A new study suggests that excess weight is associated with a higher mortality risk than previously thought, as earlier research using body mass index to study health outcomes may have produced biased results.
Extensive research has shown that conditions associated with being overweight, such as heart disease or diabetes, elevate mortality risk. Meanwhile, very few studies demonstrated that the risk increases in people with higher body mass index (BMI).
The so-called "obesity paradox" has been observed: individuals in the "overweight" (BMI 25 to 30) or "obese" (BMI 30 to 35) categories have little or no increased mortality risk compared to those with a healthy weight (BMI 18.5 to 25). The studies showed that only individuals in "underweight" (BMI less than 18.5) and "extremely obese" (BMI 35 and higher) groups are at elevated mortality risk.
However, BMI is based on weight and height only and doesn’t account for differences in body composition or how long a person has been overweight, says Ryan Masters, associate professor of sociology at the University of Colorado Boulder. For example, actor Tom Cruise, a muscular man who weighed 201 pounds at one point, had a BMI of 31.5, putting him in the "obese" category.
"It isn’t fully capturing all of the nuances and different sizes and shapes the body comes in," says Masters. When bodybuilders gain muscle, their number on the scale goes up, but they aren't overweight.
In his study considering these nuances, findings of which were published in the journal Population Studies, Masters examined data from 17,784 people, including 4,468 deaths, collected over 27 years.
He found that 20% of the sample characterized as "healthy" weight had been in the overweight or obese category in the decade prior. When set apart, this group had a substantially worse health profile than those with stable weight.
Masters notes that carrying excess weight for an extended period can lead to illness, which may result in rapid weight loss. If BMI data is captured during this time, it can skew research results.
Meanwhile, 37% of individuals in the "overweight" category and 60% of those in the "obesity" group had lower BMIs in the decade prior. As a result, those who had only recently gained weight were in better health.
The new study suggests that those with low BMI (18.5 to 22.5) have the lowest mortality risk, with no significant increase in the "underweight" category. Contrary to previous studies, estimating two to 3% of U.S. adult deaths were due to high BMI, Masters’ study pegs the toll at eight times that.
- University of Colorado Boulder. Excess weight, obesity more deadly than previously believed.
- Population Studies. Sources and severity of bias in estimates of the BMI–mortality association.