Research published Monday found that adding waist-based measurements to BMI could classify more than 75% of U.S. adults as having obesity
A new definition of obesity could nearly double the prevalence of U.S. adults with the condition, according to a study published Monday in JAMA Network Open. For decades, doctors have relied on body mass index, a tool that uses height and weight to estimate body fat, to determine obesity. A team of researchers from Beth Israel Deaconess Medical Center, Harvard University, Yale University and Yale New Haven Health found that BMI alone may significantly underestimate how many U.S. adults have obesity.
Using a new definition that includes waist-based measurements, the team found that more than 75% of adults may meet criteria for obesity compared to 40% when using BMI alone. The research used data from the National Health and Nutrition Examination Survey representing 237.7 million adults between 2017 and 2023.
Lancet Commission proposes revised definition
Earlier this year, The Lancet Diabetes & Endocrinology Commission, a partnership between the medical journal and the health clinic King’s Health Partners Diabetes, Endocrinology and Obesity, proposed a revised obesity definition that included waist circumference, waist-to-hip ratio and waist-to-height ratio. More than 70 medical organizations around the world have endorsed the definition but its use in practice has not been evaluated until this study.
Dr. Erica Spatz, a cardiologist at Yale School of Medicine and co-author of the new study, noted that BMI is the standard measure for determining criteria for being obese and the most widely known metric. However, BMI alone does not account for adipose tissue, which stores energy, insulates organs and produces hormones that regulate appetite.
Adipose tissue drives health risks
Adipose tissue is less visible than other types of fat but is more associated with high blood pressure, diabetes and heart disease according to the research team. The study applied the Lancet Commission’s proposed obesity criteria along with BMI to data from more than 14,000 participants. They found an estimated 75.2% of U.S. adults met criteria for obesity using BMI and additional body measurements compared to 40% when BMI alone was reviewed.
Nearly four in 10 adults with a normal BMI were found to have excess body fat when waist-based measures were also applied. This finding highlights the limitations of relying solely on height and weight calculations to assess health risks associated with obesity.
Age and ethnicity show variations
The study found that obesity prevalence increased sharply with age and was higher among Hispanic adults, though rates were similar between men and women. Because nearly all adults age 50 and older were classified as having obesity under the new definition, age-specific thresholds are needed as well. The authors highlighted the limitations of BMI as a standalone screening tool and suggest that incorporating waist measurements could improve obesity detection.
The research team stressed that because the new criteria will likely raise the current obesity estimates, more research should be done before broadly adopting the new definition. The expanded classification system represents a significant departure from current diagnostic standards that have been in place for decades.
Clinical implications require consideration
Dr. Fatima Cody Stanford, an associate professor of medicine and pediatrics at Massachusetts General Hospital and Harvard Medical School who was not involved in the study, stated that the findings provide a sense of how significant obesity is in the United States. The research highlights obesity as by far the most significant chronic disease in human history, with the Lancet Commission giving clinicians a sense of how much needs to be done to improve treatment and ensure proper training to identify not only obesity itself but the over 230 chronic diseases associated with it.
The expanded definition could have far-reaching implications for healthcare systems, insurance coverage, and public health initiatives designed to address obesity-related conditions. Medical professionals will need to determine how to implement the new criteria while maintaining practical screening methods.
Implementation challenges remain
The authors acknowledged several limitations in their study. The dramatic increase in obesity prevalence under the new definition raises questions about practical implementation in clinical settings where time and resources are already strained. Additional research is needed to establish appropriate cutoff points for different age groups and potentially for different ethnic populations given the variations observed in the current study.
Healthcare providers will need to balance the improved accuracy of the expanded definition with the ease of use that made BMI such a widely adopted tool. The transition to incorporating waist-based measurements alongside BMI will require updated training, equipment and protocols across medical practices, hospitals and public health programs throughout the country.