In a study from the University of Colorado Boulder, scientists found excess weight or obesity boosts the risk of death by anywhere from 22% to 91%—much more than previously believed.
They also found the mortality risk of being slightly underweight has likely been overestimated.
The findings counter prevailing wisdom that excess weight boosts mortality risk only in extreme cases.
The study also shines a light on the pitfalls of using Body Mass Index (BMI) to study health outcomes, providing evidence that the go-to metric can potentially bias findings.
In the study, researchers analyzed data from nearly 18,000 people. After accounting for those biases, it estimates that about 1 in 6 U.S. deaths are related to excess weight or obesity.
Previous studies have shown that heart disease, high blood pressure and diabetes (which are often associated with being overweight) elevate mortality risk.
But very few have shown that groups with higher BMIs have higher mortality rates.
Instead, in what some call the ‘obesity paradox,’ most studies show a U-shaped curve: Those in the “overweight” category (BMI 25 to 30) surprisingly have the lowest mortality risk.
Those in the “obese” category (30 to 35) have little or no increased risk over the so-called “healthy” category (18.5-25).
And both the “underweight” (less than 18.5) and extremely obese (35 and higher) are at increased risk of death.
The current study found that a full 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 much worse health profile than those in the category whose weight had been stable.
The team pointed out that a lifetime carrying excess weight can lead to illnesses that, paradoxically, lead to rapid weight loss. If BMI data is captured during this time, it can skew study results.
Meanwhile, 37% of those characterized as overweight and 60% of those with obese BMI had been at lower BMIs in the decade prior. Notably, those who had only recently gained weight had better health profiles.
By including people who had spent most of their life at low-BMI weight in the high-BMI categories, previous studies have inadvertently made high BMI look less risky than it is.
Collectively, the findings confirm that studies have been “strongly affected” by BMI-related bias.
When re-crunching the numbers without these biases, he found not a U-shape but a straight upward line, with those with low BMI (18.5-22.5) having the lowest mortality risk.
Contrary to previous research, the study found no significant mortality risk increases for the ‘underweight’ category.
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The study was conducted by Ryan Masters et al and published in the journal Population Studies.
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