Objective: To determine whether Mexican Americans have an increased incidence of non-insulin-dependent. (type II) diabetes mellitus relative to non-Hispanic.
Steven M. Haffner, MD, MPH Helen P. Hazuda, PhD Braxton D. Mitchell, PhD Judith K. Patterson, PhD Michael P. Stern, MD
Increased Incidence of Type II Diabetes Mellitus in Mexican Americans
Objective: To determine whether Mexican Americans have an increased incidence of non-insulin-dependent (type II) diabetes mellitus relative to non-Hispanic whites. Currently, no study has reported on the incidence of this disorder in Mexican Americans. Research Design and Methods: We determined the 8-yr incidence of type II diabetes in 617 Mexican Americans and 306 non-Hispanic whites who participated in the San Antonio Heart Study, a population-based study of diabetes and cardiovascular disease. Forty Mexican Americans (6.5%) and 6 non-Hispanic whites (2%) developed type II diabetes, as defined by World Health Organization criteria. The age-adjusted ethnic odds ratio (OR; Mexican Americans/non-Hispanic whites) for diabetes incidence was 8.13 (95% confidence interval [C1] 1.10-59.9) in men and 3.62 (95% Cl 1.37-9.55) in women. We adjusted for age, sex, ethnicity, body mass index, and level of educational attainment with multiple logistic regression analyses. Results: Mexican Americans continued to show a statistically significant increase in diabetes incidence (OR 2.72, 95% Cl 1.02-7.28). Obesity and age were also positively related to diabetes incidence in this analysis (P < 0.001). In addition, subjects with at least some college education had a lower incidence of diabetes than those with less than a high school education (OR 0.51, 95% Cl 0.26-0.99). Conclusions: The incidence of type II diabetes in Mexican Americans is greater than in non-Hispanic whites, a difference that is not explained by ethnic differences in obesity, age, or level of educational attainment. Diabetes Care 14:102-108, 1991
From the Division of Clinical Epidemiology, Department of Medicine, University of Texas Health Science Center at San Antonio, San Antonio, Texas. Address correspondence and reprint requests to Steven M. Haffner, MD, Division of Clinical Epidemiology, Department of Medicine, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78284. Received for publication 25 May 1990 and accepted in revised form 30 August 1990.
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everal studies have documented that Mexican Americans have an increased prevalence of noninsulin-dependent (type II) diabetes mellitus relative to non-Hispanic whites (1-6). The MexicanAmerican population is also characterized by increased adiposity (1,2,7,8), a more centralized distribution of body fat (7,8), hyperinsulinemia (9), and insulin resistance (10). However, their greater degree of adiposity and adverse body-fat distribution does not fully explain their higher prevalence of type II diabetes (11,12). Moreover, Mexican Americans are predominantly of lower socioeconomic status (1), which, in this group and several other populations, has been shown to be inversely related to the prevalence of diabetes (13-15). Although an increased prevalence of a disorder usually reflects an increased incidence, it could also be due to increased survival of patients with that disorder. The latter possibility is at least plausible with regard to diabetes prevalence among Mexican Americans, because Mexican-American men have decreased cardiovascular mortality relative to non-Hispanic white men (16-18). In addition, diabetic Mexican Americans of both sexes and nondiabetic Mexican-American men have a decreased prevalence of nonfatal myocardial infarction relative to their non-Hispanic white counterparts (19). In this study, we report the 8-yr incidence of type II diabetes in Mexican Americans and non-Hispanic whites examined in the San Antonio Heart Study, a population-based study of diabetes and cardiovascular disease. In addition, we have examined the relationship of diabetes incidence to age, sex, obesity, and educational attainment (an indicator of socioeconomic status) and have attempted to ascertain whether ethnic differences in these risk factors account for the ethnic differences in diabetes incidence. DIABETES CARE, V O L . 14, N O . 2, FEBRUARY
1991
S.M. HAFFNER AND ASSOCIATES
RESEARCH DESIGN AND METHODS The San Antonio Heart Study is a population-based study of diabetes and cardiovascular disease in Mexican Americans and non-Hispanic whites. From 1979 to 1982, we randomly sampled households from several San Antonio census tracts: two low-income (barrio) census tracts (99% Mexican American), two middle-income (transitional) census tracts (60% Mexican American, 40% non-Hispanic white), and a cluster of high-income (suburban) census tracts (10% Mexican American, 90% non-Hispanic white; 1). Only Mexican Americans were sampled in the barrio. Stratified random sampling was used in the middle-income and suburban census tracts to ensure the inclusion of approximately equal numbers from each ethnic group in the study sample from these neighborhoods. All men and nonpregnant women 2564 yr of age residing in the randomly selected households were eligible for the study. A total of 1288 Mexican Americans and 929 non-Hispanic whites were included in the 1979-1982 survey. This study, however, concerns the 8-yr follow-up of participants from the first four census tracts (2 low income, 1 middle income, 1 upper income) that