Recent research suggests that the traditional measurement of obesity using only body mass index (BMI) may overlook a significant number of individuals with the condition. This updated approach to defining obesity indicates that more people may have the disease than previously believed.

Currently, it is estimated that 40% of adults in the United States have obesity. Many of these estimates are based on BMI, which calculates weight relative to height. However, BMI is increasingly seen as an inadequate measure because it does not differentiate between excess weight, body fat, and muscle or bone mass.

Last year, after much debate about recognizing obesity as a disease and determining the best way to measure it, an international commission defined “clinical obesity” as a chronic illness characterized by tissue and organ dysfunction due to excess body fat. Dr. Brian P. Lee, a hepatologist at Keck Medicine of the University of Southern California, along with his research team, investigated how this new definition could alter the classification of individuals with obesity.

Utilizing a nationwide sample of 5,600 adults, Dr. Lee and his colleagues evaluated the prevalence of excess body fat using waist and other body size measurements along with BMI, in alignment with the commission’s guidelines. They also assessed organ dysfunction and physical limitations in daily activities. Their initial findings suggested that about half of the U.S. adults currently considered overweight, but not obese by BMI standards, might be classified as having “clinical obesity.” The study also indicated that some individuals with normal BMI could have obesity.

Dr. Francesco Rubino, chair of the international Lancet Diabetes & Endocrinology Commission on obesity, appreciated the effort to gauge the disease’s true prevalence. However, he expressed concern that the study might have overestimated the condition’s prevalence. He mentioned that the study’s classification method, which included anyone with excess body fat and clinical characteristics like liver fibrosis, heart failure, or walking difficulties, contributed to this potential overestimation.

The research published in the Annals of Internal Medicine adds to the ongoing conversation about how obesity is defined and measured, highlighting the complexity of accurately identifying and addressing the disease.