L-carnitine

About L-carnitine

A compound synthesised from lysine and methionine that transports long-chain fatty acids into mitochondria. Red meat is the main dietary source; supplements use L-carnitine tartrate or the free form.

Carnitine is sold on a mechanism that is correct and a conclusion that does not follow: it is required to move fatty acids into mitochondria, but muscle carnitine is not normally the limiting step in fat oxidation, and supplementing it does not increase muscle carnitine content unless a large carbohydrate load is co-ingested to drive insulin-mediated uptake. Weight-loss meta-analyses report about a kilogram of difference, mostly from trials of mixed quality. The more consequential finding runs the other way: gut bacteria convert carnitine to trimethylamine, which the liver oxidises to trimethylamine N-oxide, a metabolite associated with cardiovascular events in cohort studies.

Sold as

Capsule, Tablet, Liquid, L-carnitine L-tartrate, Prescription levocarnitine

Standardised to

Grams of L-carnitine; the tartrate salt is roughly 68% carnitine by weight — a product without this marker may not resemble what was studied.

Also known as

Carnitine, L-carnitine tartrate, Levocarnitine

Regulation: Sold as a dietary supplement with no pre-market efficacy review, and separately available as prescription levocarnitine for primary and secondary carnitine deficiency, which is a genuine metabolic disorder. Not prohibited in sport when taken orally, though intravenous infusion above 50 ml per 12 hours falls under the WADA prohibition on infusions.

What the research used

The amounts, forms and durations published trials of L-carnitine actually administered.

When it was taken

Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.

In the food catalogue

The whole-food form, where one exists. The evidence for a concentrated extract rarely transfers to the food, or the other way round — these are cross-links, not equivalents.