To investigate the effect of oxidized fish oil on fish muscle metabolic responses and flesh quality, Nile tilapia ( Oreochromis niloticus ) weighing 13.73 ± 0.31 g were fed two diets for 12 weeks: a fresh fish oil (FFO) and a highly oxidized fish oil (OFO) diet. The peroxide value of the FFO and OFO diets was 2.2 meq/kg and 120.6 meq/kg, respectively. The OFO diet resulted in a decrease in growth, muscularity, nutritional value of fatty acids in the muscle, and density of myofibers. In the OFO group, the mRNA expression levels of lipolysis gen...