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Almost 70% of adults in the US would be deemed to have obesity based on a new definition, research suggests.
The traditional definition of obesity, typically based on having a body mass index (BMI) of 30 or greater, has long been contentious, not least as it does not differentiate between fat and muscle.
In an effort to tackle the issue, in January medical experts from around the world called for a new definition to be adopted. This would encompass people either with a BMI greater than 40; or those with a high BMI and at least one raised figure for measures such as waist circumference, waist-to-hip ratio, or waist-to-height ratio; or those with two such raised figures regardless of BMI; or those with direct measures of excess body fat based on scans.
In addition, they said obesity should be split into two categories: clinical obesity – where there are signs of illness – and pre-clinical obesity, where there are not.
Now research suggests the revamped definition could result in a dramatic rise in the prevalence of obesity among adults in the US.
Dr Lindsay Fourman, the first author of the study, from Mass General Brigham in the US, said the increase in obesity prevalence based on the new definition was striking, and was largely driven by people who would not have been considered to have obesity based on their BMI alone.
“Recognising people with [this type of] obesity can lead to more accurate health risk stratification,” she said. “For example, someone with BMI 23 but excess abdominal fat could benefit from lifestyle interventions such as improving diet and increasing physical activity, even though their BMI is in the “normal” range. Their physician might also more closely monitor for obesity-related complications such as pre-diabetes or fatty liver.”
Writing in the journal Jama Network Open, Fourman and colleagues describe how they applied the traditional and new definitions of obesity to a nationally representative group of 301,026 participants, aged 18 to 80, who had enrolled in a US-based health study between the end of May 2017 and the end of September 2023.
While traditional BMI-based criteria would mean 128,992, or 43%, of the participants were deemed to have obesity, the team found this increased by nearly 60% using the new definition, encompassing 206,361, or 69%, of participants. For those aged 70 years or older, prevalence under the new definition was 78%.
Almost all those with obesity under the traditional definition also had obesity under the new definition, with only 678 participants no longer considered to have obesity. However, the new definition meant 78,047 participants who would not previously have been considered to have obesity based on BMI alone would be deemed to have the condition.
“Our data show that these individuals have a heightened risk of organ dysfunction and long-term health complications, suggesting that their classification as having obesity is clinically appropriate,” said Fourman.
Of those deemed to have obesity without a high BMI, 48% had clinical obesity. For those who had obesity based on their BMI together with other measures, this figure was 56%.
The researchers noted that while the prevalence of obesity under the new definition was similar between men and women, men showed a greater increase upon the use of the new definition, adding that while prevalence increased by a similar number of percentage points across racial groups, the greatest relative rise was among Asian individuals.
Naveed Sattar, a professor of metabolic medicine at the University of Glasgow, who was not involved in the study, described it as thought-provoking and said further research was now needed, but noted that the newly proposed definition of obesity had yet to be adopted.
“Regardless of definitions, it’s clear that current obesity levels pose major challenges especially in high-income countries like the UK, with rising weight levels underscoring the urgent need for more effective strategies to support healthier living,” he said.
