The kynurenine pathway can alternatively be extrahepatically initiated by indoleamine 2,3-dioxygenase (IDO) expressed in peripheral tissues and induced by hallmarks of obesity such as inflammatory signals (i.e., TNFα, IL-6] as well as oxidative stress [19,23,24,25,26,27,28]. Here, IDO2 is linked to obesity due to melanocortin 4 receptor deficiency.