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 [ 1 , 2 ]. 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 [ 1 ].
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 Academies formerly National Academy of Sciences reviewed studies on the functions of carnitine in and concluded it was not an essential nutrient [ 3 ]. 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 [ 1 , 3 , 5 ]. The carnitine content of several foods is listed in Table 1. 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 [ 1 , 6 ]. Adults eating mixed diets that include red meat and other animal products obtain about 60— milligrams of carnitine per day [ 6 ].
Vegans get considerably less about 10—12 milligrams since they avoid animal-derived foods. The kidneys efficiently conserve carnitine, so even carnitine-poor diets have little impact on the body's total carnitine content [ 1 , 5 ]. Rather than being metabolized, excess carnitine is excreted in the urine as needed via the kidneys to maintain stable blood concentrations. 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.
There is scientific agreement on carnitine's value as a prescription product for treating such deficiencies [ 2 ]. Carnitine has been studied extensively because it is important to energy production and is a well-tolerated and generally safe therapeutic agent [ 7 ]. 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.
Athletic performance Some athletes take carnitine to improve performance. The total body content of carnitine is about 20 grams in a man weighing pounds, almost all of it in the skeletal muscle [ 11 ]. 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 [ 10 ].
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 [ 12 ]. 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 [ 16 ]. In these studies, subjects took 1. Cardiovascular and peripheral arterial disease Several studies have examined the effectiveness of supplemental carnitine in the management of cardiac ischemia restriction of blood flow to the heart and peripheral arterial disease whose most important symptom is poor circulation in the legs, known as intermittent claudication [ 17 , 18 ].
Because levels of carnitine are low in the failing heart muscle, supplemental amounts might counteract the toxic effects of free fatty acids and improve carbohydrate metabolism [ 17 ]. In short-term studies, carnitine has had anti-ischemic properties when given orally and by injection. Treatment with L-carnitine significantly reduced mortality 5 days after randomization but did not significantly affect the risk of heart failure or death at 6 months.
The authors of a meta-analysis combined the results from this trial with those from 12 smaller trials [ 20 ]. Claudication results from an inadequate supply of oxygen-rich blood to the legs and leads to an accumulation of acetylcarnitine in muscle due to its incomplete utilization. Patients with peripheral arterial disease who develop claudication have significant impairments in exercise performance and have difficulty walking even short distances at a slow speed [ 18 ].
Research indicates that carnitine might improve the performance of skeletal muscles in the leg. A similar multicenter trial in the United States and Russia found that the same daily dose and form of carnitine administered for 6 months in patients with disabling claudication significantly improved walking distance and speed, reduced bodily pain, enhanced physical function, and improved perceived health state compared to patients in the control group [ 22 ].
The authors of a systematic review and meta-analysis that included these and 12 other randomized clinical trials concluded that propionyl-L-carnitine significantly increases peak walking distance in patients with claudication [ 23 ]. These findings suggest that L-carnitine, when administered for up to 1 year, might have beneficial effects on the cardiovascular system in certain settings.
Other research, however, has raised concerns about the cardiovascular effects of chronic exposure to carnitine. A study that included both rodents and 2, humans undergoing elective cardiac evaluation found that L-carnitine is metabolized by intestinal microbiotia to trimethylamine-N-oxide TMAO , a proatherogenic substance that is associated with cardiovascular disease risk [ 24 ].
Due to differences in intestinal bacteria composition, omnivorous study participants produced more TMAO than vegans or vegetarians following consumption of L-cartinine.
The study also found dose-dependent associations between fasting plasma L-carnitine concentration and risk of coronary artery disease, peripheral artery disease, and overall cardiovascular disease, but only among participants with concurrently high TMAO levels. The researchers noted that these findings could partly explain the link between high levels of consumption of red meat a rich source of carnitine and increased cardiovascular disease risk.
More research is needed to fully understand the effects of carnitine on cardiovascular health. Cancer Fatigue resulting from chemotherapy, radiation treatment, and poor nutritional status is common in cancer patients [ 25 ].
They may also be deficient in carnitine [ 25 ]. In both studies, most subjects were carnitine deficient before taking the supplements. Type 2 diabetes Insulin resistance, which plays an important role in the development of type 2 diabetes, may be associated with a defect in fatty-acid oxidation in muscle [ 27 ]. This raises the question of whether mitochondrial dysfunction might be a factor in the development of the disease. Increased storage of fat in lean tissues has become a marker for insulin resistance [ 27 ].
