Figure 2.

This sketch highlights how gut microbiota utilize tryptophan to produce metabolites that significantly influence physiological processes, including immune modulation, gut-brain communication, and appetite regulation. Understanding these pathways provides insights into the microbiota’s role in maintaining immune homeostasis, regulating gut physiology, and even affecting neurological and metabolic health. It offers potential therapeutic strategies targeting microbiota or tryptophan metabolism for disorders related to immunity, gut health, and brain function. The metabolism of tryptophan by gut microbiota involves several pathways that lead to the production of various metabolites. Gut microbiota can convert tryptophan into indole and its derivatives have been implicated in various physiological processes, including modulation of immune responses. Indole derivatives can activate the aryl hydrocarbon receptor (AhR) and it plays a role in immune modulation. Gut enterochromaffin cells can convert tryptophan into serotonin (5- HT), influencing gut physiology and other functions and also, influence the release of GLP-1 (glucagon-like peptide-1). Both GLP-1 and serotonin are involved in the regulation of appetite and satiety. Related to kynurenine Pathway, the kynurenine pathway can be used to metabolize tryptophan, producing a number of metabolites, including kynurenic acid, which is implicated in immunological modulation and has been linked to neurotransmission disorders. The figure created with BioRender.com.