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By Mandy Oaklander
March 17, 2015
TIME Health
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Anyone who’s considered lighting up knows smoking’s skinny-making reputation, and a new study of 80,000 people shows there’s truth to the claim. Researchers found that smokers weighed about 5 pounds less than people who had never smoked, according to new research in the International Journal of Epidemiology. The cause of that lighter weight, the researchers say, is tobacco.

Those findings are a direct contradiction to several observational studies that have linked smoking to just the opposite: higher body weight and BMI. But researchers at Copenhagen University Hospital wanted to look at the link genetically, in a way that wouldn’t be plagued by confounding lifestyle factors that often go along with smoking. They took weight and BMI measurements of 80,342 people, along with blood samples that they analyzed for DNA.

They looked at a genetic variant associated with higher tobacco consumption, close to a gene called CHRNA3—”the smoking craving gene,” says study author Børge G. Nordestgaard, MD, professor at the University of Copenhagen in the department of clinical biochemistry. Smokers with this genotype weighed almost three pounds less than smokers who didn’t inherit this genetic variant. But in people who had never smoked or formerly smoked, there was no link between CHRNA3 and a lower body weight.

“That’s really the proof that smoking causes it,” Nordestgaard says.

That doesn’t mean that smoking will give you a better figure. In the study, smoking only affected total body weight, not body shape or fat distribution.

The weight loss effect may be due to a laundry list of chemicals in cigarettes, the authors say—some studies have found nicotine to suppress appetite and increase resting metabolic rate. “There’s a possibility that many of these chemicals may influence weight in some pathway we don’t know about yet,” Nordestgaard says.

That’s obviously no reason to start—or continue—smoking, the researchers caution. “From what we know so far, the hazards of smoking much overweigh the slight benefit of having a lower body weight,” Nordestgaard says. “But when smokers tell you they won’t stop smoking because they’re afraid of gaining weight, I think it’s important to know that this is real—so we can try at the same time to help them quit smoking and keep a lower body weight.”

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