L-Carnitine
A naturally occurring amino-acid derivative essential for transporting long-chain fatty acids into mitochondria for beta-oxidation. Marketed as a fat-loss and endurance supplement; genuine clinical benefit is limited to deficiency states and modest recovery effects, with the fat-burning claims largely unsupported at oral doses.
01 Overview
L-carnitine is synthesised endogenously and obtained mainly from red meat. Its core physiological role is shuttling long-chain fatty acids across the inner mitochondrial membrane via the carnitine palmitoyltransferase system, making it central to fat metabolism. Supplementation clearly benefits primary and secondary carnitine deficiency and some patients on haemodialysis.
In healthy athletes the evidence is weaker: oral bioavailability is low and muscle carnitine content is hard to raise without co-ingested carbohydrate and insulin. Some studies suggest reduced exercise-induced muscle damage and improved recovery, but consistent fat-loss or performance benefits are not established. A separate concern is that chronic high intake raises TMAO, a metabolite linked in observational data to cardiovascular risk.
02 Mechanism
Serves as the obligatory carrier for long-chain fatty acids across the inner mitochondrial membrane (via CPT-1/CPT-2), enabling beta-oxidation; also buffers the mitochondrial acyl-CoA/CoA ratio.
03 Dosing
| Tier | Dose | Route | Notes |
|---|---|---|---|
| Supplement | 1–3 g/day | Oral | Typical supplemental range, often with carbohydrate to aid muscle uptake. |
| Dialysis/deficiency | 10–20 mg/kg | IV | Clinical IV dosing for deficiency states. |
04 Effects
| Effect | Magnitude | Evidence | |
|---|---|---|---|
| Corrects carnitine deficiencyClearly effective in primary and secondary carnitine deficiency. | Clinical | ||
| Reduced exercise-induced muscle damageSome trials show lower markers of muscle damage and improved recovery. | modest | Observational | |
| Fat loss / performancePopular fat-burner claims are largely unsupported at practical oral doses. | Disputed |
05 Side effects
| Effect | Severity | Frequency | Evidence | Countermeasures |
|---|---|---|---|---|
| Fishy body odourTrimethylamine metabolites cause a fishy odour at higher doses. | Mild | Dose-related | Clinical | |
| Gastrointestinal upsetNausea, cramps and diarrhoea at high oral doses. | Mild | Common | Clinical | |
| Raised TMAO / cardiovascular concernChronic high intake raises trimethylamine-N-oxide, associated with atherosclerosis in observational studies. | Moderate | Uncertain | Observational |