You need to be a big believer in carnitine and its cellulite zapping abilities if you buy Nivea's Good-bye Cellulite (as featured on the Tyra Banks Show). There is precious little else in this potion apart from tea (from the South American mate tree) and vitamin E. The rest of the ingredients are thickeners and the like.
Still, carnitine is interesting and Nivea wants you take it topically in a gel and as an oral supplement up to three times a day. Carnitine is associated with (amongst other things like lowering cholesterol) muscle strength and fat breakdown. L-carnitine transfers long-chain fatty acids, such as triglycerides, into mitochondria (a cell's energy powerhouse), where they may be oxidized to produce energy. Because it is thought to burn fat, increase energy, and improve resistance to muscle fatigue, it is often taken as a supplement by body builders.
According to the University of Maryland, there is no evidence that it improves athletic performance. Several studies have reported that supplementing rats with both L-carnitine and alpha-lipoic acid blunts the age-related increases in oxidation. However, the rats were given extremely large doses and no one knows if relatively lower does would work on humans.
I could only find claims that carnitine breaks down fat. I couldn't find any actual evidence, however.
Incidentally, the Nivea supplement is mostly safflower oil and 30mg of L-carnitine.