Prelude to Disease? $\mathrm{P}{\mathrm{M}}_{2.5}$ and Markers of Diabetes Risk in Nondiabetic Adults

The list of health conditions believed to be affected by air pollution has grown well beyond lung and cardiovascular disease. It now includes diabetes, whose prevalence in the United States has nearly quadrupled during the last four decades.1 In this issue of Environmental Health Perspectives, a new study estimates the effect of air pollutants on markers of diabetes risk in more than 4,000 nondiabetic Germans between 45 and 75 years of age.2

First author Sarah Lucht, a PhD student at the Heinrich Heine University in Düsseldorf, Germany, says, “The goal of our study was to home in on the relationship between exposure duration and glucose metabolism before the onset of diabetes. Since no previous studies had systematically evaluated different temporal windows from acute to long-term exposure, important associations may have been missed.”

Wherever you find combustion engines, you will find particulate air pollution. Image: © justhavealook/iStock.

The study participants lived in three cities in the Ruhr region of Germany. Air quality was assessed with a comprehensive, validated model that provides hourly pollutant estimates around residential addresses. These estimates are based on emissions and meteorological data as well as knowledge about the transport and activity of chemicals in the atmosphere.3 The authors of the new report estimated that exposure to fine particulate matter ($\mathrm{P}{\mathrm{M}}_{2.5}$) over a period of months was associated with higher levels of blood glucose and glycated hemoglobin A1c (HbA1c).

The researchers used mixed linear regression to account for correlated levels measured during baseline (2000–2003) and followup (2006–2008) exams. An increase in estimated $\mathrm{P}{\mathrm{M}}_{2.5}$ exposure of $5.7\mathrm{\mu g}/{\mathrm{m}}^3$ during a 28-day period was associated with a $0.9\mathrm{mg}/\mathrm{dL}$ average increase in blood glucose. An increase in estimated exposure of $4.0\mathrm{\mu g}/{\mathrm{m}}^3$ during a 91-day period was associated with an average 0.07% increase in HbA1c. The association was the strongest for exposure windows of 28–60 days for blood glucose, and of 75–105 days for HbA1c. That finding is consistent with the distinct nature of these two metabolic markers: Blood glucose fluctuates from day to day, whereas HbA1c is a more stable marker of average glucose levels.

Associations were adjusted for factors potentially related to both air pollution and glucose metabolism, including socioeconomic status, physical activity, nutrition, body weight, smoking, traffic noise, and traffic density. According to the authors, a plausible underlying mechanism is that inhalation of $\mathrm{P}{\mathrm{M}}_{2.5}$ causes oxidative stress and inflammation in the lungs that may spread to adipose tissue and impair glucose metabolism long before a clinical diagnosis of diabetes.

Sung Kyun Park, an associate professor of epidemiology at the University of Michigan, says the study advances the field because it correlates air pollutants with continuous markers of high blood glucose and provides a comprehensive evaluation of relevant exposure windows. He says that increases of $4\mathrm{–}6\mathrm{\mu g}/{\mathrm{m}}^3$ in $\mathrm{P}{\mathrm{M}}_{2.5}$ are meaningful for the United States. Because air pollution is such a ubiquitous exposure, he explains, the estimated shift in the blood glucose distribution suggests an important effect size at the population level that could result in many additional diabetes cases. Park was not involved in the research.

For Tanya Alderete, a postdoctoral fellow at the University of Southern California who was not involved in the study, the take-home message is that moderate levels of air pollution may have adverse effects on metabolically healthy individuals. “The fact that an association with the pollutants themselves persisted even after adjustment for numerous confounders is very compelling,” says Alderete, who has studied air pollution and markers of glucose metabolism in Latino children.4 “It adds further support to the hypothesis that the inhalation of fine particulate matter increases the risk of diabetes.”

Biography

Silke Schmidt, PhD, writes about science, engineering, and the environment from Madison, Wisconsin.