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obesity epidemic 'toll' hidden by reliance on bmi: study

researchers say the use of body mass index to measure health is masking the impact of weight on health.

by dave yasvinski

feb 28 2023
read time 4 minute read
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a new study has found that obesity and excess weight are far more dangerous than previously thought and may raise the risk of mortality by as much as 91 per cent.

the research, published in the journal population studies , cast a harsh glare on the conventional wisdom that excess weight only presents a mortality risk in extreme cases. it also questions the wisdom of using body mass index (bmi) as a metric of human health.

“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,” said ryan masters , author of the study and an associate professor of sociology at cu boulder. “this study and others are beginning to expose the true toll of this public health crisis.”

obesity is a ‘public health crisis’

although many studies have shown that high blood pressure , heart disease and diabetes all raise the risk of mortality, few have shown that groups with higher bmis face similar risks. instead, most studies have revealed a u-shaped curve that has become known as the “obesity paradox.” according to the paradox, people in the “overweight” category (bmi 25 to bmi 30) have the lowest risk of mortality and “obese” groups (bmi 30 to bmi 35) have only a small or slight increase in risk over the “healthy” category (18.5 to 25). those considered “underweight” (bmi less than 18.5) and “extremely obese” (bmi 35 or higher) face the greatest increased risk of death.

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“the conventional wisdom is that elevated bmi generally does not raise mortality risk until you get to very high levels and that there are actually some survival benefits to being overweight,” said masters. “i have been suspicious of these claims.”

although viewed as an important metric of health, bmi is based only on weight and height and does not consider how long a person has carried excess weight or any differences in body composition .

“it’s a reflection of stature at a point in time. that’s it,” masters said. “it isn’t fully capturing all of the nuances and different sizes and shapes the body comes in.”
to see what would happen when this information is incorporated, the researcher examined the national health and nutrition examination survey (nhanes) from 1988 to 2015, a database encompassing 17,784 people and 4,468 deaths. his analysis revealed that a full 20 per cent of people characterized as having a “healthy” weight by bmi were considered overweight or obese 10 years earlier. when these people were isolated and studied together, they were found to have a considerably worse health profile than those in the same category who had maintained a stable weight over time.

studies relying on bmi data likely to have skewed results

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when you consider that a lifetime of carrying excess weight can lead to illnesses that result in rapid weight loss, masters said, studies that rely on bmi data are likely to have skewed results.
“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 recently,” he said.
similarly, he found that 37 per cent of those considered overweight and 60 per cent of those categorized as obese in the survey had recorded lower bmis 10 years earlier. those who had only recently gained weight had healthier profiles than those who maintained their excess weight over time.
“the health and mortality consequences of high bmi are not like a light switch,” said masters. “there’s an expanding body of work suggesting that the consequences are duration-dependent.”
studies that put people who have typically carried a low bmi into high-bmi groups have made having a high bmi not seem as risky as it really is, he said. according to the researcher, studies have been “significantly affected” by bmi-related bias.
when masters recompiled the numbers without this bias, he ended up with a straight upward line instead of the u-shape associated with the obesity paradox. those with a low bmi (18.5 to 22.5) had the lowest mortality risk and there were no significant risks associated with the underweight category.
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and while conventional wisdom attributes roughly two to three per cent of adult deaths to having a high bmi, masters’ study found the number to be eight times greater. he hopes researchers will exercise caution in the future when drawing conclusions based on bmi and that people will become aware of the threat obesity presents to human health.
“for groups born in the 1970s or 1980s who have lived their whole lives in this obesogenic environment, the prospects of healthy aging into older adulthood does not look good right now,” he said. “i hope this work can influence higher-level discussions about what we as a society can do about it.”
 

dave yasvinski is a writer with  healthing.ca

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