IDO1 and metabolic syndrome: Because an enzyme activated during inflammation (indoleamine 2,3-dioxygenase, IDO) catalyses one pathway of kynurenine synthesis from tryptophan [22], an increase in kynurenine can be associated with low-grade inflammation as reported during the OGT in horses [7], in lame cows [23], and in humans with metabolic syndrome [5].