Carnitine is synthesized in the body from essential amino acids lysine and methionine and was first discovered in muscle tissue in 1905. The natural and only form with biological activity is the L-stereoisomer, whose chemical structure was established in 1927. (In case your chemistry is rusty, isomers are molecules with the same types and numbers of atoms, but different structures.) L-carnitine is found in meat and dairy products while limited amounts are found in fruits, vegetables, grains and eggs.
Function
About 95 percent of the body's L-carnitine is found in skeletal muscle where its primary function is as a carrier molecule. It transports fatty acids across the mitochondrial membrane for Beta-oxidation, also called fatty acid oxidation. This is the process in which fatty acids are broken down to yield energy. Mitochondria are organelles within cells responsible for energy production, often referred to as the powerhouse of the cell.
Diet
Carnitine is not a required dietary nutrient as a normal diet provides adequate amounts. While no clinically recognized nutrition-related carnitine deficiency exists, one group who may be at risk would be strict vegetarians. However, as carnitine is made by two essential amino acids---essential meaning must be obtained from the diet---as long as the vegetarian obtained dietary lysine and methionine they would not be at risk for deficiency. Deficiency could occur secondary to a genetic or acquired disorder, but in these cases carnitine supplementation would be pharmacologic versus nutritional.
Possible Ergogenic Aid
Carnitine is a popular ingredient in dietary supplements, especially those promoting weight loss and improved sports performance. Due to its role in Beta-oxidation, studies have suggested L-carnitine can improve athletic performance by increasing fatty acid oxidation during exercise sparing muscle glycogen and reducing lactate production thus accelerating recovery from strenuous exercise. Some studies found supplementation to enhance performance while some have not. Existing data neither proves carnitine as beneficial or harmful.
Safe Supplementation
Like all supplements, carnitine is not approved by the Food and Drug Administration. The packaging may give suggested recommendations, but supplements are not like medicine. They are not based on long-term clinical trials where the drug has gone through many levels of testing. Additionally, as supplements are not regulated, it's not entirely known if what's in the bottle is actually what the label claims it is. When purchasing supplements in general, make sure they are coming from a reputable company.
Dosing Guidelines
A risk assessment performed on carnitine and results published by Hathcock and Shao in 2006 found "evidence from well-designed randomized, controlled human clinical trials indicates that the Upper Level for Supplements (ULS) for carnitine is 2000 mg L-carnitine equivalents per day." According to "Modern Nutrition in Health and Disease," large quantities consumed orally, about 5 g per day by an adult, may cause diarrhea but nothing toxic has been identified.
References
- "Modern Nutrition in Health and Disease;" Maurice E. Shils, Moshe Shike, A. Catharane Ross, Benjamin Caballero, Robert Cousins; 2006
- "Nutrition for Sport & Exercise;" Jacqueline R. Berning, Suzanne Nelson Steen; 1998
- "Nutrition;" Supplementation of L-Carnitine in Athletes: Does it Make Sense? Heidrun Karlic, Alfred Lohninger; 2004
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