Excess weight boosts mortality risk more than previously believed, while the mortality risk of being slightly underweight has likely been overestimated, according to new research from the University of Colorado Boulder. 

Obesity boosts the risk of death by anywhere from 22% to 91%, the research findings, which were published Feb. 9 in the Population Studies journal, concluded. 

This information countered previous beliefs that excess weight boosts mortality risk only in extreme cases, according to a news release about the study.

The findings came from statistical analyses of nearly 18,000 people and pointed to pitfalls of using Body Mass Index (BMI) to study health outcomes, according to the CU Boulder release.

“Existing studies have likely underestimated the mortality consequences of living in a country where cheap, unhealthy food has grown increasingly accessible and sedentary lifestyles have become the norm,” author Ryan Masters, associate professor of sociology, said in the release. “This study and others are beginning to expose the true toll of this public health crisis.”

Masters spent his career studying mortality trends and has always been skeptical of the previous belief that people in the “overweight” and “obese” BMI categories have the lowest mortality risk, whereas the “underweight” and “extremely obese” BMI categories are at increased mortality risk.

BMI is based on weight and height only and does not account for differences in body composition or how long a person has been overweight, according to the release.

Masters looked at data for 17,784 people, including 4,468 deaths, from 1988 to 2015 from the National Health and Nutrition Examination Survey and discovered that 20% of the people characterized as “healthy” weight had been in the overweight or obese category in the decade prior, according to the release.

The aforementioned group had a worse health profile than people whose weight had been stable, Masters discovered. A lifetime of carrying excess weight leads to illnesses that lead to rapid weight loss, according to the release.

“I would argue that we have been artificially inflating the mortality risk in the low-BMI category by including those who had been high BMI and had just lost weight,” Masters said. 

Thirty-seven percent of people characterized as overweight and 60% of people characterized as obese had been at lower BMIs in the decade prior. People who recently gained weight had better health categories and, by including them in the high-BMI categories, studies made high BMI look less risky than it is, according to Masters.

Without the BMI-related biases, Masters found that those with low BMIs had the lowest mortality risk.

Previous researched estimated that about 2-to-3% of adult deaths in the U.S. were due to high BMI. Master’s study estimates the deaths to be about eight times that, according to the release.

Masters hopes his research will make scientists “extremely cautious” when making conclusions based on BMI, according to the release. He also hopes the study will bring attention to a public health crisis “fueled by an unhealthy or ‘obesogenic’ environment in the U.S.,” according to the release.

The full study is available in Population Studies.