Nevertheless, administration of exogenous Kyn significantly exacerbated HFD-induced obesity and insulin resistance (Fig. 1f–i), and whole-body knockout of Ido1 restored circulating Kyn levels and protected mice from obesity (Fig. 1j–m and Supplementary Fig. 2k–q), which were opposite to the assumption. Here, IDO1 is linked to obesity due to melanocortin 4 receptor deficiency.