LEP and obesity due to melanocortin 4 receptor deficiency: Patients with sarcopenic obesity had significantly higher serum leptin levels compared to patients with obesity alone and normal weight and, in multivariable regression models, serum leptin was negatively correlated with vitamin D. Based on the available evidence, it appears that the relationships among adipose tissue, bone and skeletal muscle are strictly entangled, and that vitamin D might play a role in modulating, at least in part, this interorgan crosstalk (Figure 2).