RISK FACTORS OF TYPE 2 DIABETES IN MEXICAN AND U.S. PIMA INDIANS: ROLE OF ENVIRONMENT

Abstract

Introduction. Pima Indians living in the United States (U.S.) have the highest prevalence of type 2 diabetes mellitus in the world. Their Mexican counterparts, living a traditional lifestyle in the mountain of Sonora, Mexico, have at least five times less diabetes than the U.S. Pima Indians. The effects of a traditional lifestyle in reducing type 2 diabetes risk factors and the association of factors to type 2 diabetes were evaluated in a sample of 1211 genetically related Pima Indians living different lifestyles (224 from Mexico and 887 from U.S.). Subsets of these populations were used to address specific questions. First, differences in insulin resistance between subjects with normal glucose tolerance (194 Mexican versus 449 U.S. Pima) were evaluated. Second, the effect of physical activity and obesity explaining differences in metabolic syndrome prevalence were evaluated in 224 and 447 Mexican and U.S. Pima Indians. Third, factors associated with type 2 diabetes were evaluated in each Pima Indian population (224 from Mexico and 887 from U.S.).Methods. Demographic, physical, biochemical, and lifestyle factors were measured in 1996 in a cross-sectional study of Pima Indians 20 years of age or older living in Maycoba, Sonora Mexico and contrasted to results from a sample of U.S. Pima Indians participating in an ongoing epidemiological study that used similar methods and selection criteria. Insulin resistance was estimated by both fasting insulin and homeostasis model assessment-insulin resistance (HOMA-IR). Metabolic syndrome was defined using the Third Report of the National Cholesterol Education Program's Adult Treatment Panel III (NCEP III) criteria. Body mass index (BMI) was calculated by dividing weigh in kilograms by the square of height in meters (Kg/m2). Physical activity was measured using a questionnaire developed for the U.S. Pima Indians and adapted to the Mexican Pima Indian population. Type 2 diabetes was defined according to the 1999 WHO criteria after an oral glucose tolerance test. Multiple linear regression analysis was used to answer the first question (related to differences in insulin resistance) and multiple logistic regressions analysis to answer the second (related with differences in metabolic syndrome) and third questions (related to factors associated with type 2 diabetes).Results. Insulin resistance was much lower in the Mexican Pima Indians than in genetically related U.S. counterparts, even after controlling for differences in obesity, age and sex. In addition, the unadjusted prevalence of metabolic syndrome was 24.1% and 56.6 % in the Mexican and U.S. Pima Indians, respectively. However, most of the difference in metabolic syndrome prevalence was explained by differences in obesity and physical activity. Furthermore, in Mexican Pima Indians, type 2 diabetes was independently associated with age, fasting insulin, and waist circumference. In the U.S. Pima Indians, type 2 diabetes was associated with with age, sex, fasting insulin, total cholesterol, blood pressure and physical activity.Conclusion. The findings underscore the importance of lifestyle in the prevention of type 2 diabetes risk factors, such as insulin resistance and metabolic syndrome, even in individuals with high propensity to develop diabetes.