Gastric inhibitory polypeptide (GIP) release and incretin effect after oral glucose in obesity and after jejunoileal bypass.

Twelve morbidly obese patients and 17 patients treated for obesity by jejunoileal shunt operation were studied. A 50-g oral glucose load (OGTT) and an intravenous glucose infusion were carried out to study a) the relation between the plasma gastric inhibitory polypeptide (GIP) levels after oral glucose and the type of jejunoileal bypass performed and b) the importance of endogenous GIP as an incretin in man. The GIP release during OGTT and incretin effect were normal in the obese patients. After jejunoileal shunt, measuring 48 cm and with a ratio of 3:1 between the jejunal and ileal segments, the GIP release and the incretin effect were significantly reduced. Incremental increase in plasma GIP and OGTT was significantly correlated to the incretin effect in these patients. After jejunoileal shunt with the reverse ratio of proximal and distal intestine the incretin effect was significantly higher in spite of a comparable GIP release. Five patients after ileoascendostomia for familial hypercholesterolemia had significantly supernormal GIP release during OGTT but normal incretin effect. The findings indicate the insulinotropic effect of GIP and are in accordance with the concept that incretins other than GIP are released from the distal intestine.