Unexplained hyperinsulinemia in normal and "prediabetic" Pima Indians compared with normal Caucasians. An example of racial differences in insulin secretion.

The pattern of insulin response to oral and/or intravenous glucose has been claimed to be characteristic of diabetes and even prediabetes. To determine if differences in insluin secretion might explain the exceptionally high prevalence of diabetes in the Pima Indians, 26 genetically normal Pimas (nondiabetic offspring of nondiabetic parents), 32 genetically prediabetic Pimas (nondiabetic offspring of diabetic parents), 10 diabetic Pimas, and 29 normal Caucasians were studied. All subjects received an intravenous glucose tolerance test (IVGTT) to examine the acute-phase insulin response, and all nondiabetic subjects received an oral glucose tolerance test (OGTT) and arginine infusion (AI). The prediabetics also received a cortisone-primed oral glucose tolerance test (CGTT) and were classified by the result of this test. While acute-phase insulin release during the IVGTT was absent in the diabetics, there was a rapid response in all nondiabetics. Prediabetic Pimas with normal or abnormal CGTT had insulin levels similar to normal Indians during the IVGTT, OGTT, and AI. Thus, no evidence of impairment of acute- or late-phase insulin release was found. The normal and prediabetic Indians had fasting and stimulated insulin levels during all the tests two-to-threefold greater than the Caucasians. Differences in insulin levels between the two races could not be explained by differences in glucose level, age, or obesity.