Carnitine is an amino acid responsible for transporting fatty acids into a cells mitochondria. Because it is required to burn fat, it has been widely speculated that increasing carnitine intake would also increase fat oxidation.
The following study showed no difference between carnitine and placebo. Carnitine is synthesized within the body from the amino acids lysine or methionine. Apparantly this is done at a satifactory rate and carnitine is not a limiting factor in fat loss.