People who need more energy and want to avoid stimulants that will make them edgy should consider taking supplements of carnitine or acetyl-l-carnitine, two forms of the same nutrient found naturally in protein. Carnitine works by transporting fats in cells to where they are burned for energy. By boosting the activity of the body's cellular furnaces, carnitine can energize the heart, brain, and muscles to do more. Doctors have also found carnitine and acetyl-l-carnitine helpful in treating many conditions, including heart failure, muscle weakness and failing memory. Carnitine can also enhance physical training.