INS and obesity due to melanocortin 4 receptor deficiency: 2009; Frias et al. 2011). Further analysis showed that obesity developed only in a subgroup of animals sensitive to the HFD and this was associated with elevated insulin levels. The HFD resistant animals remained lean with normal insulin, consistent with our published findings in studies using this model (McCurdy et al. 2009; Frias et al. 2011).