Obesity is associated with increased post-prandial GIP levels which are not reduced by dietary restriction and weight loss.

Obesity is characterised by fasting and post-prandial hyperinsulinaemia. One factor which may contribute to this is overactivity of the enteroinsular axis. Glucose tolerance, beta-cell response and GIP profiles were therefore compared during oral glucose (OGTT), mixed meal (MTT) and intravenous glucose tolerance tests (IVGTT) in both lean (IBW less than 120%) and obese (IBW greater than 120%) healthy subjects. The tests were repeated in the obese group after a period of dietary restriction and weight loss. Fasting GIP concentrations were similar, but postprandial levels were significantly greater in the obese subjects during both the OGTT and MTT. Glucose profiles were similar but associated with basal and stimulated hyperinsulinaemia in the obese subjects indicating insulin resistance. GIP levels did not change during the IVGTT and were similar in the two groups throughout the test. Following diet and weight-reduction there was a significant decrease in both fasting and post-prandial insulin levels in the obese subjects but there were no significant changes in glucose or GIP concentrations. In conclusion the endogenously stimulated plasma GIP response is exaggerated in obese healthy subjects but this increased response is not decreased by short term diet and weight loss. The increased GIP concentrations may contribute the observed hyperinsulinaemia in obesity, but its contribution is likely to be small in view of the decrease in insulin concentrations following diet and weight-loss which was independent of any change in GIP.