L-Carnitine is the active form (and the form sold as a supplement) of carnitine, an amino acid-like substance manufactured by the liver and kidneys. Carnitine is found mainly in heart and skeletal muscle, where it is necessary for fatty acid metabolism.
Some believe that because carnitine metabolizes fatty acids, taking L-Carnitine will help burn excess fat and facilitate weight loss. Others hypothesize that supplementing with L-Carnitine during exercise will help increase endurance by breaking down fats before muscle glycogen (the primary source of fuel for most muscles).
(Source: Natural Health magazine, online at www.naturalhealthmag.com.)