The intestinal flora regulates TRP metabolism through two pathways, kynurenine/indole: (i) IDO1-mediated activation of the kynurenine pathway is associated with depression and immune tolerance, and its metabolites promote atherosclerosis; and (ii) indole derivatives maintain the intestinal barrier and inhibit inflammation through the AHR, and are protective in IBD and autoimmune diseases. This evidence concerns the gene AHR and inflammatory bowel disease.