INS and obesity due to melanocortin 4 receptor deficiency: Obesity in the Charles River fa/fa rats at 32 and 40 weeks of age was accompanied by mild glucose intolerance and increased insulin compared to the controls; however, insulin level did not reach the same values as in the fa/fa rats from Envigo, and also QUICKI showing peripheral IR was not as decreased, even it was significantly decreased compared to control saline group [16].