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ABSTRACT:The ketogenic diet (KD) provides ketones from the degradation of free fatty acids for energy metabolism. It is a therapeutic option for pharmacoresistant epilepsies. Carnitine is the carrier molecule that transports fatty acids across the mitochondrial membrane for degradation into ketones. The integrity of this transport system is a prerequisite for an adequate ketogenic response. For monitoring of tissue metabolism with KD, we used the sampling method of s.c. microdialysis (MD), which permits minimally invasive, frequent, and extensive metabolic monitoring independent of blood tests. By using this new method, we monitored changes in carnitine metabolism induced by KD, particularly in free carnitine (C0), acetylcarnitine (C2), and hydroxybutyrylcarnitine (C4OH). Correlation of microdialysate and tissue concentrations for carnitines in vitro was about 85%. Carnitine metabolism was monitored in seven children started on a KD for pharmacoresistant epilepsy after a conventional initial fasting period. Detected metabolic changes consisted of a slight decrease in s.c. C0 and a marked increase in C2/CO and C4OH/CO levels. The levels of s.c. C4OH strongly correlate with ␤-hydroxybutyrate (␤-OHB) levels in plasma providing an additional parameter for the carnitine reserve of the body and reflect an optimal ketogenic energy supply. Subcutaneous MD allows close and extensive monitoring of metabolism with a KD. A high-fat, low-carbohydrate KD has been proven to be efficient for intractable childhood epilepsies (1). The metabolic effects of the KD are comparable to prolonged fasting. Glucose substrates are replaced by ␤-OHB, acetoacetate, and free fatty acids. Carnitine plays a major role in the degradation of fatty acids. As a trimethylated amino acid, it facilitates translocation of fatty acids into the mitochondrion and is therefore an essential cofactor in fatty acid oxidation and ketogenesis (2). In mammals, changes in the carnitine pattern in plasma and several tissues have been demonstrated with alterations in nutritional state. Studies in humans have shown a delayed decrease in plasma free carnitine and a rapid increase in long-and particularly short-chain acylcarnitines during fasting or diabetic ketosis (3)(4)(5). A study in children demonstrated that the changes in acylcarnitines during fat load (ingestion of sunflower oil) are more or less comparable to those during fasting (6). However, studies on the dynamics of carnitine metabolism, in particular C4OH, during initiation of a KD have not been reported so far.The technique of MD is a potent tool for the study of tissue metabolism. The method is based on the diffusion of substances through a semipermeable dialysis membrane implanted in the tissue of interest. It allows repeated measurement of the concentrations of tissue molecules that have crossed the membrane. Water-soluble analytes with a molecular weight below the exclusion size of the catheter cross the membrane until their concentrations in the extracellular fluid and the microdialysate are equal ...

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