CYP2R1 and obesity due to melanocortin 4 receptor deficiency: Obesity is associated with the repression of the enzyme CYP2R1, which is primarily responsible for 25-hydroxylation of vitamin D in the liver, leading to decreased serum levels of 25(OH)D (the active form of vitamin D) due to reduced production of this metabolite; essentially, obesity inhibits the key step in the activation of vitamin D, resulting in lower circulating levels of the active form [45,46].