PYY and obesity due to melanocortin 4 receptor deficiency: Results from such studies serve as evidence of the receptor’s involvement in obesity development, which may in part be mediated by the release of satiety hormones glucagon-like peptide 1 (GLP-1) and peptide YY (PYY) from enteroendocrine cells (17), free fatty acid oxidation of adipose tissue (18), and control of energy expenditure (16).