Additionally, it was reported that dysregulated metabolites of tryptophan and activated indoleamine 2,3-dioxygenase (IDO) by inflammation were crucial parts of depression and aging regulated by the gut-brain axis.402 Supplementing mitochondria decreased the IDO activity and attenuated anxiety- and depression-like behaviors, highlighting the potential to treat psychiatric disorders.397. Here, IDO2 is linked to depressive disorder.