Read the ingredient label on any energy drink and odds are fairly good that, among other arcane items, you’ll come across something called L-carnitine.
Also sold in supplement form, L-carnitine is often touted as a miracle molecule that boosts energy and helps burn fat naturally.
The claims appear to stem largely from studies showing that the molecule may improve endurance and energy in people with heart or vascular disease. But to conclude that it would do the same in healthy people is a big leap, says Dr. Ziv Haskal, professor of radiology and surgery at the University of Maryland Medical Center in Baltimore.
Clinical evidence of L-carnitine’s effects on the heart and vascular system has been emerging from a number of small but provocative studies over the last few decades, many of them conducted in Europe, particularly Italy.
In a 1992 study of 160 heart attack patients, for instance, Italian researchers reported that patients who took 4 grams of L-carnitine daily for a year, in addition to their regular medications, had less chest pain and greater improvements in blood rate and blood pressure during that time than patients who did not take the supplement. The study was published in the journal Drugs Under Experimental and Clinical Research.
A study by a separate group of Italian researchers, published in the same journal in 1991, found that in patients who suffered chest pain during exercise, taking 2 grams of L-carnitine a day for six months improved their blood pressure and ability to tolerate exercise.
More recently, researchers have been studying slightly modified forms of the molecule, including acetyl-L-carnitine and propionyl-L-carnitine. A small study published in the journal Hypertension in July showed that taking 2 grams of acetyl-L-carnitine twice a day appeared to lower blood pressure and improve insulin resistance in 32 patients identified as at risk for diabetes and cardiovascular disease.
And a study of 75 patients with limb pain due to narrowed arteries, published in the journal Cardiovascular Drugs and Therapy in August, found that patients who took 1.2 grams of propionyl-L-carnitine daily had less pain and smaller skin ulcers, and were able to walk longer distances compared with those who did not take the supplement.
Despite the evidence, “other products and walking programs work better” for patients, particularly those with vascular disease, says Haskal, who was one of several physicians who reviewed the evidence on L-carnitine for the American College of Cardiology Foundation and the American Heart Assn. in 2005. They found that the molecule showed promise for people with intermittent claudication, or muscle pain caused by peripheral artery disease, but concluded that the findings were still preliminary.
Nonetheless, studies hinting at L-carnitine’s promise have encouraged some doctors to prescribe it to a variety of patients. Dr. Mary Hardy, medical director of the Simms/Mann UCLA Center for Integrative Oncology in Los Angeles, sometimes recommends it to cancer patients to combat fatigue and increase energy levels. Dr. Joyce Frye, a professor of family and community medicine at the Center for Integrative Medicine at the University of Maryland School of Medicine in Baltimore, says she recommends the supplement to some of her heart patients.
But neither recommends the supplement to the average athlete looking for an energy boost or greater exercise capacity. “It’s kind of a leap, to conclude that it would have a similar effect in healthy people,” Hardy says.
Indeed, in the few studies that have investigated L-carnitine’s ability to improve endurance in healthy people, results have been mixed. Studies conducted in the 1990s suggested the supplement improved oxygen uptake and power, but more recent studies have not consistently shown any significant difference in performance between athletes who took the supplement and those who did not.
A molecule derived from two amino acids, L-carnitine occurs naturally in the body, where it’s made by the liver and the kidneys. Its job is to carry, or “shuttle,” fatty acids into cells, where they’re burned for energy. Carnitine deficiencies are rare; most people make enough of the molecule on their own. Carnitine also occurs naturally in a variety of foods, including red meat, tempeh, avocados and asparagus.
Getting too much L-carnitine has its drawbacks, says Roger Clemens, a professor of pharmacology at the USC School of Pharmacy, whose research has focused on functional foods, food processing and nutrition. Many supplement makers recommend as much as 3 to 4 grams of L-carnitine a day, but more than that can cause nausea or diarrhea.
These may be side effects that patients with serious disease are willing to endure. But for those after a faster jog around the block, risking diarrhea and nausea -- absent any proof of benefit -- just might not be worth it.
“It’s another fad supplement,” Haskal says. “It won’t hurt the healthy, but it won’t help you, either.”