Carnitine is a trimethylated amino acid derivative that is required in humans for the transport of fatty acids into mitochondria for β-oxidation. Based on this role, carnitine is sold as a dietary supplement for increasing fat oxidation (fat burning). However, just because a substance is involved in a physiological process does not mean that a supplementary intake must increase the rate of that process. In humans, carnitine is synthesised endogenously from L-lysine and L-methionine, and production is under tight regulatory control. Therefore the ability of carnitine to increase fatty acid uptake and oxidation pivots on the efficiency of dietary carnitine to increase concentrations of carnitine within the intermembrane space of the mitochondria of skeletal muscle tissue. This is important because only carnitine in the intermembrane space can increase rates of β-oxidation. A number of studies have reported on the effects of carnitine supplementation, and generally it appears that humans can synthesise all the carnitine they require. Therefore dietary carnitine supplements may not increase skeletal tissue levels.
For example, one study1 investigated the association between dietary carnitine intake and the plasma and skeletal muscle concentrations of carnitine in healthy men and women. Using a 24 hour dietary recall questionnaire, the authors determined that there was a significant association between the carnitine and the protein content of the diet. This suggests that those who consume more protein have higher dietary intakes of carnitine due to the presence of carnitine in meat. When the authors measured plasma levels of carnitine they found that the intake of dietary carnitine and protein was associated with the plasma levels of carnitine in the subjects, suggesting that dietary supplements could raise plasma carnitine levels. However, when the authors took tissue biopsies from the vastus lateralis and assessed the carnitine concentrations of the skeletal muscle tissue, they found no association with either dietary protein intake or dietary carnitine intake. Therefore dietary carnitine does not increase skeletal muscle concentrations of carnitine, which explains the inability of carnitine supplements to increase β-oxidation rates in humans.
