The rising prevalence of obesity and high prevalence of diabetes among Pima Indians suggest that the incidence of diabetes

OBJECTIVE The rising prevalence of obesity and high prevalence of diabetes among Pima Indians suggest that the incidence of diabetes has risen over time. We examined trends in the incidence rate of type 2 diabetes among Pima Indians between 1965 and 2003. RESEARCH DESIGN AND METHODS Incidence rates were computed independently in three 13-year time periods in Pima Indians aged ≥5 years old. Diabetes was defined by the presence of at least one of two criteria 1) 2-hour plasma glucose concentration ≥200 mg/dl (11.1 mmol/l), or 2) hypoglycemic treatment. RESULTS Among 8,236 subjects without diabetes at baseline, 1,005 incident cases occurred during follow-up. Age-sex-adjusted incidence rates of diabetes were 25.3 cases/1,000 pyrs (95%CI=22.5-28.0) in 1965-1977, 22.9 cases/1,000 pyrs (95%CI=20.0-25.8) in 1978-1990 and 23.5 cases/1,000 pyrs (95%CI=20.5-26.5) in 1991-2003 (p=0.3). The incidence rate in subjects aged 5-14 years was 5.7 (95%CI=1.9-17.4) times as high in the last as in the first period, but the rate declined in those 25-34 years old (incidence rate ratio=0.6, 95%CI=0.4-0.8). Sex-adjusted prevalence increased significantly over time only in those 5-24 years of age (ptrend<0.0001). CONCLUSION The overall incidence of diabetes among Pima Indians remained stable over the past four decades, with a significant rise occurring only in the youth. Diabetes prevalence is increasing worldwide, especially in populations transitioning from traditional to modern lifestyles (1, 2). While prevalence and incidence of type 2 diabetes are highest among adults, youth are increasingly affected (3, 4). Whether the rising prevalence of type 2 diabetes is due to increasing incidence, declining mortality, or both, is unclear (5-7). Projections based on increasing life expectancy, population growth, and progressive urbanization predict substantial increases in the prevalence of diabetes for decades to come (5). These estimates may be too low, however, since they do not account for the increasing frequency and magnitude of obesity and other major risk factors for diabetes. We examined secular trends in the incidence rate of type 2 diabetes over the past 40 years in the Pima Indians, a population with a very high prevalence and incidence of diabetes (8). Previous observations (9, 10) suggest that diabetes was either rare or largely unrecognized among Pimas around the 1900s. At that time, increasing settlement of the area by people of European derivation led to diversion of the Pimas’ water supply and disruption of their agriculture (10). The loss of water resulted in curtailment of subsistence farming and led to fundamental changes in their way of life (11). In the late 1930s, a review of medical records from the hospitals serving the population identified 21 Pima Indians with diabetes. The author concluded that the prevalence of diabetes was similar to that in the US population (12). By the 1950s, many more Pimas were known to have diabetes (13) and since then, a rising prevalence of obesity (14) suggests that the incidence of diabetes might continue to rise. Since 1965, systematic testing for diabetes was performed as part of a longitudinal study, using the same methods for diagnosing diabetes throughout the study. RESEARCH DESIGN AND METHODS Pima and the closely related Tohono O’odham (Papago) Indians who live in a part of the Gila River Indian Community in Arizona and are ≥5 years of age participate in a research examination approximately every 2 years regardless of health. These biennial examinations include an oral glucose tolerance test (OGTT), measurement of height and weight, and assessment for diabetes complications. Venous blood is collected in NaF 2-h after ingestion of 75 g of carbohydrate. Plasma was separated and stored at –20°C until glucose measurement by the alkaline potassium ferricyanide method (Technicon AutoAnalyzer, Tarrytown, NY) through October 1991 and by the hexokinase method (Ciba-Corning, Palo Alto, CA) thereafter. Glucose concentrations were 3.6 mg/dl higher, on average, by the latter method in 194 samples measured simultaneously by both methods. Because this difference was small and quarterly external proficiency testing confirmed the accuracy of both methods over time, no adjustment was made between the methods. The 75 g load was used throughout the study, regardless of age and body size. Since 1975, all subjects were asked to fast overnight prior to the OGTT. Body mass index (BMI) was defined as weight divided by the square of height (kg/m2). The study population included subjects aged ≥5 years old who were at least half-Pima or Tohono O’odham heritage, attended research examinations, and resided in the community at any time during follow-up. Statistical analysis Incidence rates of diabetes were computed independently in three 13-year time periods (January 1965-December 1977, January 1978December 1990, January 1991-December 2003). Diabetes was defined by at least one of