Can smog make you fat?
new study has found that lab rats exposed to Beijing’s notoriously polluted air grew significantly more obese than rats exposed to filtered air.
“No data have directly supported a link between air pollution and non-diet-induced weight increases,” reported the authors of the study, which will be published in the March issue of the Journal of the Federation of American Societies for Experimental Biology. “In a rodent model, we found that breathing Beijing’s highly polluted air resulted in weight gain and cardiorespiratory and metabolic dysfunction.”
Researchers randomized pregnant rats into two groups. One group was placed in a chamber with unfiltered air coming directly from outside. The other group was placed in a chamber outfitted with a high-efficiency particulate filter.
Even though the rats were fed the same diet, the group exposed to pollution was 7 percent heavier on day 14 and 15 percent heavier on day 19 than rats in the filtered chamber. The lungs of the unfiltered group were 25 percent heavier on day 19 and displayed inflammation around blood vessels and the bronchial wall.
These rats also had 50 percent higher LDL cholesterol, 46 percent higher triglycerides, and 97 percent higher total cholesterol and increased levels of insulin resistance, a precursor of type 2 diabetes, than the control group.
Similar results were found at eight weeks of age among offspring of the pregnant rats, which were kept in the same chambers as their mothers.
There is emerging evidence that chronic exposure to particulate matter with a diameter smaller than 2.5 microns, or PM2.5, leads to increased lipid deposits in fatty tissue, a precursor of type 2 diabetes, the authors wrote.
How does that happen? There are at least two mechanisms at play, according to senior author Junfeng "Jim" Zhang, a professor of global and environmental health at Duke University.
The first is inflammation.
“We have tissue pathology and different sets of markers showing that dirty air causes more lung inflammation compared to controls,” Zhang said. “That inflammation we think can spill over to the whole body system, and we saw inflammation indicators in the blood increase after dirty-air exposure and also fat-tissue inflammation. Particle exposure caused the entire body to increase its inflammation status, and chronic inflammation is a well-established risk factor for obesity.”
The second mechanism is increased LDL cholesterol in the blood and increased insulin resistance, meaning the ability of the body to break down sugars is impeded, increasing the likelihood for weight gain, Zhang said.
Of course, rats are not people, and more research is needed in humans.
“If translated to and verified in humans, these findings will add support to the urgent need to reduce air pollution exposure, given the growing burden of obesity and related metabolic abnormalities in today’s highly polluted world,” the authors wrote.

Zahn said he was considering a large-scale study of people in highly polluted areas versus cleaner locations, but with similar diets, lifestyles, and ages, to compare cholesterol, body mass index, and other inflammation markers.
Norman Edelman, senior scientific adviser to the American Lung Association, called the paper a “neat little study” but said it was not clear if the results can be translated directly to humans.
Some epidemiological studies have suggested that air pollution is a risk factor for metabolic syndrome—a condition marked by high blood pressure and blood-sugar levels, excess body fat around the waist, and abnormal cholesterol—and diabetes, “but they don’t all agree,” Edelman said.
“It probably requires a relatively high level of pollution, and whether or not pollution levels in the United States are a risk factor or not is unclear. But there probably is some sort of’s biologically plausible.”
That doesn’t mean the new study will bring about policy changes, at least in the U.S.
“Nobody is going to take a study on rats exposed to pollution in Beijing to create policy in this country,” Edelman said. “But it is a cautionary tale.”
Zhang said the research might help encourage further pollution-control measures in heavily polluted places, such as Beijing.
But he worries about the influence it could have on individuals.
“I don’t want this to become an excuse, where someone says, ‘Hey, I’m living in a polluted place, so I’m getting fat,’ ” he said. “The most important factors in obesity are diet and physical activity.”
Even so, people should try to reduce exposures through air purifiers and face masks, Zhang said. “Protect your lungs and heart,” he said, “but also protect your body so it’s not messed up by metabolic dysfunction.”