Naturally occurring amino acid. Deficiencies of this small but essential component can result in muscle loss and a multitude of other problems. Research abounds for carnitine, especially acetyl-L-carnitine, which is considered to have more bioavailability in terms of its effect on aging and brain function. Research into how this amino acid affects skin when applied topically is limited, though a few studies indicate it can be an antioxidant (Sources: Medical Science Monitor, June 2005, pages 176–180; and International Journal of Biochemistry and Cell Biology, February 2003, pages 149–156).