contributed 921 Mexican Americans and 391 non-Hispanic whites to the overall survey. The overall response rate was 61% in Mexican Americans and 68% in non-Hispanic whites. Mexican Americans were defined as individuals whose ancestry and cultural traditions derived from a Mexican national origin (20). A detailed description of the 1979-1982 survey has been published previously (1). The study was approved by the Institutional Review Board of the University of Texas Health Science Center at San Antonio, and all subjects gave informed consent. At the baseline examination, blood specimens were obtained after a 12- to 14-h fast and 1 and 2 h after administration of a 75-g glucose-equivalent load (Glucola, Ames, Elkhart, IN). Plasma glucose concentrations were measured with an Abbott Bichromatic Analyzer (South Pasadena, CA). Height and weight were determined after each participant had removed his/her shoes and upper garments and donned an examination gown. Body mass index (BMI) was calculated as weight (kg) divided by height squared (m2). Level of educational attainment was used as an indicator of socioeconomic status and categorized as less than high school, high school, and greater than high school. Beginning in October 1987, an 8-yr follow-up study was begun to determine the incidence of type II diabetes and cardiovascular disease. Vital status was ascertained on 97.6% (899 of 921) of Mexican Americans and 99.2% (388 of 391) of non-Hispanic whites from the first four census tracts. The follow-up examination consisted of a home or telephone interview, followed by a medical examination performed in a mobile clinic located in the participant's neighborhood. Thirty-seven Mexican Americans and 12 non-Hispanic whites died before the follow-up interview. Two Mexican Americans and two non-Hispanic whites were ineligible for DIABETES CARE, VOL. 14, N O . 2, FEBRUARY 1991
the interview due to physical or mental disabilities. The initial interview was completed by 96.9% (855 of 882) of surviving eligible Mexican Americans and by 98.1 % (370 of 377) of surviving eligible non-Hispanic whites. One Mexican American and one non-Hispanic white died after completing the initial interview but before completing the medical examination. There were an additional two Mexican Americans who completed the home interview but who were then considered ineligible for the medical examination due to physical disabilities. The response rate to the medical examination was 82.7% (705 of 852) of Mexican Americans and 87% (321 of 369) of non-Hispanic whites who completed the home interview. Thus, the overall response rate was 80.1% for Mexican Americans (0.969 x 0.827) and 85.3% for non-Hispanic whites (0.981 x 0.870). This study was restricted to the 617 Mexican Americans and 306 non-Hispanic whites who were free of diabetes at the baseline examination conducted from 1979 to 1982 and who attended the medical examination 8 yr later. At the follow-up examination, plasma glucose was measured in the fasting state and 2 h after administration of a 75-g glucose-equivalent load (Koladex or Orangedex, Custom, Baltimore, MD). The methods used for glucose and anthropometric measurements were identical to those used in the baseline examination. A complete description of the follow-up procedures and response rates has been published previously (21). Diabetes mellitus was diagnosed according to the criteria of the World Health Organization (fasting plasma glucose level >7.8 mM and/or 2-h plasma glucose >11.1 mM; 22). In previous studies (1,11,12), we used the National Diabetes Data Group criteria (23), which require an additional glucose determination at an intermediate point between the fasting and the 2-h value. Concordance for diabetes between the two criteria in the overall San Antonio Heart Study population was 98%. Because only fasting and 2-h post-glucose load blood specimens were obtained at the follow-up examination, we used the simpler World Health Organization criteria in this study. Diabetic subjects who did not meet the World Health Organization plasma glucose criteria, but who were being treated with either oral antidiabetic agents or insulin, were also considered to have diabetes. Diabetic subjects who were not taking insulin were considered to have type II diabetes. Insulintaking diabetic subjects were also considered to be type II if they had both an age at onset of >40 yr and a BMI >30 kg/m2. The remaining insulin-taking subjects were considered to have insulin-dependent diabetes or to be unclassifiable. Subjects who had diabetes at baseline (i.e., prevalent cases) were excluded from these analyses. Ethnic-specific means for continuous variables were computed for subjects who converted to diabetes and those who remained free of disease by two-way analysis of variance (24; Table 1). Ethnic differences in diabetes incidence stratified by age, sex, level of education, and obesity were evaluated by the Mantel-Haenszel procedure (Tables 2-4). Multiple logistic regression analysis 103
INCIDENCE OF DIABETES IN MEXICAN AMERICANS
TABLE 1 Baseline characteristics of converters and nonconverters to non-insulin-dependent diabetes in San Antonio Heart Study Mexican Americans
Age (yr) M/F Body mass index (kg/m2) Education (yr)
Converters
Nonconverters
Converters
Nonconverters
Ethnicity
46.0 ± 1.7 15/25 30.6 ± 0.6 1.1 ± 1.6
41.5 ± 0.4 238/339 26.9 ± 0.2 13.0 ± 1.6
51.8 ± 4.3 1/5 26.2 ± 1.9 10.7 ± 0.2
44.5 ± 0.6 124/176 24.7 ± 0.3 14.3 ± 0.2
