New criteria point to even higher rates of obesity among Americans

Obesity affects about 1 in 8 people worldwide–and that number, which includes children and adolescents, is growing ever higher as countries experience shifts in diet and physical activity. Described by the WHO as a “global epidemic,” obesity is associated with death by noncommunicable diseases such as cardiovascular disease, diabetes, cancer, neurological disorders, chronic respiratory diseases, and digestive disorders. 

Since the 1970s, body mass index (BMI), a measure derived from a person’s weight and height, has been the primary diagnostic for defining and classifying obesity. Simple, inexpensive, and widely available, BMI has served as a useful screening tool for identifying individuals at risk of obesity-related complications. But BMI, originally developed on a population of white men, has always had limitations—and in January 2025, Lancet Diabetes & Endocrinology published new recommendations and diagnostic criteria by a commissioned body of international researchers to provide a more comprehensive framework for defining obesity. 

While BMI correlates with population-level risk, it does not distinguish between fat mass and lean mass, capture body fat distribution, or reflect the functional impact of excess adiposity on organs and daily activities. BMI also performs inconsistently across racial and ethnic groups; for example, some populations, such as non-Hispanic Asians and Hispanics, are more prone to accumulating visceral fat at lower BMIs. These individuals may face substantial metabolic risk despite having a “normal” or “overweight” BMI. At the same time, others with high muscle mass may be misclassified as having obesity based on BMI alone.

Instead, the commission proposed incorporating multiple body size measurements, direct assessments of body fat, and evidence of obesity-related organ dysfunction or activity limitations. They also developed a more nuanced framework that distinguishes between preclinical and clinical obesity, rather than treating obesity as a single, uniform diagnosis. By establishing objective criteria for disease diagnosis, the commission hoped to improve clinical decision making, interventions, and strategies for improving public health.

Profound impacts of new criteria

Now a new study by researchers at the University of Chicago suggests the epidemiologic impact of these criteria could be profound: applying the Lancet commission definition to nationally representative US data from 1999 to 2023 increases the estimated prevalence of obesity from about 40% of adults to nearly 60%.

Using data from the National Health and Nutrition Examination Survey (NHANES), the study tracked obesity prevalence over more than two decades. Because not all measurements were consistently available across survey cycles, the analysis anchored prevalence estimates to measures that were collected throughout the entire period, particularly waist circumference and waist-to-height ratio. 

“Even using this limited set of measurements, obesity prevalence increased substantially,” says Jennifer Hwang, DO, who led the study. While most individuals who already met BMI-based obesity criteria also met the new definition. The biggest change came from people who previously fell below the BMI threshold—those with normal or overweight BMIs but elevated abdominal adiposity or other high-risk body measurements.

While the new criteria are informative on a population level, Hwang notes that the guidelines need refinement for clinical use with patients.

“Obesity medicine has been moving away from BMI-only definitions for years, and this framework represents a meaningful step toward a more pathophysiology-based understanding of obesity. The Lancet commission is trying to address gaps by incorporating additional body measurements and direct fat assessments, which I think is the right direction,” she said.

“That said, there are still significant issues that need clarification. The list of obesity-related organ dysfunctions is extensive and often vague, and it’s unrealistic to expect primary care clinicians to assess all of these reliably. There’s also no clear guidance on how to determine whether a given dysfunction is causally attributable to obesity.”

Pressing policy questions

Beyond diagnosis, the findings raise pressing policy questions. Hwang’s study provides a first estimate of how large the affected population might be under this new definition. Expanding the population defined as having obesity has enormous implications for health systems, especially as highly effective—but still costly—obesity medications become more widely available.

“If the eligible population grows this much, the issue isn’t just cost-effectiveness at the individual level,” Hwang said. “It’s fiscal impact at the national level. Even as medication prices decrease, treating a much larger population would place substantial strain on health systems and public payers like Medicare and Medicaid.” This reality underscores the need for better staging and prioritization to ensure that treatment resources are directed to those at greatest risk of adverse outcomes. 

In the meantime, before making any policy recommendations, Hwang plans to continue testing the results of the new criteria by using microsimulation modeling to project long-term outcomes.

“Do those classified as having obesity under the new criteria—particularly those with normal BMI—would they actually experience worse long-term health outcomes? Do they have higher risk of diabetes, cardiovascular disease, or mortality compared to those without obesity? These are important questions that we need to address,” she said.

“The Lancet commission’s excess adiposity criteria classify nearly three out of five U.S. adults as having obesity, and most already have some form of organ dysfunction. This raises urgent questions about how we identify risk, stage disease, and prioritize treatment.”