IDO1 and obesity due to melanocortin 4 receptor deficiency: Specifically, there is a feasible evidence that kynurenine (Kyn), a downstream catabolite of tryptophan (Trp), is upregulated in the plasma from certain subjects with obesity, which could be caused by the increased activity of indoleamine 2,3-dioxygenase 1 (IDO1)9–11.