CYP27B1 and obesity due to melanocortin 4 receptor deficiency: Other theories suggest that individuals who are lean have a better ability to activate vitamin D in adipose tissue than those who are obese and that in obesity, there is a lower expression of the enzymes 1α-hydroxylase (mitochondrial CYP27B1) and 25-hydroxylase (CYP2J2) in adipocytes compared to people who are lean [24,39,40,41,42].