker-sponsored: modest help quitting smoking, relief in a low-acid stomach condition. Higher blood cysteine tracks with body fat and diabetes; whether supplements carry this risk is unknown. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Fasting plasma glucose | 70–99 mg/dL | Safety check: a rise prompts stopping |
| HbA1c | 4.0–5.6% | Safety check: a rise prompts stopping |
| Serum creatinine and eGFR | Creatinine about 0.6–1.3 mg/dL | Safety check: reduced function stops use |
| Ferritin | About 20–300 ng/mL | Safety check: high values change timing or stop use |
| Urinalysis for cystine crystals | Negative | Safety check: crystals stop use |
| Plasma total cysteine | No established target; track change from own baseline | Expected to change with supplementation |
| Whole-blood or red-cell glutathione | No established target; track change from own baseline | Expected to change with supplementation |
| Body weight and waist circumference | No upward drift | Early warning for unwanted fat gain |
Cadence: Baseline panel before starting; weight and waist monthly for the first 3 months, then every 3 months; fasting glucose, HbA1c, creatinine and ferritin at 8–12 weeks, then every 6–12 months while use continues; specialty cysteine or glutathione tests, if used, repeated once at 8–12 weeks