Carnitine is an amino acid responsible for transport of fatty acids into a cell's mitochondria. It is often sold as a nutritional supplement.
As with all amino acids used directly in the metabolism, carnitine exists in the left-handed form. This isomer is expressed as L-carnitine, as it is usually marketed.
Carnitine can be synthesised within the body from lysine or methionine. It has been said though, that during growth or pregnancy the use of carnitine exceeds natural production. The best source of natural carnitine is in red meat and dairy products.