Neuronal decanoic acid oxidation is markedly lower than that of octanoic acid: A mechanistic insight into the medium-chain triglyceride ketogenic diet

Objective: The medium-chain triglyceride (MCT) ketogenic diet contains both octanoic (C8) and decanoic (C10) acids. The diet is an effective treatment for pharmacoresistant epilepsy. Although the exact mechanism for its efficacy is not known, it is emerging that C10, but not C8, interacts with targets that can explain antiseizure effects, for example, peroxisome proliferator-activated receptor-c (eliciting mitochondrial biogenesis and increased antioxidant status) and the alpha-amino-3-hydroxy- 5-methyl-4-isoxazolepropionic acid receptor. For such effects to occur, significant concentrations of C10 are likely to be required in the brain. Methods: To investigate how this might occur, we measured the beta-oxidation rate of 13 C-labeled C8 and C10 in neuronal SH-SY5Y cells using isotope-ratio mass spectrometry. The effects of carnitine palmitoyltransferase I (CPT1) inhibition, with the CPT1 inhibitor etomoxir, on C8 and C10 beta-oxidation were also investigated. Results: Both fatty acids were catabolized, as judged by 13 CO2 release. However, C10 was beta-oxidized at a significantly lower rate, 20% that of C8. This difference was explained by a clear dependence of C10 on CPT1 activity, which is low in neurons, whereas 66% of C8 beta-oxidation was independent of CPT1. In addition, C10 beta-oxidation was decreased further in the presence of C8. Significance: It is concluded that, because CPT1 is poorly expressed in the brain, C10 is relatively spared from beta-oxidation and can accumulate. This is further facilitated by the presence of C8 in the MCT ketogenic diet, which has a sparing effect upon C10 beta-oxidation.