GCG and metabolic syndrome: Among them, AAs such as tryptamine, indole, and indoleacetic acid (IAA) can enhance epithelial barrier functions via decreased intestinal permeability, possibly mediated by the pregnane X receptor in vitro [31]; similarly, indole can improve the glucose metabolism of patients with metabolic syndrome through modulation of glucagon-like peptide-1 (GLP-1) secretion.