Insulin Secretion and Insulin Sensitivity Pattern Is Different in Isolated Impaired Glucose Tolerance and Impaired Fasting Glucose

Abstract

OBJECTIVE—Isolated impaired fasting glucose (IFG) and impaired glucose tolerance (IGT) are two risk categories for type 2 diabetes. This study compared both categories with respect to the degree of insulin secretion abnormalities and insulin resistance. RESEARCH DESIGN AND METHODS—This is a crossover comparison of a population at high risk for type 2 diabetes. The subjects were recruited from the Risk Factor in Impaired Glucose Tolerance for Atherosclerosis and Diabetes (RIAD) study. They underwent a 75-g oral glucose tolerance test, with measurement of specific insulin, C-peptide, proinsulin, and free fatty acids at baseline and every 30 min after load for 2 h. Factor analysis was performed to evaluate the importance of insulin resistance and secretion abnormalities in both categories. RESULTS—All categories of prediabetic hyperglycemia had a higher cardiovascular risk factor level when adjusted for sex, age, and BMI compared to control subjects with normal glucose tolerance. Subjects with isolated IFG were more insulin resistant than those with IGT. By contrast, subjects with isolated IGT exhibited a more severe deficit in early- and late-phase insulin secretion versus IFG subjects. As shown with factor analysis, in IFG the insulin resistance factor explained 28.4% of the variance, whereas in IGT the insulin secretion factor was dominant, explaining 31.1% of the total variance. CONCLUSIONS—Our cross-sectional data from the RIAD study demonstrate that isolated IFG and isolated IGT are different with respect to the degree of insulin resistance and anomalies in insulin secretion, and that subjects with IGT exhibit a deficit in the early and late phases of insulin secretion. This finding may be important for a differential approach in primary prevention of type 2 diabetes.