Fig. 1.

Natural ligands for aryl hydrocarbon receptor. Kynurenine (KYN) is converted from tryptophan in host metabolism. Kynurenic acid (KYNA) is also an AHR ligand, converted from KYN by kynurenine aminotransferase (KAT). There are three groups for microbiota-mediated AHR ligands: (1) tryptophan metabolites derived by microbiota, (2) bacterial virulence factors, and (3) short chain fatty acids. Short chain fatty acids are not direct ligands for AHR, but those facilitate AHR effects. Flavonoids, stilbenes, carotenoids, and indoles from plants are dietary ligands for AHR