Serum immunoreactive insulin (IRI), plasma free fatty acids, and blood glucose were measured during an intravenous, an oral, and a cortisone-primed oral glucose tolerance test in each of eleven normal and five prediabetic (offspring of two diabetic parents) nonobese males, age twentyfive to thirty-four years. All normals and prediabetics had entirely normal blood glucose levels during each of the three tests. There were no significant differences between mean levels of serum insulin at any time period in any of the three types of tests. Serum IRI and blood glucose were significantly correlated in both normal and prediabetic groups during each of the three types of glucose tolerance tests. A linear regression equation of serum IRI (y) on blood glucose (x) was computed as an index for each type of test for each group and the regression coefficients (IRI in µU./ml per mg. per cent blood glucose) compared. A significantly lower slope was found for the oral and the cortisoneprimed oral glucose tolerance test in the prediabetic group. This suggests a diminished insulin secretory response to glucose in prediabetics who still maintained normal glucose tolerance. There was no evidence of a delay in the rise of serum insulin following a glucose load. There were no significant differences in mean plasma free fatty acids during each glucose tolerance test comparing prediabetics to normals. DIABETES 17:17-26, January, 1968.

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