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These rats' babies showed similar issues: Eight weeks after birth, the pollution-exposed female rats were 10% fatter than their clean-air counterparts, while males were 18% chubbier.



Yowza.



I got in touch with Junfeng "Jim" Zhang at Duke University, one of the lead authors of the study. Jim, I said, could it be that it's Beijing's air, and not my shameful lack of willpower, that's to blame for my Chinese spare tire?



"Biologically, you have evidence," he said. (Jim, did I mention I love you?)



Although the primary factor in weight gain is still eating too much and exercising too little, he said, pollution is a serious behavioral and biological factor.



"Think about how many days children [in Beijing] are advised not to go outside and exercise" because of pollution, he said. "That contributes to weight gain, and then there's this evidence that you inhale these things and it gets your body messed up, with the fat metabolism and sugar metabolism. That will slow things down, and energy will accumulate in the body."



OK, I said, but how is all this really working in my body?



"The particles go to the lungs, and the lung will get an impact; we measure this as lung inflammation," he explained. "The inflammation of the lung spills over; think of this as chain reaction. The lung is not isolated, it's related to the systemic circulatory system. We measure molecules in blood and we see whole body inflammation from the dirty air exposure.



"Then we look into the fat tissue.... Fat tissue also shows a higher level of inflammation. That can directly translate into body weight gain" as more lipids are deposited into fat tissue.



Adding insult to injury, metabolic function — basically how efficiently we break down our food and turn it into energy — is also affected.



"We saw that in the dirty-air rats, it increases their LDL — that's bad, because it increases the fat level in the bloodstream, and also decreases insulin metabolism, which is related to the sugar metabolism; you don't want too much sugar accumulating in the body," he said. "So dirty air causes more fat to accumulate in the body, and also decreases sugar metabolism, which is directly related to weight gain."



I called Marie Ng, the lead author of the Lancet study and a former University of Washington researcher who now works at IBM's Watson Health, to see what she thought about the fat rats.



"It's a very interesting study," she agreed, though she cautioned that "generalizing from rats to humans, we have to be careful."



"Obesity is really complex," she added, noting that a host of factors contribute to the condition, from increased wealth and increasingly sedentary lifestyles, to the introduction of fast food and the way pollution limits outdoor activities.



Still, she said, it is curious that South Korea and Japan have industrialized and modernized even more than China has but have not experienced the same explosion of obesity as China.



"There might be something unique about China," she said, "but I still have not found a study that can pinpoint certain aspects of it."



Whether air pollution might be that "X Factor" remains to be seen. In the meantime, said Zhang, research indicating that smog might make you fat might help persuade heretofore recalcitrant Beijing residents to don masks or install air purifiers in their homes and offices.



"We know air pollution is bad for lungs… but there are people who say, 'I'm tough, maybe I'm even a smoker, why should I worry?' But if you say air pollution will make you fat if you are continuously exposed to it, maybe people will respond to that. Of course, cancer is more threatening, but to some people, appearance means a lot."



As for me, I've already decided to see whether cutting back on smog can help me slenderize.



So pass me a new mask, and crank up the air purifier to 11. Swimsuit season is just around the corner.