L-Carnitine is a vitamin-like compound found naturally in meats. It has a role in energy use by the muscle. Claims for the supplement include that it improves performance during endurance activities.
Studies show that long-term carnitine supplementation (6 months) increases muscle carnitine content, but shorter periods of supplementation have no effect. Studies on carnitine's effect on performance show no enhancement on fuel use or endurance during activity. This means that simply taking several grams per day of L-carnitine will not improve activity performance.
L-Carnitine appears to be safe, although large doses may cause diarrhea. Most studies using carnitine as a supplement for athletes reported no adverse effects with doses ranging from 500 mg per day to 6 grams per day taken for 1 day to 4 weeks. Typical dosages in studies on athletes is 2 grams daily. Supplements vary in purity.