https://www.sciencedirect.com/science/article/abs/pii/S0278584620304619 Highlights • Among several possible tryptophan metabolism routes, the kynurenine pathway can be influenced by the gut microbiota. • Changes in gut biodiversity may cause increased gut permeability and cause systemic and central nervous system inflammation. • Proinflammatory cytokines induce kynurenine pathway resulting in formation of metabolites linked with some mental disorders. Abstract The intestine and the gut-associated limphoid tissue constitute the largest immunity organ of the human body. Among several possible tryptophan metabolism routes, the kynurenine pathway can be influenced by the gut microbiota. Disturbances of gut biodiversity may cause increased gut permeability and cause systemic inflammation, also related to central nervous system. Proinflammatory cytokines induce kynurenine pathway enzymes resulting in formation of neuroactive metabolites, which are being associated with several psychiatr...