Peptide YY is an endogenous neuropeptide Y-related peptide that activates Y1 and Y2 receptors. Peptide YY is an appetite-regulating hormone secreted from the gastrointestinal tract that exhibits anorexigenic and gastrointestinal motility modulating activities. Peptide YY decreases osteoblast activity, contributing to bone loss in clinical settings in which patients have recently lost weight. Peptide YY also decreases levels of AMPK and phosphorylated acetyl CoA carboxylase in the liver, decreasing lipid oxidation and/or increasing lipogenesis. Peptide YY also induces slight improvements in insulin response during glucose challenge in animal models. In other in vivo models, peptide YY induces mucosal anti-secretory activity, inhibiting colonic and upper gastrointestinal transit.