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A review of carnitine and weight loss

The amino acid l-carnitine has been studied for weight loss, with confusingly mixed results. Researchers pooled previous studies together in this meta-analysis to get a clearer picture.

Study under review: The effect of (L-) carnitine on weight loss in adults: a systematic review and metaanalysis of randomized controlled trials

Introduction

Most adults in the U.S. are overweight or obese[1]. While there are a variety of dietary strategies that can lead an individual to weight loss, pharmacotherapy always sounds alluring. After all, taking a pill that promotes weight loss requires almost no work on the part of the user. This is why there are so dietary supplements claiming to promote weight loss. The vast majority of these are ineffective, and the rest seem to only have marginal effects. One supplement in particular that has been touted for weight loss is L-carnitine.

L-Carnitine is synthesized in the liver from the amino acids methionine and lysine, but (as shown in Figure 1) can also be consumed through a variety of food sources, mostly animal products. About 98% of the carnitine in the human body is found in skeletal and heart muscle, and it plays a role in both glucose and fat metabolism. It influences glucose metabolism by stimulating glycolysis[2], the breakdown of glucose. But carnitine is generally more well-known for its role in fat metabolism[3], where it shuttles fatty acids across the mitochondrial membrane for oxidation.

While this role in metabolism has led to claims of weight loss, studies have shown that ingesting up to six grams of carnitine per day for 14 days did not change muscle carnitine[4] concentrations and did not cause weight loss[5]. In contrast, other studies[6] have shown carnitine to be effective for weight loss. To evaluate the existing state of the research on L-carnitine and weight loss, researchers conducted a systematic review and meta-analysis of randomized controlled trials.

Carnitine is purported to aid in weight loss, but research on the topic has shown mixed results. To evaluate the existing state of the research on L-carnitine and weight loss, a systematic review and meta-analysis of randomized clinical trials was performed.
Figure 1: L-carnitine, common food sources and typical supplement dose

References: Rebouche CJ. Ann N Y Acad Sci. 2004 | Rebouche CJ. Carnitine. In: Modern Nutrition in Health and Disease, 9th Edition (edited by Shils ME, Olson JA, Shike M, Ross, AC). | Lippincott Williams and Wilkins, New York, 1999, pp. 505-12.

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