Figure 1.

The gastrointestinal tract is the principal location of tryptophan (Trp) metabolism. In humans, Trp is acquired exclusively through dietary intake. Dietary Trp can be processed by three main metabolism routes: (A) the kynurenine pathway (KP), which mainly occurs in both immune and epithelial cells via IDO1, leading to several kynurenine metabolites (KYNs) including ligands for AhR (B) the serotonin production pathway, taking place in enterochromaffin cells (subtype of intestinal epithelial cells) and via TpH1, and finally (C) the direct conversion of Trp by the gut microbiota into several Indoles and derivates, including ligands of the AhR.