L-carnitine is a nutrient widely available in supplement form and often marketed as a weight loss aid. Made naturally in the body, l-carnitine is essential for transporting fat into cells to produce energy. But while many supplement manufacturers claim that l-carnitine can speed up metabolism, there is little scientific support for l-carnitine's ability to promote weight loss.
Benefits of L-Carnitine for Weight Loss
To date, few clinical trials have tested l-carnitine's effectiveness as a weight loss aid. The available research includes a 2000 study from the International Journal of Sport Nutrition and Exercise Metabolism, in which 36 moderately overweight premenopausal women were given either l-carnitine or a placebo twice daily for eight weeks. Analyzing study results, researchers found no difference in total body mass or fat mass between those who took l-carnitine and those given the placebo.
A number of animal studies have also shown that l-carnitine may not support weight loss. A 2002 study from the Annals of Nutrition and Metabolism, for instance, found that rats treated with l-carnitine did not experience a greater reduction in body fat levels compared to a control group. What's more, a 2005 study published in the International Journal for Vitamin and Nutrition Research found that rats fed an l-carnitine-enhanced diet tended to gain more weight than other animals in the study (despite no difference in their intake of calories).
In some cases, l-carnitine supplements may produce certain side effects (including nausea, abdominal cramps, diarrhea, and a "fishy" body odor). In addition, l-carnitine may interact with antibiotics used in the long-term prevention of urinary tract infections (such as pivampicillin).
Brandsch C, Eder K. "Effect of L-carnitine on weight loss and body composition of rats fed a hypocaloric diet." Ann Nutr Metab. 2002;46(5):205-10.
Melton SA, Keenan MJ, Stanciu CE, Hegsted M, Zablah-Pimentel EM, O'Neil CE, Gaynor P, Schaffhauser A, Owen K, Prisby RD, LaMotte LL, Fernandez JM. "L-carnitine supplementation does not promote weight loss in ovariectomized rats despite endurance exercise." Int J Vitam Nutr Res. 2005 Mar;75(2):156-60.
Office of Dietary Supplements. "Dietary Supplement Fact Sheet: Carnitine". Last accessed February 2011.
Villani RG, Gannon J, Self M, Rich PA. "L-Carnitine supplementation combined with aerobic training does not promote weight loss in moderately obese women." Int J Sport Nutr Exerc Metab. 2000 Jun;10(2):199-207.