Disposition Index, Glucose Effectiveness, and Conversion to Type 2 Diabetes: The Insulin Resistance Atherosclerosis Study (IRAS)

OBJECTIVE: Disposition index (DI) and glucose effectiveness (S(G)) are risk factors for diabetes. However, the effect of DI and S(G) on future diabetes has not been examined in large epidemiological studies using direct measures. RESEARCH DESIGN AND METHODS: Insulin sensitivity index (S(I)), acute insulin response (AIR), and S(G) were measured in 826 participants (aged 40–69 years) in the Insulin Resistance Atherosclerosis Study (IRAS) by the frequently sampled intravenous glucose tolerance test. DI was expressed as S(I) × AIR. At the 5-year follow-up examination, 128 individuals (15.5%) had developed diabetes. RESULTS: The area under the receiver operating characteristic curve of a model with S(I) and AIR was similar to that of DI (0.767 vs. 0.774, P = 0.543). In a multivariate logistic regression model that included both DI and S(G), conversion to diabetes was predicted by both S(G) (odds ratio × 1 SD, 0.61 [0.47–0.80]) and DI (0.68 [0.54–0.85]) after adjusting for demographic variables, fasting and 2-h glucose concentrations, family history of diabetes, and measures of obesity. Age, sex, race/ethnicity, glucose tolerance status, obesity, and family history of diabetes did not have a significant modifying impact on the relation of S(G) and DI to incident diabetes. CONCLUSIONS: The predictive power of DI is comparable to that of its components, S(I) and AIR. S(G) and DI independently predict conversion to diabetes similarly across race/ethnic groups, varying states of glucose tolerance, family history of diabetes, and obesity.