Many diseases, for example, inflammation, type 2 diabetes mellitus, and obesity, lead to an altered rate of hepatic metabolism.86 Obese patients have a reduced rate of metabolism via CYP3A4 isoenzymes, whereas metabolism by uridine diphosphate glucuronosyltransferases (UGT1 and UGT2 with respective subfamilies), xanthine oxidase, N-acetyltransferase, and CYP2E1 is accelerated.87 Moreover, activities of CYP1A2, CYP2C9, CYP2C19, and CYP2D6 might also be increased, but study results are inconsistent in this regard.60,87 Only limited data are available on effects of obesity on phase II reactions. Here, XDH is linked to obesity due to melanocortin 4 receptor deficiency.