This article will most likely wrap up my supplement series at least for the time being, although if there is anyone who would like to know more information about a certain supplement feel free to ask in the comments and I can certainly take a look. Otherwise the last supplement I will be reviewing will be L-carnitine. L-carnitine is a nonessential amino acid which is abundant in the heart and muscles. Since it carries long-chain fatty acids across the inner mitochondrial membrane for oxidation it is often assumed that increasing carnitine within the muscles will increase the oxidation of fatty acids and aid in fat loss. As per usual, today we will take a look at the research to see if those claims appear to be true or not.
In one study published in the International Journal for Vitamin and Nutrition Research in 2005 researchers looked at using L-carnitine to promote weight loss in swim-trained rats. In the 5 week study the rats were ovariectomized (ovaries removed) to induce weight gain. Serum carnitine was higher in rats who received L-carnitine and the weight gain was counteracted by swimming, but L-carnitine did not reduce the weight gain or abdominal fat in the rats. (1)
Research in the Annals of Nutrition and Metabolism from 2004 looked to verify if supplementing could maximize fat loss in trained rats. They split the rats into 4 groups, control, sedentary supplemented, trained and trained supplemented. Once again they used swimming for exercise. The results from this study were similar to those of the previous study we looked at. Swimming successfully induced a decrease in fat content and the weight of adipose tissues, but even though supplementing with L-carnitine did augment carnitine content in the soleus mitochondria, the higher content did not maximize fat loss induced by swimming. (2)
Switching to human studies, research published in 2000 in the International Journal of Sport Nutrition and Exercise Metabolism looked at combining aerobic training and L-carnitine to promote weight loss in moderately obese woman. In this study 36 premenopausal women were split into 2 groups, one ingesting 2 grams twice daily of L-carnitine or a placebo. All subjects walked for 30 minutes 4 days per week. At the end of the study there was no significant changes in mean total body mass, fat mass and resting lipid utilization between the two groups. While resting energy expenditure increased significantly for all subjects, there was no difference between groups. (3)
In a longer study done on male subjects 8 healthy men took 2 grams of L-carnitine twice daily for 3 months while doing exercise tests using a bicycle ergometer. At the completion of the study researchers concluded oral treatment of healthy adults with L-carnitine is not associated with a significant increase in the muscle carnitine content, mitochondrial proliferation, or physical performance. (4)
As you can see there doesn’t appear to be much support for using L-carnitine to aid in fat loss. What I did find interesting, however, was there is some evidence carnitine can increase androgen receptor synthesis and thus potentially increasing cellular uptake of testosterone following weight training. It’s also possible it could aid in the recovery of resistance training. (5, 6, 7, 8) However, despite this research I don’t think there is enough long-term data to convince me supplementing with carnitine will do much in terms of sizable differences in muscle mass and certainly not for fat loss. Add to that the research Matt presented to us last year which shows individuals with high levels of L-carnitine may be at increased risk for heart disease and I’d have to say taking a pass on L-carnitine seems like the smart choice.