We used a combined tracer technique with the stable isotopes 13C and 15N to gain further insight into the metabolic changes that accompany supplementation of L-carnitine. The aim of the present study was to investigate whether L-carnitine supplementation can influence fat oxidation, protein turnover, body composition, and weight development in slightly overweight subjects. Twelve volunteers received an individual regular diet either without or with L-carnitine supplementation of 3 g/d for 10 days. Protein turnover and fat oxidation were investigated after administration of [15N]glycine and an [U-13C]algae lipid mixture. The 15N- and 13C-enrichment in urine and breath were measured by isotope ratio mass spectrometry. Body fat mass (BFM), total body water (TBW), and lean body mass (LBM) were calculated by using bioelectric impedance analysis. L-carnitine supplementation led to a significant increase in 13C-fat oxidation (15.8% v 19.3%; P = .021) whereas protein synthesis and breakdown rates (3.7 and 3.4 g/kg/d, respectively) remained unchanged, indicating that the increased dietary fat oxidation in slightly overweight subjects was not accompanied by protein catabolism.
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