Effect of lifelong carnitine supplementation on plasma and tissue carnitine status, hepatic lipid metabolism and stress signaling pathways and skeletal muscle transcriptome in mice at advanced age

Despite this large body of evidence from clinical studies that an improvement of carnitine status has beneficial effects on metabolic health, serious safety concerns with carnitine supplementation have been raised based on the recent observation that longterm carnitine supplementation (from weaning to 5 mo of age) in a transgenic mouse model of atherosclerosis (Apoe-/- mice) promotes atherosclerosis through gut microbiota-dependent formation of trimethylamine (TMA) and subsequent hepatic conversion into proatherogenic TMA-N-oxide (TMAO) (Koeth et al., 2013). In addition, Lu et al. (2017) found that carnitine supplementation to normal mice for 3 mo induces oxidative stress, liver inflammation, liver lipid accumulation and even liver damage as well as hypercholesterinemia. However, it has to be mentioned that the carnitine dose administered to the mice in the abovementioned studies was extremely high. In order to address whether safety concerns are founded under conditions of longterm supplementation of a carnitine dose used in clinical trials, the present study aimed to investigate the effect of almost lifelong daily administration of a carnitine-supplemented diet (1 g/kg diet) to mice from weaning to 18 mo of age. Considering a body weight of 50 g for an adult mice and a daily feed intake of 5 g, the daily intake of this diet results in a daily carnitine dose of 100 mg/kg body weight which better reflects the dose used in clinical trials. In order to evaluate possible detrimental effects of carnitine supplementation, we investigated parameters of lipid metabolism and stress signaling pathways in the liver and performed a genome-wide transcriptome analysis of skeletal muscle in the mice at advanced age. For microarray analysis of skeletal muscle, n = 5 skeletal muscle total RNA samples were randomly selected each from the control group and the carnitine group. RNA samples were processed at the “Kompetenzzentrum Fluoreszente Bioanalytik” (Regensburg) using Affymetrix GeneChip Mouse Gene 2.0 Sense Target arrays as described.