demonstrated that Bacteroides fragilis was decreased and the bile acid glycoursodeoxycholic acid (GUDCA) was increased in newly diagnosed T2DM individuals treated with metformin, and the benefits of metformin were abrogated in HFD-fed mice colonizaed with Bacteroides fragilis, implicating that Bacteroides fragilis–GUDCA–intestinal farnesoid X receptor (FXR) axis mediated the glucose-lowering effect of metformin (67). This evidence concerns the gene NR1H4 and type 2 diabetes mellitus.