L-carnitine, a natural compound sold as a dietary supplement, helps cells turn fat into energy; related forms extend that reach to the brain and blood vessels. Its clearest benefit is correcting a real shortage; beyond that, gains in body weight, blood sugar, cholesterol, exercise recovery, and fatigue in older adults are modest. Gut bacteria also turn it into a byproduct some studies tie to hardening of the arteries. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Plasma free carnitine | 25–50 µmol/L | Confirms whether a true deficit exists and whether it is corrected |
| Acyl:free carnitine ratio | < 0.4 | Flags functional deficiency or impaired fatty-acid handling |
| Plasma TMAO | < 6 µmol/L (lower is better) | Central longevity safety marker; rises with supplementation |
| Fasting glucose | 70–85 mg/dL | Tracks the glucose-lowering benefit |
| HbA1c | < 5.4% | Longer-term glucose control |
| Fasting insulin / HOMA-IR | Insulin < 6 µIU/mL; HOMA-IR < 1.5 | Detects insulin-sensitivity gains |
| Lipid panel + lipoprotein(a) | LDL < 100 mg/dL; triglycerides < 90 mg/dL; Lp(a) < 30 mg/dL | Captures lipid and Lp(a) changes |
| hs-CRP | < 1.0 mg/L | Tracks the anti-inflammatory effect |
| TSH / free T4 | TSH 0.5–2.5 mIU/L; free T4 mid-range | Detects thyroid antagonism |
| eGFR / creatinine | eGFR > 90 mL/min/1.73 m² | Carnitine and TMAO clearance is renal |
Cadence: Baseline panel before starting, recheck at roughly 8–12 weeks, then every 6–12 months during continued use; INR at 1–2 weeks if on warfarin, thyroid labs at 6–8 weeks if on thyroid medication. Testing is done fasting where indicated.