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L-CARNITINE
Carnitine is an amino acid with a key role in the pathway by which long-chain fatty acids (the most common type in the diet) are oxidized to yield energy. Thus, carnitine supplements have been marketed extensively as aids to weight-loss and to enhance physical performance (e.g., endurance). Carnitine is a trimethylated amino acid synthesized in the liver and exported to peripheral tissues, such as muscle, for use. Carnitine is formed from lysine and methionine, and from trimethylysine obtained following the breakdown of tissue proteins.
The principle energy-yielding pathway of fatty acid oxidation occurs in the mitochondria of the cell. Known as beta-oxidation, this pathway becomes increasingly important with fasting or during prolonged exercise in tissues such as skeletal muscle, liver and the heart (Manaerts and Debeer, 1982). In order to cross the mitochondrial membrane and reach the site of beta-oxidation, long-chain fatty acids must be transferred to carnitine. This process is catalyzed by an enzyme complex, carnitine palmitoyltransferase (CPT) (McGarry and Brown, 1997).
CPT I, located on the outer surface of the outer mitochondrial membrane, is considered the rate-limiting step in the oxidation of long-chain fatty acids. It is reversibly inhibited by malonyl-CoA the first committed intermediate in fatty acid synthesis. Under conditions of exercise and/or low-calorie dieting, it has been theorized that carnitine supply might impair fatty acid oxidation by limiting entry into mitochondria by CPT I. Heo et al. (2000) relate:
"Since the discovery of carnitine in the early 1900s, considerable research has elucidated its biochemical role in the transport of fatty acids across the inner mitochondrial membrane (see McGarry and Brown 1997 for review). Given that carnitine is a cosubstrate of carnitine palmitoyltransferase, a pivotal regulatory enzyme in the pathway of fatty acid oxidation, carnitine status could conceivably affect utilization of fatty acids as metabolic fuel. This has led to interest and unsubstantiated claims among athletes and body-builders in the use of carnitine to enhance performance (Heinonen 1996). Similarly, interest in production agriculture has stemmed from the desire to partition nutrients away from fat accretion and toward muscle deposition. Several studies in a variety of species have reported improved nitrogen (N) balance, reduced body fat and/or increased protein accretion upon L-carnitine supplementation (Bohles et al. 1984b, Hongu and Sachan 2000, Ji et al. 1996, Penn et al. 1997 , Rabie and Szilagyi 1998); however, the limited studies evaluating carnitine effects in growing pigs (Cho et al. 1999, Hoffman et al. 1993, Owen et al. 1996) have yielded inconsistent results." |
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