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Carnitine
- Carnitine, derived from an amino acid, is found in nearly all cells of the body. Its name is derived from the Latin carnus or flesh, as the compound was isolated from meat. Carnitine is the generic term for a number of compounds that include L-carnitine, acetyl-L-carnitine, and propionyl-L-carnitine.
Carnitine plays a critical role in energy production. It transports long-chain fatty acids into the mitochondria so they can be oxidized ("burned") to produce energy. It also transports the toxic compounds generated out of this cellular organelle to prevent their accumulation. Given these key functions, carnitine is concentrated in tissues like skeletal and cardiac muscle that utilize fatty acids as a dietary fuel.
The body makes sufficient carnitine to meet the needs of most people. For genetic or medical reasons, some individuals (such as preterm infants), cannot make enough, so for them carnitine is a conditionally essential nutrient.
- What are recommended intakes for carnitine?
- Healthy children and adults do not need to consume carnitine from food or supplements, as the liver and kidneys produce sufficient amounts from the amino acids lysine and methionine to meet daily needs. The Food and Nutrition Board (FNB) of the National Academy of Sciences reviewed studies on the functions of carnitine in 1989 and concluded it was not an essential nutrient. The FNB has not established Dietary Reference Intakes (DRIs) - including a recommended dietary allowance (RDA) - for carnitine.
- What foods provide carnitine?
- Animal products like meat, fish, poultry, and milk are the best sources. In general, the redder the meat, the higher its carnitine content. Dairy products contain carnitine primarily in the whey fraction. The carnitine content of several foods is listed in Table 1.
Table 1: Selected food sources of carnitine
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Food
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Milligrams (mg)
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Beef steak, cooked, 4 ounces
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56-162
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Ground beef, cooked, 4 ounces
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87-99
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Milk, whole, 1 cup
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8
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Codfish, cooked, 4 ounces
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4-7
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Chicken breast, cooked, 4 ounces
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3-5
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Ice cream, ½ cup
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3
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Cheese, cheddar, 2 ounces
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2
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Whole-wheat bread, 2 slices
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0.2
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Asparagus, cooked, ½ cup
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0.1
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Carnitine occurs in two forms, known as D and L, that are mirror images (isomers) of each other. Only L-carnitine is active in the body and is the form found in food.
- Absorption and metabolism of carnitine
- Adults eating mixed diets that include red meat and other animal products obtain about 60-180 milligrams of carnitine per day. Vegans get considerably less (about 10-12 milligrams) since they avoid animal-derived foods. Most (54-86%) dietary carnitine is absorbed in the small intestine and enters the bloodstream.
The kidneys efficiently conserve carnitine, so even carnitine-poor diets have little impact on the body's total carnitine content. Rather than being metabolized, excess carnitine is excreted in the urine as needed via the kidneys to maintain stable blood concentrations.
- When can a carnitine deficiency occur?
- Two types of carnitine deficiency states exist. Primary carnitine deficiency is a genetic disorder of the cellular carnitine-transporter system that usually manifests itself by five years of age with symptoms of cardiomyopathy, skeletal-muscle weakness, and hypoglycemia. Secondary carnitine deficiencies may occur due to certain disorders (such as chronic renal failure) or under particular conditions (e.g., use of certain antibiotics) that reduce carnitine absorption or increase its excretion. There is scientific agreement on carnitine's value as a prescription product for treating such deficiencies.
- What are some current issues and controversies about carnitine?
- Carnitine has been studied extensively because it is important to energy production and is a well-tolerated and generally safe therapeutic agent. Researchers prefer to use acetyl-L-carnitine in research studies because it is better absorbed from the small intestine than L-carnitine and more efficiently crosses the blood-brain barrier (i.e., gets into brain tissue).
Athletic performance
Some athletes take carnitine to improve performance. However, twenty years of research finds no consistent evidence that carnitine supplements can improve exercise or physical performance in healthy subjects - at doses ranging from 2-6 grams/day administered for 1 to 28 days . (The total body content of carnitine is about 20 grams in a man weighing 155 pounds, almost all of it in the skeletal muscle.) For example, carnitine supplements do not appear to increase the body's use of oxygen or improve metabolic status when exercising, nor do they necessarily increase the amount of carnitine in muscle.
Aging
A decline in mitochondrial function is thought to contribute to the aging process. Carnitine may be involved because its concentration in tissues declines with age and thereby reduces the integrity of the mitochondrial membrane. Research in aged rats found supplementation with high doses of acetyl-L-carnitine and alpha-lipoic acid (an antioxidant) to reduce mitochondrial decay. The animals also moved about more and improved their performance on memory-requiring tasks. At present there are no equivalent studies of this kind in humans. However, a meta-analysis of double-blind, placebo-controlled studies suggests that supplements of acetyl-L-carnitine may improve mental function and reduce deterioration in older adults with mild cognitive impairment and Alzheimer's disease.
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