The amino acid L-carnitine is a cofactor that is required for the metabolism of fats and their subsequent transport into the mitochondria, where they undergo oxidation and produce energy. L-carnitine is probably best known for its cardiac benefits. In the heart muscle, L-carnitine facilitates fatty acid use, and promotes rhythm, myocardial blood flow, and cardiac output. It has also been shown to benefit athletic performance. For example, studies show improvements in running speed, increased maximal oxygen uptake, and decreases in average oxygen consumption, heart rate, and lactic acid. L-carnitine has been shown to help maintain normal levels of total cholesterol, LDL cholesterol, lipoprotein(a) (associated with LDL cholesterol), and triglycerides. It also promotes healthy sperm count and sperm motility.

In addition to supporting athletic performance, cardiac function, healthy lipid levels, and reproductive health, L-carnitine has also been shown to promote general wellbeing, appetite control, and carbohydrate metabolism. Individuals prone to being deficient in L-carnitine or for whom supplementation would be advisable, include those on dialysis or who are HIV-positive.