Tryptophan metabolism as a key integrator within the gut-lung-brain axis: Mechanistic insights and nutritional therapeutic strategies for inflammatory and neuropsychiatric disorders

The gut-lung-brain (GLB) axis is a multidirectional communication network linking the gastrointestinal tract, respiratory system, and central nervous system (CNS) through neural, endocrine, and immune pathways. Emerging evidence suggests that tryptophan (Trp) metabolism serves as a key integrating node within this axis, modulating host-microbe interactions involved in systemic homeostasis. Trp catabolism follows three divergent pathways: the kynurenine (Kyn) pathway, which is involved in immune tolerance but can generate neuroactive and potentially neurotoxic metabolites; the serotonin pathway, essential for mood and gastrointestinal motility; and the microbial indole pathway, which supports epithelial barrier function through aryl hydrocarbon receptor activation. Gut dysbiosis and chronic inflammation may disrupt these pathways and contribute to the "metabolic hijacking" of Trp, shifting its metabolism away from serotonin and indole synthesis toward increased production of Kyn pathway metabolites. This shift has been implicated in the pathogenesis of respiratory diseases, such as chronic obstructive pulmonary disease, asthma, and pulmonary fibrosis, as well as neuropsychiatric conditions, including depression and Alzheimer's disease. These alterations contribute to systemic low-grade inflammation and immune dysregulation, which further propagate cross-organ pathology within the GLB axis. This review synthesizes current evidence on how Trp metabolites may function as cross-organ mediators, contributing to the "leaky gut" and "leaky brain" phenotypes. Furthermore, we evaluate the potential of precision nutrition and therapeutic interventions, including psychobiotics, dietary phytochemicals, cofactor supplementation, and fecal microbiota transplantation, to restore metabolic equilibrium. Targeting the Trp-GLB metabolic axis may therefore offer an integrative therapeutic framework for managing interconnected inflammatory and neuropsychiatric comorbidities.