Tryptophan metabolism (Agus, Planchais & Sokol, 2018) involves three main pathways in the gastrointestinal tract: (1) tryptophan is directly converted into several metabolites by the gut microbiota, including ligands for aryl hydrocarbon receptors, which can help regulate the activity of glial cells; (2) indoleamine 2,3-dioxygenase 1, the L-tryptophan-degrading enzyme, mediates the immune and kynurenine pathway of epithelial cells; (3) tryptophan hydroxylase 1 present in enterochromaffin cells converts tryptophan to serotonin, a pathway that is usually disturbed in autism. This evidence concerns the gene TPH1 and autism.