L-Carnitine was first discovered by two Russian scientists in muscle extracts from which substance got its name (Latin word carnis – flesh or meat).  Carnitine is biosynthesized from the amino acids lysine and methionine . In the terms of bioavailability Acetyl-l-carnitine is claimed to be superior to L-carnitine .
L-carnitine has been increasingly popular as a potential ergogenic aid because of its role in the conversion of fat into energy . The most important claim relates to the role of carnitine in fat metabolism. It is often advertised that L-carnitine benefits in fat metabolism, fat mass reduction, and increases muscle mass. In other words, it is sold as “fat burner”. Its claims for a weight loss agent are based on the assumption that oral supplementation increases intracellular concentrations of L-carnitine. This would increase the fat oxidation and reduce body’s fat reserves. However, several studies have shown that oral carnitine ingestion does not change muscle carnitine levels and does not promote weight loss [6,7]. A study in animals reported that dietary carnitine stimulates carnitine palmitoyltransferase (or carnitine acyltransferase 1),  which could explain the increased appetite by L-carnitine supplementation .
So, claims that l-carnitine supplementation promotes weight loss are not sufficiently justified, especially for healthy non-obese individuals.
- Gulewitsch WKR. Zur Kenntnis der Extraktionsstoffe der Muskeln. 2. Mitteilungen über das Carnitin (extracted substances in muscle, report on carnitine). Hoppe-Seyler Z Physiol Chem 1905;45:326
- Steiber A, Kerner J, Hoppel C (2004). “Carnitine: a nutritional, biosynthetic, and functional perspective”. Mol. Aspects Med. 25 (5–6): 455–73.
- Hosein, E. A., and Jennie M. Smoly. “Biosynthesis of acetyl-l-carnityl choline.” Archives of Biochemistry and Biophysics 114.1 (1966): 102-107.
Cerretelli, P., and C. Marconi. “L-carnitine supplementation in humans. The effects on physical performance.” International journal of sports medicine 11.01 (2008): 1-14.
Karlic, Heidrun, and Alfred Lohninger. “Supplementation of L-carnitine in athletes: does it make sense?.” Nutrition 20.7 (2004): 709-715.
Barnett, C., et al. “Effect of L-carnitine supplementation on muscle and blood carnitine content and lactate accumulation during high-intensity sprint cycling.” International journal of sport nutrition 4.3 (1994): 280.
Villani, Rudolph G., et al. “L-Carnitine supplementation combined with aerobic training does not promote weight loss in moderately obese women.” International Journal of Sport Nutrition 10.2 (2000): 199-207.
Karlic, Heidrun, et al. “Dietary l-carnitine stimulates carnitine acyltransferases in the liver of aged rats.” Journal of Histochemistry & Cytochemistry 50.2 (2002): 205-212.