Furthermore, microbes in the GI tract regulate the release of various gut hormones that improve glucose homeostasis and behavioral changes, such as peptide YY (PYY); regulate the physiology of immune cells, anxiety and mood, such as neuropeptide Y; and are involved in the regulation of hunger and satiety, such as leptin, ghrelin, cholecystokinin, and glucagon-like peptide-1 [46,47]. Here, PYY is linked to Anxiety.