What is L-carnitine, and which foods contain it?

L-carnitine is a small compound derived from amino acids. The body makes it from lysine and methionine, mainly in the liver and kidneys, and it is also found in food, above all in red meat. In a table from the US NIH Office of Dietary Supplements, 3 ounces (about 85 g) of cooked beef steak has about 42 to 122 mg, a cup of whole milk 8 mg, and vegetables, fruit and grains very little. L-carnitine is not one of the nutrients in the Thai Reference Daily Intakes (Thai RDIs), and in the US it was concluded in 1989 that it is not an essential nutrient.

This article explains what L-carnitine is, how the body makes it, which foods contain it and what is known about upper amounts and effects of large amounts. It does not discuss what it does in the body.

What is L-carnitine?

The NIH Office of Dietary Supplements (ODS), part of the US National Institutes of Health, describes carnitine as a compound derived from an amino acid, and as the general name for several compounds, including L-carnitine, acetyl-L-carnitine and propionyl-L-carnitine. Carnitine is found naturally in many foods, especially foods of animal origin.

The molecule

PubChem, the chemistry database of the US National Library of Medicine, gives L-carnitine the formula C7H15NO3 and a molecular weight of 161.20. It is a small molecule with a nitrogen atom carrying three methyl groups (a trimethylammonium group), a hydroxyl group and an acid group, on a chain of four carbon atoms.

L-carnitine and D-carnitine

Carnitine comes in two mirror-image forms, like a left and a right hand. PubChem lists D-carnitine with the same formula and weight as L-carnitine; the two differ only in the 3D arrangement around one carbon atom, labelled (3R) for L-carnitine and (3S) for D-carnitine. The form made by the body and found in food is L-carnitine.

Related forms

Other carnitine compounds are L-carnitine with another group attached at the hydroxyl group.

The ODS says L-carnitine and acetyl-L-carnitine are the two main forms in dietary supplements.

How does the body make L-carnitine?

From two amino acids

A 1993 review from Ramathibodi Hospital, Mahidol University, by Tanphaichitr and Leelahagul, explains that carnitine in the human body comes from two sources: carnitine already present in food, and carnitine the body makes from the amino acids lysine and methionine. It says carnitine is made in the liver and kidneys and leaves the body mainly in urine.

Where the last step happens

A 1980 study by Rebouche and Engel measured the enzymes of this pathway in human tissues. The first three steps, which turn a modified form of lysine into a compound called gamma-butyrobetaine, took place in all the tissues studied (liver, brain, kidney, heart and skeletal muscle). The final step, which turns gamma-butyrobetaine into L-carnitine, was found only in the liver, kidney and brain. The activity of the enzyme for this final step in the liver depended on age: it was 12% of the adult average in infants and reached the adult level by age 15.

How much the body makes

The ODS says the body's own production does not appear to be affected by how much carnitine is eaten, and is enough to meet the needs of healthy people. As an example, it says a person weighing 165 pounds (about 75 kg) on a strict vegetarian diet makes about 14.4 mg a day. A 2008 French study notes that adults have about 20 g of L-carnitine in the body, that it is not broken down there but removed through the kidneys, and that what is lost is replaced by the body's own production or by food.

Which foods contain L-carnitine?

The ODS says carnitine is present in animal products, especially red meat, that poultry, fish and dairy foods provide some, and that vegetables, fruits and grains provide negligible amounts. It notes that data on individual foods are limited and that the US Department of Agriculture's FoodData Central does not list carnitine.

Carnitine per serving

Values from the ODS table, which is based on several small studies. The ranges for beef are wide, so they show the order of size rather than an exact amount.

Does cooking or freezing change it?

The 2008 study from the University of Dijon, France, measured L-carnitine in beef and salmon after freezing and after different cooking methods. L-carnitine was abundant in all the beef products analysed, the amounts in different cuts were similar, and freezing and cooking did not change the content. Salmon contained about 12 times less L-carnitine than beef, and cooking or freezing did not change it either, except in smoked salmon. The abstract does not give the exact figures for each cut.

What did a study of Thai adults find?

Bangkok and a village in Ubon Ratchathani

A 1980 study by Tanphaichitr and colleagues, the same Thai researcher as the 1993 review, measured carnitine in the blood and urine of adults living in Bangkok and in villages in Ubon Ratchathani. Average blood (plasma) carnitine was 56.6 µmol per litre in the Bangkok adults and 50.3 µmol per litre in the village adults, and urinary carnitine was 161 and 127 µmol per litre.

The researchers noted that rice, which contains little carnitine, was the main source of protein and energy for the village adults, and that their lower values could be due to a lower carnitine intake. They also found that men and women differed. The study is more than 40 years old and covered two places, so it shows how diet can relate to carnitine levels rather than today's figures for Thailand. The ODS notes that the body's own production is enough to meet the needs of healthy people.

How much carnitine does a diet provide?

Diets with and without meat

The ODS says a typical diet that includes meat provides about 24 to 145 mg of carnitine a day for a person weighing 165 pounds, while a vegan diet provides about 1.2 mg. A 1989 study from the University of Iowa compared adults and children on strict vegetarian, lacto-ovo vegetarian and mixed diets. In adults, blood carnitine and urinary carnitine were lower in both vegetarian groups than in the mixed-diet group, but the authors wrote that these small differences did not suggest a nutritionally significant difference in carnitine status. The differences were larger in children.

How much of the carnitine in food is absorbed?

A 1991 study by Rebouche and Chenard gave adults a small labelled dose of L-carnitine with a meal. They estimated that 54% to 87% of carnitine from normal Western diets is absorbed, and that the share depends on how much is eaten. Part of what is not absorbed is broken down in the gut into trimethylamine N-oxide (TMAO), 8% to 39% of the dose, and gamma-butyrobetaine.

Is there a daily amount or an upper limit?

No Thai RDI

The Thai RDIs list 33 nutrients, and carnitine is not one of them, so there is no recommended daily amount for carnitine in Thailand. The ODS says that in 1989 the Food and Nutrition Board of the US National Academies concluded that carnitine is not an essential nutrient and so set no reference intakes. For protein, the Thai RDI is 50 g a day; for the amino acids lysine and methionine, see What are essential amino acids?

Large amounts

The ODS says there is no established upper limit for carnitine, but that doses of about 3 g a day from supplements can cause nausea, vomiting, abdominal cramps, diarrhoea and a fishy body odour. It adds that some research shows bacteria in the gut turn unabsorbed carnitine into TMAO and gamma-butyrobetaine, and that studies have raised concerns about possible long-term effects of this on health. This effect appears more marked in people who eat meat than in vegetarians or vegans. The ODS says what this means is not well understood and needs more research.

Who should talk to a doctor or pharmacist?

People who take regular medication

The ODS says several types of medicines can interact with carnitine supplements, and that some medicines lower blood carnitine levels. If you take medication regularly, talk to a doctor or pharmacist before taking carnitine.

People under regular care for a health problem

The ODS notes that large amounts can cause problems for some people who are under regular care for a health problem. If you are under regular care for a health condition, ask your doctor first.

Pregnant and breastfeeding women

If you are pregnant or breastfeeding, talk to a doctor or pharmacist before taking any dietary supplement.

Dietary supplements are not intended to prevent, treat or cure any disease.