The authors fed mice a control or inulin-based, high-fibre diet for two weeks. Using rRNA sequencing and untargeted metabolomics, they found that inulin fibre altered the composition of the intestinal microbiota and their metabolites, particularly cholic acid, a common bile acid. The inulin fibre diet, or supplementation with cholic acid, was accompanied by type 2 inflammation, which involved activation of group 2 innate lymphoid cells and eosinophilia in the colon and lungs. These inflammatory effects were not present when an inulin diet was given to germ-free mice, thereby demonstrating the importance of the microbiome in mediating the effects of inulin. Further genetic analyses revealed that the Farnesoid X receptor — a receptor for bile acids — mediates signals from inulin to generate type 2 inflammatory responses.