L-Carnitine
L-carnitine is absorbed by a saturable transporter, so bioavailability from supplement doses is only about 14 to 18%, against 54 to 87% from the small amounts in food. It is both an FDA-approved drug for carnitine deficiency and a widely sold supplement ingredient.
What carnitine does
L-carnitine (161 Da) is a quaternary ammonium zwitterion that carries long-chain fatty acids into mitochondria for beta-oxidation through the carnitine shuttle (CPT1, carnitine-acylcarnitine translocase, CPT2). The body synthesizes it from lysine and methionine, and red meat is the main dietary source. Acetyl-L-carnitine and propionyl-L-carnitine are esters with their own research literature. In healthy people with normal synthesis, supplementation does not reliably raise muscle carnitine unless combined with insulin-stimulating carbohydrate. Deficiency is rare outside genetic disorders.
Regulatory status
Levocarnitine is an FDA-approved prescription drug (Carnitor) for primary systemic carnitine deficiency and for secondary deficiency, including in end-stage renal disease patients on dialysis in its injectable form. L-carnitine and its tartrate and fumarate salts are also dietary supplement ingredients in the US and permitted in food supplements in the EU. It is not on the WADA Prohibited List as a substance, but intravenous infusions above 100 mL per 12 hours are a prohibited method.
The delivery problem
Uptake in the gut is mediated by the OCTN2 transporter and saturates at supplement doses; unabsorbed carnitine reaches the colon, where bacteria convert it to trimethylamine, which the liver oxidizes to TMAO, a metabolite associated with cardiovascular risk in observational studies. L-carnitine base is highly hygroscopic, which complicates tablets and powders, and it has a fishy odor when degraded. These properties mean that simply raising the dose does not raise absorbed carnitine proportionally, while the colonic share grows.
Formulation routes in use
The tartrate salt reduces hygroscopicity. Split dosing across the day limits transporter saturation. Liquids are common for the prescription form. Liquid and powder formats need moisture-barrier packaging, and flavor systems are used to cover the characteristic taste. Controlled-release forms are less common than for other supplements, although the transporter kinetics suggest release rate should matter. Acetyl-L-carnitine and propionyl-L-carnitine have different stability and absorption profiles and should be evaluated as separate ingredients rather than as interchangeable forms of carnitine.
Key facts
- Bioavailability of L-carnitine is 54 to 87% from dietary amounts but only 14 to 18% from 0.5 to 6 g supplement doses (Rebouche 2004, Ann N Y Acad Sci 1033:30)
- Gut microbial metabolism of L-carnitine produces TMAO, which was associated with atherosclerosis in mice and cardiovascular risk in humans (Koeth et al. 2013, Nat Med 19:576)
- Levocarnitine is FDA-approved for primary and secondary carnitine deficiency (FDA label, Carnitor)
- Intravenous infusions or injections of more than 100 mL per 12-hour period are prohibited under method M2.2 (WADA Prohibited List 2026)
How our delivery technology applies
Carnitine's limit is a saturable transporter, which is a release-rate problem. A multi-layer particle that releases carnitine gradually along the small intestine keeps local concentration within OCTN2 capacity for longer, which could raise the absorbed fraction and reduce the unabsorbed share reaching colonic bacteria. The outer shell also protects the hygroscopic core. Both effects need confirmation in a PK study with urinary and plasma carnitine and TMAO as endpoints.
Discuss a controlled-release carnitine format with our formulation team.
Related reading
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