GIP and irritable bowel syndrome: Duodenum harbors the greatest diversity of gut endocrine cell types, including serotonin, secretin, cholecystokinin (CCK), gastric inhibitory polypeptide (GIP), somatostatin, and motilin.9 In cases of IBS, multiple studies have reported decreased densities of secretin, CCK, GIP, and somatostatin cells.10 Consequently, this reduction in those mentioned cell densities contributes to a decrease in CgA density in the duodenum.