Your left and right hands are basically mirror images of each other. Similarly, every amino acid except the simplest one, glycine, comes in two mirror-image molecular forms: dextro- and levo-, for right and left, respectively. The L- form, including L-carnitine, is the only one that's active in the body. Technically carnitine refers to a mix of both the D and L forms, but in most cases where carnitine is discussed in the context of nutrition and supplements, carnitine is simply shorthand for L-carnitine.
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Mirror Forms of Molecules
Proteins are composed of long chains of amino acids, which contain the elements carbon, hydrogen, nitrogen and oxygen. In these compounds a carbon molecule may be bonded to four different groups, and the same chemical formula can represent two molecules that are mirror images of each other. This property is called chirality by chemists. The two forms are called D or L in biochemistry, depending on how they interact with plane-polarized light.
L-Carnitine in the Body
L-carnitine is involved in transport of molecules in and out of the mitochondria, organelles in each of your cells that produce energy. L-carnitine transports fatty acids into the mitochondria to be turned into energy and transports metabolic waste out of the mitochondria for removal. L-carnitine is not an essential amino acid, as your body can produce all it needs in most cases.
Benefits of L-Carnitine
In cases where your metabolism or circulation is impaired, your body may not be able to produce or transport enough L-carnitine for proper fatty acid metabolism. Patients with congestive cardiovascular diseases, type 2 diabetes, kidney disease and hypothyroidism may benefit from supplementation with L-carnitine, although further research is needed. No studies have conclusively demonstrated any benefits of L-carnitine supplementation in fat loss or athletic performance in healthy individuals.
Most studies recommended L-carnitine doses of between 1 and 3 g per day. Be sure to use only L-carnitine and not a mixture of D- and L-carnitine, as D-carnitine may interfere with the effectiveness of L-carnitine. L-carnitine may interfere with a small number of medications; consult your doctor before using L-carnitine supplements if you are taking daily medication or being treated for any medical condition.