AHR and colitis: An ubiquitous negative correlation has been identified between serum tryptophan (Trp) and the systemic inflammatory marker C-reactive protein (CRP).75 Xanthurenic acid (XANA) and kynurenic acid (KYNA), metabolites of tryptophan, can reduce the severity of colitis by influencing intestinal epithelial cells and T cells, through the activation of the aryl hydrocarbon receptor (AhR) and the reconnection of cellular energy metabolism.