Fat and Sugar Metabolism During Exercise, With and Without L-carnitine in Patients With Carnitine Transporter Deficiency

The investigators wish to investigate fat and sugar metabolism during exercise with and without L-carnitine supplementation in patients with carnitine transporter deficiency (CTD). Patients with CTD have low plasma- and muscle concentrations of carnitine, which is believed to lead to an impaired fat oxidation. Presently there is no cure available for these patients, but daily intake of L-carnitine has been shown to limit the amount of symptoms. Little is known about the metabolism during exercise and the pathophysiological mechanisms causing the symptoms. Studying the fat and sugar metabolism in CTD patients will contribute to the understanding of the role of the carnitine transporter in the development of symptoms in these patients. Furthermore, knowledge about the fat and sugar metabolism in these patients can increase the understanding of the role of the carnitine transporter in the metabolism healthy persons. The investigators have included 8 patients with genetically verified CTD in the study and a group of 10 age- and sex-matched controls. Subjects will perform a 1h cycling test, exercising at a moderate intensity. By measuring the expiration of carbon dioxide (CO2) and consumption of oxygen (O2), the investigators can determine the total fatty acid and carbohydrate oxidation during cycling. At the same time the investigators will measure the patients' whole body palmitate (fat) and glucose (sugar) oxidation rates using stable isotope technique. The patient group will repeat the cycling test after 4 days without taking their usual L-carnitine treatment. During the treatment break, patients will be admitted to be continuously monitored for heart rhythm disturbances, which is a known but rarely occurring complication to untreated CTD. Since the patients have a defect in their fat metabolism, the investigators expect to find that they have a reduced ability to burn fat, which is the major source of energy during low intensity exercise. It is therefore likely, that the CTD patients will benefit from adjustments in their daily diet, whenever they have to perform physically. By learning about the metabolism of different dietary substances, fat and sugar, these studies can help to improve the treatment in terms of dietary recommendations for CTD patients. This will have a direct impact on the daily life of the patients.