GCG and type 2 diabetes mellitus: The ways in which the gut microbiota influence T2D mainly include: altering the host gut microecological structure, such as reducing the Gram-negative bacterial strains (LPS producing strains) and increasing the content of SCFAs producing strains [13,14], mediating the farnesoid X receptor (FXR) signaling to regulate the bile acid metabolism [15], regulating the secretion of intestinal hormones, such as peptide YY (PYY) and glucagon-like peptide-1(GLP-1) [16] and reducing intestinal permeability by strengthening the intestinal barrier function [17,18].