Also called L-carnitine, this amino acid helps convert fat into energy when naturally present in the human body. In the skin-care aisle, the ingredient is often found in cellulite and eye creams. Though there's little clinical data supporting its long-term effectiveness, its anti-inflammatory activity can temporarily smooth puckering and puffiness. Carnitine is a quaternary ammonium compound involved in metabolism in most mammals, plants, and some bacteria. In support of energy metabolism, carnitine transports long-chain fatty acids into mitochondria to be oxidized for energy production, and also participates in removing products of metabolism from cells. Given its key metabolic roles, carnitine is concentrated in tissues like skeletal and cardiac muscle that metabolize fatty acids as an energy source. Generally individuals, including strict vegetarians, synthesize enough L-carnitine in vivo. Carnitine exists as one of two stereoisomers (the two enantiomers d-carnitine (S-(+)-) and l-carnitine (R-(−)-)). Both are biologically active, but only l-carnitine naturally occurs in animals, and d-carnitine is toxic as it inhibits the activity of the l-form. At room temperature, pure carnitine is a white powder, and a water-soluble zwitterion with low toxicity. Derived from amino acids, carnitine was first extracted from meat extracts in 1905, leading to its name from Latin, "caro/carnis" or flesh.
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