Glucose-dependent insulinotropic peptide (GIP), the incretins, is synthesized and released from the duodenum and proximal jejunum. Continual high-fat diet powerfully stimulated GIP secretion, leading to obesity and harmful lipid deposition in islet cells and peripheral tissues, and giving rise to insulin resistance and major disturbances in the secretion of insulin. We can improve Type 2 diabetes by compromising GIP action. The exclusion of proximal small intestine and reduction of GIP secretion may be the important reasons for Type 2 diabetes after gastric bypass surgery.