is an essential nutrient that participates in transporting fatty acids across the mitochondrial membrane where it is burned as fuel.
Therefore, carnitine supplementation has been marketed as a 'fat burner'. However excitement for this effect has fizzled over the last couple decades because of a failure to show that
actually increases muscle carnitine. Researchers in the United Kingdom have shown in a series of studies that it is possible to elevate muscle carnitine when blood levels of carnitine and insulin are simultaneously elevated. In their most recent work, they had healthy endurance-trained men supplement with 80 g of carbohydrate 2 times per day for 24 weeks. One group also received 2 grams L-carnitine L-tartrate in their carbohydrate beverage. Resting muscle carnitine was unchanged after 12 weeks but increased by 21% after 24 weeks in the carnitine group. When the carnitine group exercised at a low intensity after 24 weeks of supplementation, they showed the higher muscle carnitine was linked to significant muscle glycogen sparing (55% less) compared to controls. Furthermore, several other measures of energy status were better with carnitine supplementation, and work output was 35% greater than controls. These interesting findings point to the need to reconsider the role of carnitine supplementation in enhancing muscle energetics, fat burning, and performance. A major drawback to the approach of increasing muscle carnitine is the simultaneous ingestion of high amounts of carbohydrate that stimulate insulin, a potent inhibitor of fat breakdown and fat oxidation. Identification of strategies to increase muscle carnitine that do not involve stimulating insulin would be a major breakthrough.
Wall BT, Stephens FB, Constantin-Teodosiu D, Marimuthu K, Macdonald IA, Greenhaff PL. Chronic oral ingestion of L-carnitine and carbohydrate increases muscle carnitine content and alters muscle fuel metabolism during exercise in humans: the dual role of muscle carnitine in exercise metabolism. J Physiol. 2011 Jan 4. [Epub ahead of print]