Carnitine is an amino acid derivative which, when consumed in quantity, increases the metabolism of fat, helps to build muscle mass, and can protect against heart disease.

It was first discovered in 1905 in muscle extracts, and was named after the Latin word for flesh, caro. The chemical structure was confirmed in 1927, and became commercially available in the 1980s.

According to one carnitine-marketing source, it is a "vitamin-like nutrient related to vitamins of the B-group". Carnitine is found in the bodies of animals in large quantity, and is also found in uncooked plant foods, such as mushrooms, carrots, and rice. The human body produces carnitine naturally, and is concentrated largely in the heart, and in skeletal muscle tissue. It is synthesized in the liver, and in the kidney.

Carnitine deficiency manifests itself in the forms of muscular weakness, hypoglycemia, and cardiac and liver failure. Carnitine comes in three varieties: Levocarnitine (L-Carnitine) commercially available as Carnitor, Acetyl-L-Carnitine (ALCAR), and the D-Carnitine isomer. Outside of a few particular cases, however, increasing one's intake of carnitine through supplements is considered a waste of money.