Early research suggests that supplementation with L-carnitine intravenously may improve insulin sensitivity in diabetics by decreasing fat levels in muscle and may lower glucose levels in the blood by more promptly increasing its oxidation in cells [ ]. The treatment was most effective in subjects with type 2 diabetes of short duration [ 30 ]. HIV-infected individuals often accumulate fat in some areas of the body and lose fat in others and develop high levels of blood fats hyperlipidemia and insulin resistance, which together constitute the lipodystrophy syndrome.
This syndrome may represent mitochondrial toxicity brought about by the HIV infection and the antiretroviral drugs used to treat it, and can induce a carnitine deficiency that limits mitochondrial fat metabolism [ 31 ]. The molecular mechanisms by which this occurs are poorly understood.
End-stage renal disease and hemodialysis Carnitine homeostasis balance within the body among individuals with renal diseases can be substantially impaired by several factors, particularly reduced synthesis and increased elimination of the compound by the kidneys as well as reduced intake from food due to poor appetite and consumption of fewer animal products [ 41 ].
Many patients with end-stage renal disease, particularly those on hemodialysis, become carnitine insufficient. Carnitine blood levels and muscle stores are low, which may contribute to anemia, muscle weakness, fatigue, altered levels of blood fats, and heart disorders. Numerous studies suggest that high doses of supplemental carnitine often injected in patients on maintenance hemodialysis can correct some or all of these symptoms, though most involve small numbers of patients and are not double-blinded trials.
A recent meta-analysis of these studies concludes that carnitine supplements may aid anemia management but not blood-lipid profiles, and that their effects on exercise capacity or heart stability are inconclusive [ 42 ]. Male infertility The carnitine content of seminal fluid is directly related to sperm count and motility [ 43 , 44 ], suggesting that the compound might be of value in treating male infertility.
The reported benefits may relate to increased mitochondrial fatty-acid oxidation providing more energy for sperm and reduced cell death in the testes [ 49 ]. For these reasons, L-carnitine supplements are only necessary in special cases, such as disease treatment. The main dietary sources of L-carnitine are meat, fish and some other animal products, such as milk. A healthy individual can also produce sufficient amounts within the body. For this reason, L-carnitine levels are often lower in vegetarians and vegans, since they restrict or avoid animal products 6 , Therefore, vegetarians and vegans may want to consider L-carnitine supplements.
However, no studies have confirmed the benefits of carnitine supplements in these specific populations. Older adults may also benefit from L-carnitine supplements. Research shows that your levels tend to decline as you age 63 , In one study, 2 grams of L-carnitine reduced fatigue and increased muscle function in older adults. Other research reveals that acetyl-L-carnitine may also help boost brain health and function as you age 64 , Additionally, the risk of deficiency is higher for those with diseases like cirrhosis and kidney disease.
If you have one of these conditions, a supplement may be beneficial 1 , 66 , Specific populations may benefit from L-carnitine supplements. This includes older adults and people who rarely or never eat meat and fish. Although dosage varies from study to study, here is an overview of the use and dose for each form:. Although the recommended dose varies, around —2, mg 0. L-carnitine is best known as a fat burner — but the overall research is mixed.
It is unlikely to cause significant weight loss. However, studies support its use for health, brain function and disease prevention. Supplements may also benefit those with lower levels, such as older adults, vegans and vegetarians. Of the different forms, acetyl-L-carnitine and L-carnitine are the most popular and seem to be most effective. There are several effective supplements that can help you burn body fat.
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Pork is considered a red meat. A diet high in red meat increases your risk of chronic medical conditions like cancer or heart disease, reported a study published in in the "Archives of Internal Medicine.
All fish and shellfish contain some carnitine, but cod has the highest concentration of any seafood. A 4-ounce serving supplies between 4 and 7 milligrams. The Monterey Bay Aquarium Seafood Watch suggests choosing handline-caught Atlantic cod from the northeast Arctic or Iceland or Pacific cod captured by trap, handline or bottom longline. Cod of these types are harvested in an environmentally friendly manner and have a low risk of contamination.
Pregnant or nursing women and women who plan on becoming pregnant should limit their total seafood consumption to 12 ounces of low-mercury fish, including cod, per week.
Chicken breasts aren't only high in protein, low in fat and cholesterol and a good source of vitamins and minerals, they're high in carnitine, with 3 to 5 milligrams in every 4-ounce serving of cooked meat.
Choose skinless breasts or remove the skin before eating and use only a minimal amount of added fat like polyunsaturated vegetable oil when cooking. If you're concerned about your sodium intake, look for chicken breasts that haven't been enhanced with a saline solution.
Enhanced chicken can contain over milligrams of sodium in every 4 ounces.
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