LEP and obesity due to melanocortin 4 receptor deficiency: In this study, we used the paradigm of diet-induced obesity first described by Woods et al., who showed that ad libitum feeding of adult Long-Evans rats with a semi-synthetic defined high-fat diet for 10 weeks led to increased caloric intake, body weight, percent body fat and plasma leptin, as well as insulin resistance, in both males and females, and that the impact of high-fat diet on body weight emerged after just 3 weeks31.