L-carnitine, a nutrient that carries fat into cellular energy factories, is a dietary supplement taken for metabolic fitness and mental sharpness. Gains are modest: small weight and fat loss, slightly better blood sugar and cholesterol, less muscle soreness, and, with a brain-reaching form, fewer depressive symptoms and less nerve pain. Stomach upset is common at higher doses; that form worsened nerve damage during certain chemotherapy; long-term artery safety is unknown. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Plasma free carnitine | Upper half of reference range; acyl-to-free ratio <0.4 | Confirms status and absorption |
| TMAO (fasting) | <6 µM | Tracks gut conversion of carnitine |
| LDL cholesterol | <100 mg/dL (<70 mg/dL with higher cardiovascular risk) | Lipid response |
| Lp(a) | <30 mg/dL (<75 nmol/L) | Lp(a) response |
| HbA1c | <5.4% | Glucose response |
| Fasting glucose | 75–90 mg/dL | Glucose response and low-sugar safety |
| hs-CRP | <1.0 mg/L | Inflammation response |
| TSH (with free T3 and T4) | 1.0–2.5 mIU/L | Detects blunted thyroid action |
| INR | Individual therapeutic target (commonly 2.0–3.0) | Detects anticoagulant potentiation |
| eGFR | >90 mL/min/1.73 m² | Confirms clearance before higher doses |
| Body composition | No established target; change from own baseline | Weight and fat-mass response |
Cadence: Baseline before starting; typically repeated at 8–12 weeks, then every 6–12 months while supplementation continues; for warfarin users, an INR check within 1–2 weeks of any change is standard