The decreased levels of indole compounds, metabolites derived from tryptophan through microbial decarboxylation and dehydrogenation reactions, in maternal serum during early and mid-gestation are associated with an increased risk of GDM [108], which may be attributed to reduced populations of tryptophan-metabolizing bacteria like Bifidobacterium [87] and Lactobacillus [88] in the gut microbiota of GDM patients, consequently leading to impaired synthesis of indole-3-propionic acid and secretion of incretins like GLP-1 and GLP-2, further aggravating insulin resistance in GDM [109]. This evidence concerns the gene GCG and gestational diabetes.