L-leucine is an essential amino acid, meaning you must obtain it through diet because your body can't produce it. You can acquire L-leucine by eating meat, dairy products, eggs, soy and other beans, or by taking it as a supplement. Leucine has potential benefits for weight, lean muscle mass and cholesterol levels, although research is limited and conflicting, according to physician Ray Sahelian, who has a website devoted to natural supplements. Studies have mainly involved animals rather than humans. Consult a qualified health care provider before taking leucine supplements.