INS and obesity due to melanocortin 4 receptor deficiency: There was a highly significant inverse correlation between the whole body insulin sensitivity, M value and BMI (r = −0.85; p<0.0001), lean body mass (r = −0.8; p = 0.02) and percentage fat mass (r = −.68; p = 0.01) (Figure 1), confirming that the lean patients were insulin-sensitive and that there was an increasing degree of IR relative to increasing obesity.