Basal and stimulated insulin levels: Comparison of insulinogenic effects of oral glucose and intravenous tolbutamide in nondiabetic and diabetic subjects

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Immunoreactive insulin (IRI) and glucose levels before and during standard oral glucose (100 g) and intravenous tolbutamide (1 g) tolerance tests were compared in 17 obese and nonobese subjects with normal and abnormal carbohydrate tolerance. After tolbutamide, the increases observed in IRI levels were directly related to the prestimulated or basal IRI (r = + 0.50, p < 0.05), a relationship similar to that previously shown between basal IRI and the increment in IRI levels after glucose. These findings indicate that the level to which absolute IRI increases after both oral glucose and intravenous tolbutamide in both nondiabetic and mildly diabetic subjects is closely related to the basal level. When IRI responses were expressed as per cent changes from the basal level in order to eliminate the influence of basal on stimulated IRI levels, the mean IRI response to both glucose and tolbutamide was decreased in the diabetic group. These observations are consistent with the hypothesis that the characteristic defect in diabetes mellitus is a deficient early relative insulin response.