IDO1 and metabolic syndrome: Among the three classes of metabolites quantifiedin this study,tryptophan metabolites in the kynurenine pathway were apparently alteredin MetS-MCI patients, showing a greater concentration of kynurenine.Kynurenine is the first breakdown product of tryptophan in the kynureninepathway activated by indoleamine 2,3-dioxygenase (IDO) and tryptophan2,3-dioxygenase (TDO) enzymes.