Obesity With a Normal BMI? Study Suggests It’s Common

Over a quarter of U.S. adults with a normal body mass index (BMI) met recently proposed criteria for clinical obesity, a national cross-sectional study found.

Among adults with a BMI in the normal range of 18.5-24.9, an adjusted 26.1% had clinical obesity by the Lancet Diabetes & Endocrinology commission criteria, defined by abnormal anthropometric measurements with organ or physical dysfunction, researchers led by Brian Lee, MD, of the University of Southern California in Los Angeles, reported in the Annals of Internal Medicine.

Clinical obesity prevalence jumped to 50.3% for those in the overweight BMI category (25-29.9), 65.6% for those with class I obesity (BMI 30 to <35), 77.8% for class II obesity (BMI 35 to <40), and 85.3% for class III obesity (BMI ≥40).

The Lancet commission pitched the new diagnostic framework in January 2025, advocating for a more nuanced approach than provided by the ratio of height to weight that defines BMI. The commission proposed that clinicians verify obesity status using at least one additional anthropometric measure — such as waist circumference, waist-to-hip ratio, or waist-to-height ratio — or direct fat mass measurements via dual-energy x-ray absorptiometry (DEXA) scans or bioimpedance.

According to the commission, clinical obesity is considered a state of ongoing illness involving tissue or organ dysfunction caused by excess adiposity. This dysfunction could include conditions like hypertension, knee pain, chronic fatigue, and more. Clinical obesity is separate from pre-clinical obesity, which is characterized by excess adiposity but no related ongoing illness.

Lee’s group found that 78% of adults had excess adiposity if defined using two or three abnormal body measurements. This is nearly double the prevalence detected by traditional BMI-based definitions, Lee’s team pointed out.

“These findings suggest that incorporating multiple anthropometric indicators may detect persons with physiologic consequences of excess adiposity who would otherwise be missed using BMI alone,” they wrote.

Though widely used for decades, defining obesity solely on BMI has come under fire in recent years because it may over- or underestimate body fat.

“BMI is problematic because it does not specifically measure body fat and instead reflects total body weight, which includes muscle and bone,” Lee said in a statement. “So a muscular person can have a very high BMI but not have excess fat, while someone without much muscle can have a normal BMI but have excess fat causing health problems.”

“Many people assume that if their BMI says they are not obese, they don’t have to worry about the many health problems linked to obesity,” he continued. “Our findings show that millions of Americans may already have obesity-related health impacts and may be missing needed health interventions.”

The silver lining, Lee noted, is that obesity is highly treatable through lifestyle changes, medication, or both. “The earlier we identify people at risk, the better chance we have of improving long-term health and quality of life,” he said.

However, a group of experts from the Endocrine Society recently warned that this revised framework could overcomplicate routine care by requiring clinicians to capture more physical measurements. They also argued that splitting obesity into clinical and preclinical categories could potentially bar some patients from accessing early pharmacological treatments, such as popular weight-loss medications, widening healthcare disparities.

While Lee’s team advocated for moving beyond BMI, they acknowledged that future studies need to determine whether screening via the Commission’s proposal actually improves downstream health outcomes or simply increases overdiagnosis and overtreatment.

“Given that current clinical guidelines for screening of obesity-related medical conditions and thresholds for treatment (for example, with incretins) are based on BMI, these findings, if adopted, have important implications for screening and clinical interventions,” they noted. “Further studies should assess the safety and efficacy of such interventions among persons with clinical obesity without elevated BMI.”

The researchers analyzed National Health and Nutrition Examination Survey (NHANES) responses from 5,642 nonpregnant U.S. adults ages 20 and older between 2021 and 2023. Respondents averaged 48.7 years of age, 60.6% were white, and 50.6% were female.

The authors noted that the study has limitations, particularly that NHANES data lacks the granular clinical detail required to fully operationalize the Lancet Commission’s proposal. As a result, the team likely underestimated the true prevalence of clinical obesity, which can vary depending on how adiposity and organ and physical dysfunction are defined.