A new analysis has revealed that nearly 70 percent of adults in the United States could be classified as obese under recently proposed definitions that move beyond the traditional body mass index (BMI). This expanded definition incorporates broader indicators of health risk such as waist circumference, waist-to-height ratio, and waist-to-hip ratio, providing a more comprehensive understanding of body fat distribution and its impact on health.
Published in JAMA Network Open, the study emphasizes that focusing solely on BMI—a metric derived by dividing an individual’s weight in kilograms by height in meters squared—fails to paint a complete picture of metabolic health. Researchers from Mass General Brigham say their findings signal a major public health shift, one that could redefine how obesity is diagnosed, prevented, and treated.
Beyond BMI: Rethinking How We Measure Health
For decades, BMI has served as the dominant measure in assessing body weight relative to height. However, its simplicity is also its greatest limitation. While BMI can indicate average population trends, it doesn’t distinguish between muscle and fat mass, nor does it reveal where fat is stored in the body—a key factor in determining health risk.
As previous research has confirmed, visceral fat (fat stored around internal organs) poses far more danger to heart health and metabolism than fat located on the hips or limbs. A normal BMI can therefore conceal dangerous fat accumulation, while a “high” BMI might misclassify muscular individuals as obese.
In recognition of these shortcomings, the King’s College London-led commission earlier this year proposed an updated system for classifying weight and health. Their recommendations centered on incorporating anthropometric measures that better capture fat distribution patterns and corresponding risk levels.
Testing the New Framework
To evaluate the real-world effects of this new definition, researchers analyzed health records from 301,026 adults in a large longitudinal database. Participants were tracked over an average of four years. Under traditional BMI-based categories, 42.9 percent of participants qualified as obese. But when researchers applied the new multi-factor definition—including waist and height ratios—the share surged to 68.6 percent.
That means nearly three out of every four American adults could meet clinical obesity criteria under the updated framework. Among individuals over age 70, the figure climbed even higher, with almost 80 percent falling into the obese category.
“We already thought we had an obesity epidemic,” said Dr. Lindsay Fourman, lead author and endocrinologist at Mass General Brigham. “But these findings are astounding. If 70 percent of adults are classified as having excess fat, we urgently need to rethink our treatment priorities and prevention strategies.”
A Better Reflection of Health Risk
The expanded criteria not only capture more people but appear to do a better job identifying who faces serious health complications. The study found people newly classified as obese under the revised definition were also at significantly increased risk of diabetes, cardiovascular disease, and premature death compared to those without obesity.
Dr. Steven Grinspoon, senior author and endocrinologist at the same institution, noted that this may help bring appropriate intervention to individuals who previously fell through diagnostic gaps. “Seeing higher rates of heart disease and diabetes in this newly defined obesity group points to a major improvement in identifying who truly needs early treatment or monitoring,” he said.
Broad Institutional Support for the Shift
The proposed definition has now been endorsed by 76 medical and research organizations, including the American Heart Association and The Obesity Society. However, it has not yet been officially adopted by US health agencies or government advisory panels.
If implemented, the classification change could influence clinical guidelines, prescription decisions for new anti-obesity medications, and insurance coverage for preventive care. It might also prompt earlier intervention for at-risk groups once overlooked under BMI-driven protocols.
Public health experts believe such a shift could reshape how obesity is discussed, measured, and managed—moving toward a more individualized approach focused on biological and structural factors rather than scale weight alone.
Why BMI Doesn’t Tell the Whole Story
BMI remains a convenient screening tool because it’s easy to calculate and widely validated across large populations. But its single-number simplicity often obscures the complexities of human physiology. Age, sex, muscle mass, genetics, and race all affect how BMI relates to true body fat percentage.
A BMI of 25, for example, can represent body fat ranging from 14 to 35 percent for men and 26 to 42 percent for women, depending on physical composition and other factors. Athletes, aging adults, and certain ethnic groups may therefore fall into the “overweight” or “obese” category despite having normal metabolic profiles.
Moreover, contemporary research suggests that some individuals with BMI above the traditional cutoff remain metabolically healthy—displaying normal blood pressure, cholesterol, and insulin sensitivity levels. By contrast, people with “normal” BMIs can harbor harmful fat clustering around their organs, increasing risk for metabolic disorders despite appearing lean.
Implications for Public Health and Medical Practice
The reclassification carries profound implications for healthcare delivery and health policy. Clinicians may need to expand screening procedures to include waist measurements and other simple physical assessments alongside traditional BMI calculation. Such inclusion could identify patients vulnerable to type 2 diabetes, fatty liver disease, and heart failure earlier in life.
Dr. Fourman and colleagues urge caution, however, noting that labeling nearly 70 percent of the population as obese could also challenge current healthcare capacity. “We need more nuanced strategies,” she said, “that combine prevention, education, and equitable access to emerging obesity treatments without adding stigma.”
As powerful new GLP-1-based medications such as semaglutide (Ozempic) and tirzepatide (Mounjaro) become widely available, defining the right threshold for medical intervention becomes increasingly important. The updated framework could help direct these costly treatments toward those with measurable metabolic risk rather than weight status alone.
The Future of Obesity Assessment
The researchers hope their findings will support a more scientifically grounded, patient-centered approach to body composition assessment. “Body composition matters—it’s not just pounds on a scale,” Fourman emphasized. “Identifying excess body fat more accurately helps clinicians focus on meaningful interventions that improve long-term health outcomes.”
While BMI remains a valuable population-level tool, the medical community is steadily moving toward precision-based indicators that reflect real physiological risk. These could include a combination of waist-to-height ratio, relative fat mass, and visceral fat imaging, offering a more comprehensive understanding of metabolic health.
If adopted, the new standards could reshape both clinical practice and public health messaging in the coming years—possibly redefining how Americans think about obesity, health, and the body itself.
