More recently obesity has been shown to decrease hepatic 25-hydroxylase activity causing low 25(OH)D concentrations due to decreased expression of CYP2R1, the principal hepatic vitamin D 25-hydroxylase, while the vitamin D endocrine system is now felt to be far more complex than originally thought.32 Here, CYP2R1 is linked to obesity due to melanocortin 4 receptor deficiency.