Therefore, in this study, we aimed to investigate the effects and mechanisms of different PUFAs and MUFAs in improving the SFA-induced lipotoxicity of pancreatic islet β cells in an in vitro model in the rat insulinoma cell line INS-1E stimulated with SFAs, and in an in vivo mouse model of high fat diet (HFD)-induced obesity with impaired glucose tolerance, to compare the effects of diets with high and low doses of fish oil (FO) or olive oil (OO) on overall insulin sensitivity and glucose-lipids metabolism. Here, INS is linked to obesity due to melanocortin 4 receptor deficiency.