New definition of obesity indicates dramatic increase in prevalence of obesity

The prevalence of obesity in the US could rise sharply under a new definition of obesity released earlier this year by the Lancet Diabetes and Endocrinology Commission. Researchers from Mass General Brigham found that when applying the new criteria, which expands upon the traditional use of body mass index (BMI) to include measures of body fat distribution, the prevalence of obesity increased from about 40 percent to about 70 percent among over 300,000 people included in their study.

New definitions of preclinical and clinical obesity (Credit: The Lancet Diabetes & Endocrinology)

The rise was even more pronounced among older adults. Additionally, the researchers found that those newly added individuals also had a higher risk of adverse health outcomes.

“We already thought we had an obesity epidemic, but this is astounding,” said co-first author, Dr Lindsay Fourman, an endocrinologist in the Metabolism Unit in the Endocrinology Division of the Mass General Brigham Department of Medicine. “With potentially 70 percent of the adult population now considered to have excess fat, we need to better understand what treatment approaches to prioritise.”

Traditionally, obesity has been defined by BMI, which estimates body fat based on a person’s weight and height. But other anthropomorphic measures - such as waist circumference, waist-to-height ratio, or waist-to-hip ratio - may further account for fat distribution and aid in differentiation between muscle and fat mass.

Under the new framework, a person is classified as having obesity if they have a high BMI plus at least one elevated anthropometric measure (a condition the authors term “BMI-plus-anthropometric obesity”), or if they have a normal BMI and at least two elevated anthropometric measures (a condition termed “anthropometric-only obesity”). The new definition also distinguishes between preclinical and clinical obesity with clinical obesity defined as the presence of obesity-related physical impairment or organ dysfunction. At least 76 organisations have endorsed the new guidelines.

The study analysed participants in the National Institutes of Health All of Us Research Program’s cohort of over 300,000 Americans. Obesity prevalence was 68.6 percent with the new definition, versus 42.9 percent under the traditional BMI-based definition. This increase was entirely driven by inclusion of individuals with anthropometric-only obesity. Obesity rates varied by sex, race and especially by age affecting nearly 80 percent of adults over 70.

Importantly, the study found that those with anthropometric-only obesity – who would not have been classified as having obesity by the traditional definition – had a higher risk of diabetes, cardiovascular disease and mortality than people without obesity. About half of all individuals who met the new obesity criteria had clinical obesity and this proportion was only slightly lower in the anthropometric-only obesity group, compared with the BMI-plus-anthropometric obesity group.

“We have always recognised the limitations of BMI as a single marker for obesity because it doesn't take into account body fat distribution,” said senior author, Dr Steven Grinspoon, Chief of the Metabolism Unit in the Endocrinology Division of the Mass General Brigham Department of Medicine. “Seeing an increased risk of cardiovascular disease and diabetes in this new group of people with obesity, who were not considered to have obesity before, brings up interesting questions about obesity medications and other therapeutics.”

The researchers emphasise that further studies are needed to better understand the causes of and optimal treatments for anthropometric-only obesity. The research team previously developed a therapeutic that reduces waist circumference and plans to explore the utility of different treatment strategies in this newly defined population.

“Identifying excess body fat is very important as we’re finding that even people with a normal BMI but with abdominal fat accumulation are at increased health risk,” Fourman said. “Body composition matters, it’s not just pounds on a scale.”

The findings were featured in the paper, ‘Implications of a New Obesity Definition Among the All of Us Cohort’, published in JAMA Network Open. To access this paper, please click here