Carnitine is a chemical found in food sources such as red meat and dairy products. The biological role of carnitine is fatty acid metabolism: it transports fatty acids from the intracellular fluid to the mitochondria to generate metabolic energy.
As a dietary supplement it is found in the form of either acetyl-L-carnitine or L-carnitine. It is also sold and marketed as a so-called "fat-burner" (i.e. it will help metabolizing your body fat quickly, thus making you slim and fit), or a fat-loss supplement. According to the University of Maryland Medical Center,
"there is no scientific evidence to show that it works. Some studies do show that oral carnitine reduces fat mass, increases muscle mass, and reduces fatigue, which may contribute to weight loss in some people." —UMMC
The most widespread chemical form of carnitine is L-carnitine, and this form is usually sold as a weight-loss supplement. L-carnitine can be derived from L-tartrate.
A 2011 study published in peer-reviewed scientific journal The Journal of Physiology found that a L-carnitine L-tartrate supplement increases muscle carnitine content which in turn reduces muscle carbohydrate utilization and increases muscle lipid use. The net result is reduced fatigue during exercise and increased athletic performance.
A 2014 study of several nutriceuticals including Juvenon® (α-lipoic acid and acetyl-L-carnitine mixture) found that "The results are consistent with epidemiological studies suggesting that dietary supplements are not beneficial and even may be harmful for otherwise healthy individuals."
- Lifespan effects of simple and complex nutraceutical combinations fed isocalorically to mice by Stephen R. Spindler et al. (2014) Age 36:705-718.