How dangerous is it to be obese, really? A new study published in the journal Proceedings of the National Academy of Sciences says we can't quite answer that, at least not with current statistics. Study authors from the Boston University School of Public Health (BUSPH) and the University of Pennsylvania found past studies linking obesity to mortality were vastly underestimated. This discovery brings attention to the flaws in how obesity research is conducted — one of the biggest being when participants are weighed.

For the study, researchers analyzed a large pool of data collected by the National Health and Nutrition Examination Survey between 1988 and 2010. Participants were asked to report the heaviest they've ever weighed, and how much they weighed at the time of the survey. Of those in the normal weight range at the time of the survey, 39 percent had once been overweight or obese in the past. This indicated they had once been an unhealthy weight and lost the weight, transitioning them into a healthier, normal weight range.

The finding also, however, distorted the statistics. It's possible a person who was once overweight or obese may have an increased risk of mortality due to obesity-related heart damage or diabetes risk. So by focusing only on the amount participants weighed at the time of the survey, they missed the hidden mortality risk increase of the 39 percent among those in the normal weight range.

"The risks of obesity are obscured in prior research because most of the studies only incorporate information on weight at a single point in time," explained the study's lead author Andrew Stokes, an assistant professor of global health at BUSPH, in a press release.

If researchers had instead looked back and took into account participants' weight history, they may have seen a stark difference between those who were always a normal weight and those with a history of being overweight or obese. Instead, the two groups were mixed together under the normal weight category. And when compared to those who were overweight or obese at the time of the survey, there's less of a mortality rate difference.

"The simple step of incorporating weight history clarifies the risks of obesity and shows that they are much higher than appreciated," Stokes said. "These distortions make overweight and obesity appear less harmful by obscuring the benefits of remaining never obese."

According to the Centers for Disease Control and Prevention, more than one-third of American adults are obese, which increases their risk for some of the leading causes of preventable death, including heart disease, stroke, type 2 diabetes, and certain types of cancer. Researchers urge more studies to be performed using weight histories, adding that they should also include tobacco smoking history, in order to improve the accuracy and precision of statistical evidence for mortality in the future.

Source: Stokes A and Preston SH. Revealing the burden of obesity using weight histories. Proceedings of the National Academy of Sciences. 2016.