A new study suggests that body mass index, the tool doctors have relied on for decades to measure obesity, is significantly undercounting the number of Americans who actually have the condition. The findings, published June 1 in the Annals of Internal Medicine, conclude that about a quarter of people with a normal BMI reading may already meet the clinical standard for obesity, and that half of those classified as overweight should be reclassified entirely.
According to Healthline, researchers from Keck Medicine at the University of Southern California analyzed data from more than 5,600 participants with an average age of 49. Each participant had been tested for BMI and also had hip and waist circumference measurements taken, including the waist-to-hip ratio, known as WHR.
The results were striking. Researchers concluded that 26% of people with a normal BMI and 50% of people with an overweight BMI actually qualified as having obesity when abdominal fat measurements were factored in.
The lead researcher on the study, Brian P. Lee, MD, a hepatologist and liver transplant specialist with Keck Medicine and the study's principal investigator, pointed to a core flaw in how BMI works. "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."
The practical consequences of that flaw are significant. Patients who are misclassified based on BMI may not qualify for pharmacologic or surgical treatments for obesity. Their doctors may also not flag them as needing lifestyle changes. Lee addressed this directly. "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 said in a statement. "Our findings show that millions of Americans may already have obesity-related health impacts and may be missing needed health interventions."
David Cutler, MD, a family medicine physician at Providence Saint John's Health Center in California, who was not involved in the study, noted that BMI's limitations show up even in elite athletes. Based on BMI alone, some professional athletes could be classified as overweight or obese even when they are in peak physical condition. "All you have to do is watch a football game to see that," he told Healthline.
The researchers argue that measurements of abdominal fat, particularly the waist-to-hip ratio, are more reliable indicators of obesity and its associated health risks than total body weight. Excess fat concentrated around the abdomen is more closely tied to chronic conditions such as type 2 diabetes, cardiovascular disease, and liver disease than overall body weight alone.
The study adds to a growing body of evidence that has been pushing back against BMI as a primary diagnostic tool. Medical professionals and researchers have raised concerns about it for years, citing its inability to distinguish between fat mass and lean mass, and its inconsistent reliability across different body types and ethnic groups. The new findings put a number on the scale of potential misdiagnosis, suggesting the problem affects millions of people across the United States who may be moving through the health care system without receiving appropriate interventions.
