The effect of glucose-dependent insulinotropic polypeptide infused at physiological concentrations on the release of insulin in man

Blood glucose and the responses of insulin and glucose-dependent insulinotropic polypeptide (GIP) were measured in 12 healthy, normal weight subjects after drinking 50 g glucose. On a subsequent occasion each subject was given a simultaneous infusion of GIP and glucose to mimic the plasma concentrations observed in the first test. The peak GIP concentration after oral glucose was 22.3 +/- 1.9 pmol/l (mean +/- SEM), but was higher after GIP infusion at 36.3 +/- 4.6 pmol/l, (p less than 0.005). The blood glucose levels following oral glucose peaked at 6.3 +/- 0.5 mmol/l which was the same as seen after intravenous glucose. The insulin response to oral glucose was, however, far higher (431.3 +/- 58.2 pmol/l) than that obtained after GIP and glucose infusion (191.6 +/- 30.9 pmol/l, p less than 0.001). Thus it has not proved possible to explain completely the oral enhancement of insulin release by the action of GIP alone.