What carnitine is: Sold in the forms L-carnitine or acetyl L-carnitine, carnitine is an ingredient in many supplements marketed for healthy aging, weight loss, exercise and heart health, as well as in some energy drinks. Carnitine is found in meat and other foods, but healthy people don’t have to consume it, since the body makes it from amino acids, the building blocks of protein.
The body needs carnitine to produce energy, and it plays a key role in the cells' energy-producing mitochondria, as well as in our muscles—including heart muscle. As an antioxidant, carnitine helps neutralize cell-damaging molecules called free radicals. Carnitine levels and the function of the cells' mitochondria decrease during the aging process, which may contribute to some adverse effects of aging—such as declines in energy, muscle strength and brain function—as well as to some diseases.
Claims, purported benefits: Can help treat or prevent many disorders, including heart disease, diabetes, dementia, male infertility, fibromylagia and HIV infection. It’s supposed to increase energy, aid memory and exercise performance, help with weight loss, and fight off the negative health effects of aging.
Carnitine: the latest research
Heart health. Carnitine is concentrated in heart muscle, and some research suggests it can help prevent or reduce the damage from heart attacks. Two Italian studies found that high doses given to cardiac patients can reduce the risk of heart failure and death. And a study published in the American Journal of Clinical Nutrition in 2009 found that carnitine reduced oxidation of LDL (“bad”) cholesterol; oxidation makes LDL more damaging to arteries. According to the Natural Standard, which reviews complementary/alternative therapies, there’s good—though not strong—evidence for its use in chronic but stable angina and peripheral vascular disease.
A 2013 analysis published in Mayo Clinic Proceedings examined 13 clinical trials that tested L-carnitine supplements in study subjects who had already had a heart attack. The analysis found that the supplements cut the likelihood of angina and serious arrhythmias, and lowered the risk of premature death.
In contrast, a 2013 study published in Nature Medicine suggested that L-carnitine (which is in red meat) is a "previously unknown factor" in heart disease. The study, by researchers from the Cleveland Clinic and other institutions, found that L-carnitine is broken down in the intestines into another compound, TMA. This can then be converted by the liver into a third substance, TMAO, which is tied to the increased creation of arterial plaque. People with high blood levels of carnitine and TMAO were also found to be at higher risk of heart disease, whether they had other cardiovascular risk factors or not.
Diabetes. An Italian study found that carnitine supplements can improve insulin sensitivity in people with diabetes. Some other studies have suggested benefits for blood sugar or diabetes complications; in some, the carnitine was administered intravenously.
Cognitive impairment and dementia. A 2003 British meta-analysis concluded that carnitine may benefit people with mild cognitive impairment and early Alzheimer’s. But a review by the Cochrane Collaboration, also in 2003, found the evidence unconvincing.
Exercise performance. Since carnitine plays an important role in energy production in muscles, it has been theorized that supplements can improve muscle function, reduce fatigue and boost performance in other ways. However, 20 years of research has produced no convincing evidence that carnitine supplementation can improve athletic performance.
Weight loss. There’s no strong evidence that carnitine produces significant long-term weight loss. Studies have been small and short, and results contradictory. A study in the International Journal of Sport Nutrition and Exercise Metabolism, for instance, found that carnitine did not help overweight women lose weight.
Healthy aging. An Italian study published in Archives of Gerontology and Geriatrics in 2008 found that taking carnitine supplements for six months reduced physical and mental fatigue in people over 70. A 2007 study from the same researchers, published in the American Journal of Clinical Nutrition, looked at physical and mental fatigue and cognition in centenarians. Those taking carnitine supplements had less fatigue and improved cognitive function as well as reduced cholesterol and body fat and increased muscle mass.
Side effects: Though carnitine appears to be safe, the long-term effects of large doses are unknown. According to the Natural Standard, it can interact with a variety of medications—for instance, reducing the doses of certain drugs needed for heart conditions and high blood pressure. It may interfere with sleep if taken in the evening.
Bottom line: Despite some promising research on carnitine, larger, longer, well-designed studies on humans are needed. Most marketing claims are overblown. Don’t take it for a medical condition such as heart disease without talking to your doctor first.