Study questions use of BMI, finding overweight people no more likely to die than ‘healthy’ counterparts

There’s an often-cited statistic in the health world – that at least 2.8 million adults die each year as a result of being overweight or obese. This would make being overweight or obese the fifth leading risk for global deaths.

But is that actually true?

A new study published in PLOS ONE looked at more than half a million US adults and found that for overweight people and some obese people, this is seemingly not the case.

“The US prevalence of overweight and obesity has risen dramatically over the last 25 years, with more than half of all adults in the US now overweight or obese,” the researchers write in their new paper.

“It is well-established that elevated BMI can contribute to several cardio-metabolic conditions, including diabetes, hypertension, and coronary artery disease, which are among the leading causes of premature death in the US.”

Most research into BMI and all-cause mortality – that is death from any cause – has been undertaken on data from the 1900s, particularly from the 60s to the 90s. But the US population has changed since then. For example, there’s been a 10-year increase in life expectancy, and the US is much more ethnically diverse.

So, the researchers looked at newer data – the 1999–2018 National Health Interview Survey, tracking 554,332 adults for an average of 9 years. Within that timeframe, 75,800 people who were part of the study died.

But the results don’t seem to back up the global death statistics.

You can see in the figure below that those who were underweight (a BMI under 18.5) had some of the highest rates of mortality, with this then dropping for those at a normal weight (between 18.5 – 24.9). Those who were overweight (between 25 to 29.9) had some of the lowest numbers, with the curve only slowly rising again for obese people (a BMI above 30).

This was the case whether or not the people had ever smoked.

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The figures above show the hazard ratios for BMI categories, relative to a ‘healthy’ BMI of 22.5–24.9. Credit: PLOS ONE

“The risk of all-cause mortality was similar across a wide range of BMI categories … These results persisted after restriction to healthy never-smokers and exclusion of subjects who died within the first two years of follow-up,” the researchers wrote in their paper.

“Older adults showed no significant increase in mortality between BMI of 22.5 and 34.9, while in younger adults this lack of increase was limited to the BMI range of 22.5 to 27.4.”

Similar results have been seen in some studies around the world, and it’s come to being referred to as the obesity paradox – a seemingly lower mortality rate for overweight or obese people within certain subpopulations. It’s not yet known if this paradox is real, and if it is real, why it happens at all.

“There are several plausible reasons why participants with higher BMI (25.0–34.9) may have all-cause mortality risk similar to those with conventionally normal BMI (18.5–24.9),” the researchers write.

“Overweight individuals may have survival advantages in various adverse circumstances, such as critical illness, major morbidities, and severe infection that are not offset by the increased risk of chronic metabolic diseases, overweight individuals without disease may be metabolically healthy and have a more favourable body composition consisting of higher lean mass.”

These results will feed into a growing movement to stop using BMI as an individual measure of health. The study suggests that waist circumference, weight change over time, and abdominal fat are all more accurate measures of individual health.

The researchers say their work highlights “the potential limitations of BMI in capturing true adiposity and limitations of its clinical value independent of traditional metabolic syndrome criteria.”

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