We used a combined tracer technique with the stable isotopes (13)C and (15)N 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 [(15)N]glycine and an [U-(13)C]algae lipid mixture. The (15)N- and (13)C-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 (13)C-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. Wutzke KD, Lorenz H. Metabolism 2004 Aug;53(8):1002-6.