Agmatine produced by gut microbiota — specifically, Bacteroides vulgatus — activates the farnesoid X receptor (FXR) in intestinal epithelial L cells in a bile-acid-independent manner, which inhibits host glucagon peptide 1 (GLP-1) secretion and leads to polycystic ovary syndrome (PCOS) in mice. Supplementing mice with the GLP-1 receptor (GLP-1R) agonist liraglutide or inhibiting the production of agmatine reverses the PCOS phenotype.