Tryptophan metabolism and the intestinal microbiota: Implications for inflammatory bowel disease

Tryptophan metabolism exerts a pivotal influence on inflammatory bowel disease (IBD) involving intestinal microbiota. Tryptophan (Trp) undergoes several metabolic processes that result in the formation of several bioactive compounds, such as 5-hydroxytryptamine (5-HT), kynurenine (KYN), and indole analogs, and these metabolites have significant roles in maintaining intestinal health and modulating immune function. The components and ability of the intestinal microbiota affect the metabolic balance of tryptophan, and dysbiosis may lead to disorders of tryptophan metabolism, which may exacerbate the condition of IBD. In this paper, we went over how the intestinal microbiota and tryptophan metabolism interact and the mechanism of tryptophan metabolism and its products in IBD, explored the regulatory roles of the aryl hydrocarbon receptor (AhR) signaling pathway, immune cells, and immune factors in this regard, and proposed a strategy for the treatment of IBD based on tryptophan metabolism. We also mentioned that tryptophan metabolites exhibit distinct functions between Crohn's disease (CD) and ulcerative colitis (UC), and vary across different stages of the disease. Looking forward to the future, in-depth study of the interaction between tryptophan metabolism and intestinal microbiota can give new ideas to the clinical management of IBD.