Key search terms included “obesity,” “type 2 diabetes mellitus,” “GLP-1,” “GLP-2,” “proglucagon,” “glucagon,” “oxyntomodulin,” “glicentin,” “major proglucagon fragment or MPGF,” “glicentin-related pancreatic polypeptide or GRPP,” “GIP,” “incretins,” “peptide YY,” “cholecystokinin,” “secretin,” “amylin,” “ghrelin,” “obestatin,” “fasting,” “postprandial,” and “meal.” Only original research articles and meta-analyses published in English were included. This evidence concerns the gene PYY and obesity due to melanocortin 4 receptor deficiency.