Early Response of Plasma Insulin to Small Doses of Intravenous Glucose: Effect of Obesity

The early response of plasma insulin (IRI) to successive intravenous doses of 1.0, 2.5 and 5.0 g glucose was determined in obese nondiabetic and healthy lean subjects. The magnitude of insulin response was significantly correlated with glucose dose and with 2-min blood glucose increment (r = +0.52). However, when the increment of insulin over basal level was related to the corresponding blood glucose change (ΔIRI/Δglucose) it appeared that small hyperglycemic stimuli elicit the highest relative insulin response and that this decreases progressively with increase of hyperglycemia even if the latter is far from maximal. These results suggest that the basal steady state blood glucose level is determined by the glucose threshold of beta cell insulin release. This threshold is similar in obese and lean people but the mean early insulin response to any minimal hyperglycemic stimulus is twice as high in obese than in lean subjects. The result is compatible with the concept that in obesity the number of insulin secretory units is increased but their sensitivity to glucose is normal.