A special glucose infusion test was used to provide successive steplike increments of glucose stimulation in six normal-weight and seven obese subjects. This “staircase” method of glucose infusion demonstrates that insulin responds in “spike” fashion despite maintenance of a continuous, fixed, submaximal glucose stimulation. Further, the “spike” response recurs as the glucose concentration is stepped up.
Obese subjects showed an exaggeration of both the first phase “spike” pattern and the more gradual second phase of insulin release. Mathematical analysis of early phase insulin release indicates that obese persons have greater total amounts of insulin available for release at all glucose concentrations than do normals, yet the proportion of totally available insulin released to a given glucose stimulus is not increased in obesity.
These findings imply that the early phase, hyperinsulin response to glucose in obesity is due to a greater quantity of insulin available for release rather than to an increased sensitivity of the beta cell to glucose.