Carnitine is an amino acid-like substance that synthesizes in the body from the amino acid lysine. It is present in many foods and high concentrations can be found in red meats, such as beef or lamb. In comparison, there is little to no carnitine in white meats and vegetables, so vegetarians maybe at risk of deficiency.
Carnitine supplements are available in several chemical forms- only the L-form of carnitine is biologically active (the D-form is inactive). Reduction of carnitine levels can occur from high fat diet, certain drugs (e.g. valproic acid antibiotics) or metabolic stress.