L-Cysteine is a sulfur-containing amino acid sold as a supplement to build the body's main internal antioxidant and to neutralize the toxic by-product of alcohol and tobacco smoke. The chemical role is better supported: a slow-release lozenge measurably strips that by-product from saliva and improved the odds of giving up smoking. The antioxidant role is more contested, and people with more cysteine tend to carry more body fat. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Plasma total cysteine | 200–260 µmol/L | Total sulfur-amino-acid exposure; the marker linked to fat mass |
| Plasma total homocysteine | Below 8 µmol/L | Shows whether the pathway that makes cysteine is flowing |
| Whole-blood glutathione, reduced-to-oxidized ratio | No established target; track change from own baseline, looking for a rising ratio | Confirms cysteine is reaching its main destination |
| Fasting insulin and HOMA-IR | Insulin below 5 µIU/mL; HOMA-IR below 1.5 | Insulin resistance is the route through which the adiposity signal runs |
| Hemoglobin A1c | 4.8–5.4% | Confirms the glucose picture over three months |
| Waist circumference | Below 94 cm (men), below 80 cm (women) | Body fat is the outcome most consistently associated with high cysteine |
| Serum bicarbonate (carbon dioxide) | 24–28 mmol/L | Detects the acidifying effect of the hydrochloride salt |
| 24-hour urinary cystine | Below 100 mg per day | Identifies cystinuria before it produces a stone |
Cadence: Baseline panel before any daily regimen; metabolic markers repeated at 12 weeks, then every 6 months, with waist circumference every 3 months. Deliberately sparse for event-driven use.