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Dear Doctor: A recent study found that air pollution increases the risk of Type 2 diabetes. How can that be? Air pollution can't possibly raise blood sugar levels.
Dear Reader: I can understand your disbelief. My first thought when I saw this study was that it was correlative, not causative. In other words, air quality is simply worse in cities, which are more likely to have large numbers of people with lower socioeconomic status, who, in turn, tend to have greater rates of obesity and diabetes. Then I looked more closely.
Air pollutants assessed in the studies include nitrogen dioxide and particulate matter between 0.1 to 2.5 microns in size (PM 2.5). Early epidemiologic studies showed a correlation between areas of greater pollution and diabetes, but did not control for socioeconomic status. Then came a 2010 study that used census data from three counties in the United States, and assessed both rates of diabetes and Environmental Protection Agency data regarding PM 2.5 pollution.
The authors took into account such socioeconomic variables as median income, high school completion, male sex and ethnicity -- all markers for diabetes risk. The authors found a 16 to 20 percent increase in the prevalence of diabetes in the areas that had the highest amounts of PM 2.5 versus those areas with the lowest amounts, even with all other factors being equal.
Then came this year's study published in the journal Diabetes that included 314 obese Latino children in Los Angeles. The authors analyzed where the children lived; the amount of pollution to which they were exposed, measured by both nitrogen dioxide and PM 2.5; and their parents' socioeconomic data. The children were followed for 3.4 years, during which time they underwent tests of blood sugar, two-hour glucose tolerance, insulin and insulin response to glucose. The authors found that the markers for future diabetes were much greater in those exposed to greater amounts of pollutants. This was independent of socioeconomic status, and even independent of the level of obesity.
The cause isn't completely clear yet. Studies in rodents exposed to pollutants have shown increased inflammation within fat cells, accumulation of cholesterol in the liver and decreased ability of muscles to use sugar. This leads to both metabolic dysfunction and obesity. Further, pollutants can lead to systemic inflammation that in turn leads to insulin resistance, and thus the inability to bring sugar into the cells of the body, leaving it to sit in the bloodstream.
What is clear is that there does appear to be an association between pollution and diabetes, but more studies are needed to evaluate the degree of the association. Regardless, pollution is obviously unhealthy and needs to be controlled within society. The measurements of pollution provided in these studies come from the Environmental Protection Agency, whose existence is to safeguard the health of current and future societies.