Skeletal Muscle Carnitine Modulates Fuel Metabolism

NOTTINGHAM, England--Researchers from the University of Nottingham determined carnitine levels in skeletal muscles play an important role in the metabolism of carbohydrates and fats as fuel during exercise. Published in the December issue of the Journal of Clinical Endocrinology & Metabolism (91, 12:5013-18, 2006), their study involved seven healthy, non-vegetarian young men with body mass indexes (BMIs) around 26, who received intravenous saline or carnitine; serum insulin was maintained at a physiologically high concentration during two randomized visits. Researchers then measured skeletal muscle pyruvate dehydrogenase complex (PDC) activity and associated muscle metabolites.

The combination of hypercarnitinemia (600 µmol/L) and hyperinsulinemia (160 mU/L) increased muscle total carnitine (TC) content by 15 percent and was associated with decreases in both pyruvate dehydrogenase complex activity and muscle lactate content of 30 percent and 40 percent, respectively. Further, there was an overnight increase in muscle glycogen and long-chain acyl-coenzyme A content, 30 percent and 40 percent, respectively, compared with control.

The scientists reported an acute increase in human skeletal muscle TC content results in an inhibition of carbohydrate oxidation in conditions of high carbohydrate availability, possibly due to a carnitine-mediated increase in fat oxidation. They concluded these results may have important implications for investigation into the regulation of muscle fat oxidation, particularly during exercise, when carnitine availability may limit fat oxidation, as well as in obesity and type 2 diabetes where it is known to be impaired.

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