Inhibited carnitine synthesis impairs adaptation to high-fat diet in Nile tilapia (Oreochromis niloticus)

High-fat diet (HFD) caused some adverse effects in farmed fish, thus the adaptation to HFD in fish is an important research topic. This study was conducted to examine the roles of carnitine and its related mitochondrial fatty acid beta-oxidation (FAO) in the adaptation to HFD in Nile tilapia. Nile tilapias were fed with HFD (13 %), HFD + mildronate (HFD + MD, inhibitor of carnitine synthesis, 1000 mg/kg body per day) or a normal fat diet (NFD, 7 %) for eight weeks. After the feeding trail, the fish fed with HFD showed higher hepatic free carnitine content and FAO activities, and similar levels of serum triglyceride (TG) and whole body fat. However, the HFD + MD-fed fish remarkably decreased carnitine content and FAO efficiency in tissues than those in the HFD-fed fish, and increased contents of serum free fatty acids (FFA) and TG, whole body fat and hepatic TG. Moreover, the HFD-fed fish upregulated the expressions of the genes associated to FAO, lipid transport and lipolysis. Nevertheless, the fish fed with HFD + MD showed lower transcriptional levels of the genes related to lipolysis and lipid transport, and higher lipogenesis genes. These results indicate that the adaptive changes in the fish fed with HFD were eliminated by dietary MD supplementation, and show that carnitine and its related FAO activity play important roles in the adaptation to HFD in fish. This study illustrates that in the practical usage of HFD in aquaculture, the endogenous carnitine concentration and mitochondrial FAO activities should be important checkpoints.