karnitine

Karnitine, commonly spelled carnitine, is a naturally occurring quaternary ammonium compound that exists mainly as the L-enantiomer in humans. D-carnitine is largely inactive and may interfere with L-carnitine uptake. Carnitine occurs in two main forms: free carnitine and acylcarnitines formed when carnitine binds acyl groups.

Its key role is to transport long-chain fatty acids from the cytosol into the mitochondria, enabling beta-oxidation and energy production. The process, known as the carnitine shuttle, involves carnitine palmitoyltransferase I (CPT I), carnitine-acylcarnitine translocase, and CPT II.

Humans acquire carnitine from diet and synthesize it endogenously from the amino acids lysine and methionine, with requirements for vitamins C, B6, B3, and iron. The liver and kidneys produce it, and most body stores reside in skeletal muscle. Rich dietary sources are red meat and dairy products; plant-based diets may provide less carnitine.

Clinically, carnitine is used to treat primary and secondary carnitine deficiency and to manage certain metabolic disorders where fatty-acid oxidation is impaired, such as specific urea cycle disorders. It is also used as adjunct therapy in some mitochondrial diseases and cardiac or neuromuscular conditions, though indications vary by region and guideline.

Among healthy individuals, the effects of carnitine supplementation on exercise performance, weight loss, or fat loss are inconsistent and generally small. Acetyl-L-carnitine and propionyl-L-carnitine have been studied for cognitive function and vascular health, with mixed results. Safety profiles are favorable at recommended doses but may include GI upset and a fishy odor; long-term effects and interactions require caution, particularly in kidney disease. Some evidence links high carnitine intake to increased trimethylamine-N-oxide (TMAO) via gut microbiota, but causal risk remains debated.