Recent headlines have suggested that air pollution may contribute to growing obesity rates. It's a speculative hypothesis based on bad science.
Struggling to lose weight? Exposure to air pollution might be partially to blame. That's what recent headlines would have you believe, anyway. “A recent study reopens the conversation about obesity — and how to address it,” Desert News reported on October 27:
In a nutshell, researchers discovered that the more air pollution participants were exposed to, the likelier it was that they became obese. Specifically, women who were exposed to air pollution had a body fat increase of 4.5%
As always, there is much more to the story. Air pollution contributes to all sorts of nasty diseases, but obesity probably isn't one of them. Let's examine some of the important details that didn't make the headlines.
Pollution levels down, obesity rates up
The first and most significant problem is this: the six most common air pollutants have declined by roughly 80 percent in the US since 1970, according to the EPA, with corresponding improvements in public health. Similar trends have been observed in other developed countries. Around the same time, obesity rates began to climb. “The worldwide prevalence of obesity nearly tripled between 1975 and 2016,” the World Health Organization reported last June. In sum, we started getting fatter as the air got cleaner.
Broad trends in obesity rates and pollution levels provide helpful context, but what does the peer-reviewed literature actually show? It depends on which study you read; you can find one to support almost any conclusion because the results are inharmonious. According to a 2018 review of the available research:
Among a total of 66 reported associations between air pollution and body weight status, 29 (44%) found air pollution to be positively associated with body weight, 29 (44%) reported a null finding, and the remaining eight (12%) found air pollution to be negatively associated with body weight.
The more cynical among us could use this data to argue that pollution actually protects against obesity. If you're skeptical of that possibility, I don't blame you. The point is simply that this evidence is weak enough that it can be cited in support of hypotheses that flatly contradict each other. Even if we look at the association between specific pollutants and obesity-linked health outcomes, say particulate matter 2.5 (PM2.5 ) and stroke, we don't find clear relationships. As the authors of another 2018 paper explained:
… [P]revious meta-analyses have also reported that the direction of the association between PM 2.5 levels and the risk of different types of stroke is not consistent across studies. These meta-analyses have suggested this high heterogeneity may be attributable to differences in the definition of outcomes, different techniques for measuring the PM 2.5 , using different confounders in the models for evaluation, and applying different databases with varying levels of socioeconomic factors ...
The other crucial issue is the cross-sectional design of much of this research, which only compares transient pollution measurements to self-reported BMI rates at a single point in time. These studies are nothing more than single snapshots based on unreliable data. Pollution may still contribute to obesity and related diseases, defenders of this hypothesis could push back. Perhaps our measurement tools aren't yet sophisticated enough to clearly detect the association. Maybe. But this faith-based claim raises a critical question: if air pollution does contribute to obesity, how does it do so?
Pick a mechanism, any mechanism
Lots of mechanisms have been proposed over the years. Pollution in high enough concentrations probably does increase the risk of cardiovascular diseases, respiratory diseases, and cancer, which may have downstream effects on body weight. Another possibility is that pollution discourages outdoor exercise and, thus, caloric expenditure. Some studies have found that pollution alters the expression of genes that regulate adipose cells, resulting in higher visceral fat mass, at least in rodents. It's really anybody's guess how this works; the takeaway, for now, is that we have low-grade epidemiological studies and no solid mechanism to link pollution to obesity.
With all this in mind, the Diabetes Care paper should be interpreted with skepticism. The underlying numbers came from the Study of Women's Health Across the Nation (SWAN), which enrolled 3,302 women ages 40-55 between 1996 and 1997. Since then, the data has been mined every which way to identify correlations between various exposures and health outcomes, generating more than 600 papers.
The Diabetes Care study included just 1,654 of these individuals and analyzed their health outcomes from 2000 to 2008. Their annual air pollution exposures “were assigned by linking residential addresses with hybrid estimates of air pollutant concentrations.” At the time of publication, the paper's results were already 14 years old and based on speculative exposure data for just over 1,500 women prone to gain weight, given their age.
Additionally, the researchers "observed weaker adverse effects of PM2.5 and NO2 [Nitrogen Dioxide] on body composition in participants with more intense physical activity." This result suggests, in line with the research outlined above, that pollution has little or no effect on obesity rates. Indeed the authors themselves said as much. Citing several recent studies, they wrote that
... the benefits of physical exercise, particularly in terms of obesity and related health outcomes, may exceed the risks associated with air pollution exposure [emphasis mine].
We then face another intriguing question: if regular exercise can offset the "obesogenic" effects of pollution exposure, why all this squabbling? Do we really need to invest more time and money investigating a correlation that seems tenous at the very best?
In the 1992 film Mr. Baseball, Tom Selleck's character tries to convince the Yankees not to cut him following his paltry .235 batting average during the previous season. “Last season, I led this club in ninth-inning doubles in the month of August!” Selleck barks at his manager:
Mr. Baseball's cherry-picked statistic was obviously no excuse for his overall underwhelming performance at the plate. The researchers pushing the pollution-obesity correlation have committed the same fallacy. Yes, they found a link between the two. But that probably has more to do with their ability to run linear mixed effects models than the possibility that pollution actually makes us fat.
Until we get better data, focus on the variables you can control, like diet and exercise, and worry less about smog making you fat.