Associations between long-term exposure to ambient particulate air pollution and type 2 diabetes prevalence, blood glucose and glycosylated hemoglobin levels in China
Introduction
Type 2 diabetes mellitus (T2DM) is a long-term metabolic disorder that is primarily characterized by insulin resistance, relative insulin deficiency, and hyperglycemia. A high fasting blood glucose level was ranked as the 7th risk factor for global disease burden and accounted for 3.4 million deaths (Lim et al., 2012). It was estimated that 590 million people would be suffering from T2DM by the year 2035 (Guariguata et al., 2014). It is also an important risk factor for cardiovascular diseases, leading to enormous health consequences. Furthermore, the development of these outcomes in people with diabetes may be exacerbated by exposure to exogenous toxic factors (Zanobetti and Schwartz, 2001).
There is growing evidence from both human and animal studies suggesting that particulate matter (PM) air pollution is an important risk factor for T2DM (Balti et al., 2014, Esposito et al., 2016, Eze et al., 2015, Liu et al., 2013, Park and Wang, 2014). Dozens of cross-sectional and cohort studies have reported a positive association between long-term exposure to ambient PM and risk for T2DM (Janghorbani et al., 2014, Li et al., 2014, Wang et al., 2014), but some other studies did not find such a relationship. For example, using two large prospective cohorts, the Nurses' Health Study and the Health Professionals Follow-Up Study, Puett et al. failed to find strong evidence of an association between exposure to PM in the previous 12 months and incident diabetes (Puett et al., 2011). Additionally, studies have focused on the effects of PM on the incidence, prevalence and mortality of T2DM, but few have explored its effects on glucose homeostatic measures, such as fasting glucose and glycosylated hemoglobin (HbA1c) levels (Rajagopalan and Brook, 2012). HbA1c is a well-acknowledged marker for measuring the average plasma glucose concentration over prolonged periods, and an elevated HbA1c level denotes an increased risk of developing diabetes and its complications (Edelman et al., 2004, Gillett, 2009). Furthermore, most studies on the associations between PM and T2DM were conducted in North America and Europe, and the evidence has been very limited in developing countries, where a steep increase in T2DM was observed in the past few decades and where the air pollution level is much higher (Nicole, 2015).
As the largest developing country, China is facing a growing prevalence of diabetes and severe air pollution problems (Yang et al., 2013). Given the vast population affected by diabetes and the ubiquitous exposure to air pollution, it is of increasing public health significance to examine the impacts of PM on diabetes. Therefore, the objective of this study was to evaluate the association of long-term exposure to PM with T2DM and with fasting blood glucose and HbA1c levels in China. This study was based on a nationally representative survey of the China Health and Retirement Longitudinal Study (CHARLS) project (Zhao et al., 2014), which aimed to provide a high-quality public database with a wide range of information to facilitate needs of scientific and policy research on ageing-related issues.
Section snippets
Study population and health data
This is a cross-sectional study based on a national baseline survey within the CHARLS project. This baseline survey was conducted from June 2011 to March 2012 and included 17,708 middle-aged and elderly participants (≥ 45 years old) (Zhao et al., 2014). Briefly, these participants were selected from 150 counties or districts from 28 provinces using a four-staged, stratified and cluster sampling method. Individual information on sociodemographic characteristics, behaviors, indoor air pollution and
Descriptive statistics
The weighted descriptive statistics on sociodemographic characteristics, behaviors, and indoor air pollution of the study population (n = 11.847) are summarized in Table 1. For example, the age range of the participants was 45 to 99, with an average of 59. There were similar proportions of males and females (48.0% VS 52.0%). According to the main definition, the prevalence of T2DM in the study population was 15.8%, which was comparable to that (11.6%) reported in another nationwide survey in 2010
Discussion
To our knowledge, this was the largest epidemiological study to explore the association between long-term PM2.5 exposure and T2DM in a developing country. Our findings showed that long-term exposure to PM2.5 was positively associated with significant increases in diabetes prevalence, fasting glucose and HbA1c levels. These associations were robust to different definitions of diabetes, exposure modeling methods and the exclusion of one person from each spouse pair.
The number of studies on the
Conclusions
In conclusion, this nationwide cross-sectional study suggested that long-term exposure to PM2.5 might increase the risk of T2DM in China. Our findings demonstrated that particulate air pollution was an important modifiable environmental factor contributing to the development and prevalence of diabetes in China. Therefore, there is an urgent need to reduce particulate air pollution in this country where the prevalence of diabetes was growing rapidly.
Acknowledgements
The study was supported by the Public Welfare Research Program of National Health and Family Planning Commission of China (201502003), National Institute on Aging (1R01AG037031-03S and R03AG049144), the National Natural Science Foundation of China (71130002 and 71450001), China Medical Board Collaborating Program (13-152 and 13-154), the World Bank (7172961), the Shanghai 3-Year Public Health Action Plan (GWTD2015S04) and the National Natural Science Foundation of China (81502774).
The authors
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Cong Liu and Changyuan Yang contributed equally to this work.