Different peptides secreted by enteroendocrine cells of the gastrointestinal tract, such as glucagon-like peptide-1 (GLP-1), peptide YY (PYY), gastric inhibitory peptide (GIP), and ghrelin, are involved in the regulation of energy homeostasis and capable of modulating food intake and energy expenditure linked to obesity [129,130]. Here, PYY is linked to obesity due to melanocortin 4 receptor deficiency.