Impaired Glucose Tolerance and Obesity as Effect Modifiers of Ethnic Disparities of the Progression to Diabetes: The San Antonio Heart Study

OBJECTIVE: The Diabetes Prevention Program (DPP) reported no racial/ethnic differences in the incidence of diabetes in individuals with impaired glucose tolerance (IGT). Therefore, it has been hypothesized that factors associated with racial/ethnic disparities act prior to the development of IGT. Because impaired fasting glucose (IFG) and obesity were also very prevalent in the DPP, we examined IGT, IFG, and obesity as effect modifiers of ethnic disparities in the San Antonio Heart Study. RESEARCH DESIGN AND METHODS: Participants were 3,015 Mexican Americans and non-Hispanic whites aged 25–64 years. The median follow-up period was 7.8 years. IGT, IFG, and diabetes were defined by the 2003 American Diabetes Association criteria, and obesity was defined as BMI ≥30 kg/m(2). RESULTS: Mexican Americans had an excess risk of incident IGT (odds ratio 1.48 [95% CI 1.16–1.89]) and incident IFG (1.71 [1.31–2.23]) compared with non-Hispanic whites. Mexican Americans also had a higher incidence of diabetes among individuals who had normal 2-h glucose (2.20 [1.48–3.29]) and IGT (1.72 [1.08–2.74]) at baseline. There was an interaction of obesity on the relationship between ethnicity and progression to IGT or diabetes (P = 0.034), with Mexican Americans having a greater risk among the nonobese (1.73 [1.36–2.21]) and a comparable risk among the obese (1.08 [0.75–1.56]). CONCLUSIONS: Ethnic differences can be detected at both the early and later stages of the diabetes disease process. However, non-Hispanic whites lose much of the ethnic advantage once they have developed obesity.