Diabetes incidence and long-term exposure to air pollution: a cohort study

OBJECTIVE

Animal and cross-sectional epidemiological studies suggest a link between air pollution and diabetes, whereas the limited prospective data show mixed results. We studied the association between long-term exposure to traffic-related air pollution and incidence of diabetes.

RESEARCH DESIGN AND METHODS

We followed 57,053 participants of the Danish Diet, Cancer, and Health cohort in the Danish National Diabetes Register between baseline (1993–1997) and 27 June 2006. We estimated the mean levels of nitrogen dioxide (NO₂) at the residential addresses of the cohort participants since 1971 and modeled the association between NO₂ and diabetes incidence with a Cox regression model, separately for two definitions of diabetes: all cases and a more strict definition where unconfirmed cases were excluded.

RESULTS

Over a mean follow-up of 9.7 years of 51,818 eligible subjects, there were 4,040 (7.8%) incident diabetes cases in total and 2,877 (5.5%) with confirmed diagnoses. Air pollution was not associated with all diabetes cases (hazard ratio 1.00 [95% CI 0.97–1.04] per interquartile range of 4.9 μg/m³ mean NO₂ levels since 1971), but a borderline statistically significant association was detected with confirmed cases of diabetes (1.04 [1.00–1.08]). Among confirmed diabetes cases, effects were significantly enhanced in nonsmokers (1.12 [1.05–1.20]) and physically active people (1.10 [1.03–1.16]).

CONCLUSIONS

Long-term exposure to traffic-related air pollution may contribute to the development of diabetes, especially in individuals with a healthy lifestyle, nonsmokers, and physically active individuals.The prevalence and incidence of type 2 diabetes are rising rapidly in both the developed and developing world, presenting one of the greatest contributors to the global burden of the disease (1). The diabetes epidemic is in large part attributable to established causes related to modern lifestyle including obesity, physical inactivity, and the growing aging populations (1). Environmental exposures linked to industrialization and urbanization, such as air pollution, have not been considered risk factors for diabetes until recently (2). In the U.S., the prevalence of diabetes correlated with the release of toxicants into the air (3), whereas diabetic people appeared more vulnerable than nondiabetic people to cardiovascular health effects associated with exposure to air pollution (4). Diabetes and cardiovascular diseases share many risk factors, and diabetic people are at a highly increased risk for heart or circulatory disorders (5). The central biological mechanisms of air pollution damage to the heart and blood vessels include inflammation (6), which is also believed to be involved in the promotion of insulin resistance and type 2 diabetes (7). An enhanced association between air pollution and inflammation, endothelial dysfunction, prothrombotic changes, and altered heart rate variability was found in diabetic people (6). A plausible biological mechanism of air pollution promoting diabetes was provided by Sun et al. (8), showing that exposure to particulate air pollution caused increased blood glucose, inflammation in adipose tissue, and insulin resistance in high-fat diet–fed mice. Recent study confirmed that prolonged exposure to air pollution leads to insulin resistance and impaired glucose tolerance in rats and that this association is not limited to high-fat diet rats (9). Epidemiological evidence is sparse. Short-term exposure to air pollution was linked to exacerbations of diabetes leading to death (10–14) and hospitalizations (13). Prevalence of diabetes was linked to air pollution (14,15). Two prospective diabetes studies investigated the link with air pollution, with one reporting significant associations among a small number (n = 87) of women (16) and another failing to detect association in two large cohorts, except with a single traffic proximity proxy in women (17). Limited and mixed evidence precludes conclusions about causality between air pollution and diabetes and merits more study.We studied the association between traffic-related air pollution levels at the residence and the risk for diabetes in an elderly Danish cohort and tested for an effect modification by lifestyle, education, and comorbid conditions.