Roux-en-Y Gastric Bypass Is Associated With Hyperinsulinemia But Not Increased Maximal β-Cell Function

Context: Roux-en-Y gastric bypass (RYGB) is associated with postprandial hyperinsulinemia. Objective: This study assessed whether increased blood insulin levels may be due to an increase in maximal β-cell function. Design Setting and Participants: We performed a cross-sectional study at Columbia University Medical Center, New York, New York. Subjects without a history of diabetes were studied after surgery (n = 12) and were compared with nonsurgical controls (n = 10) who were mean matched for body mass index, insulin sensitivity, and hemoglobin A1c and with nonobese controls (n = 8). Methods: Subjects underwent a mixed-meal tolerance test and on a separate day an intravenous glucose tolerance test followed by a hyperglycemic clamp (450 mg/dL; 25 mM blood glucose) and arginine stimulation. The main outcome measure was maximal insulin secretion quantified after arginine stimulation (AinsRmax). Results: The RYGB group exhibited greater peak postprandial glucose levels and fourfold greater peak insulin levels than control groups; however, there were no significant differences in insulinogenic index or AinsRmax. Another finding was significantly greater postprandial glucagon levels in the RYGB group compared with controls. Conclusions: Our results suggest that after RYGB, the increase in postprandial levels of insulin are not due to changes in maximal β-cell function but appear to be an appropriate response to altered nutrient flow and absorption.