Oral administration of glucose stimulates insulin secretion to a greater extent than does glucose administered as an isoglycaemic intravenous glucose infusion. This phenomenon is called the incretin effect and is caused by the two incretin hormones GIP and GLP-1. In patients with type 2 diabetes, the incretin effect is impaired. The mechanisms of the impaired incretin effect have been found to involve reduced secretion of GLP-1 and a severely impaired effect of GIP. It is currently unknown whether these defects are consequences of the diabetic state or primary pathogenetic factors.