INS and obesity due to melanocortin 4 receptor deficiency: Insulin resistance and hyperinsulinemia are typical features of abdominal obesity, and, interestingly, inverse associations between 25(OH)D levels and fasting insulin concentrations [11–13] or insulin resistance (measured by HOMAIR index) [11–13] or insulin sensitivity index [14, 15] have been reported in most of the studies, even independently of BMI or obesity [13].