Abstract
Tryptophan metabolism connects the nervous, immune, and microbial systems and influences the onset and progression of chronic diseases such as cancer, neurodegeneration, and autoimmunity. This review summarizes the three major metabolic routes: the kynurenine pathway, the serotonin pathway, and microbial indole production. It outlines how their metabolites shape neural activity, immune regulation, and host-microbiota interactions. We further discuss the relevance of these metabolites as biomarkers and their potential therapeutic implications. By integrating recent insights into tryptophan-associated signaling networks, this review provides a concise framework for understanding their roles in chronic disease and guiding future precision medicine strategies.