Glucose-induced GIP levels in patients with insulinoma.

Gastric inhibitory polypeptide (GIP) levels during an oral glucose tolerance test (OGTT) have been determined in eight patients suffering from a single benign beta-cell adenoma. GIP hypersecretion and an absent correlation between the insulin: glucose ratio and plasma GIP concentration have been observed. Our data show that the insulinotropic effect of GIP is almost abolished in insulinoma patients. This finding may help explain blood glucose levels and the inappropriate plasma insulin response to oral glucose often observed in these patients.