A new Harvard and Mass General Brigham study suggests that nearly 70 percent of U.S. adults could be classified as obese under updated medical guidelines that go beyond body mass index, or BMI, to include waist and fat distribution.
Researchers say the broader definition could better identify hidden health risks, especially for diabetes and heart disease, among people who appear to have a “normal” weight.
The findings, published this month, come as obesity continues to rise nationwide and disproportionately affects Black Americans. According to the U.S. Department of Health and Human Services Office of Minority Health, about 50 percent of non-Hispanic Black adults are classified as obese, the highest rate of any racial group in the country.
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The Harvard team analyzed data from more than 300,000 participants in the National Institutes of Health’s All of Us Research Program, according to Newsweek. They found that the prevalence of obesity in the United States jumped from 40 percent to 68.6 percent when the new definition was applied, a 60 percent increase.
Dr. Lindsay T. Fourman, an endocrinologist and professor at Harvard Medical School, told Newsweek that BMI alone does not give a full picture of body fat or health risk.
“Traditionally, obesity has been defined by a high BMI, where BMI reflects weight relative to height,” Fourman said. “However, BMI is an imprecise measure of body composition as it does not distinguish between fat and muscle or indicate fat distribution.”
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The new definition includes waist circumference and waist-to-hip ratio to better capture body composition. People may now be classified as having obesity if they show excess abdominal fat even with a normal BMI.
“The latter group, which we term ‘anthropometric-only obesity,’ represents a major paradigm shift,” Fourman said. “It recognizes that even individuals with normal weight may face obesity-related health risks if they have excess abdominal fat.”
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What the new definition means for health equity
Experts say the new definition could have major implications for how obesity is diagnosed and treated, especially in communities of color, Newsweek noted.
Black Americans are more likely to experience conditions linked to obesity, such as hypertension, Type 2 diabetes, and heart disease. They also face higher exposure to social and environmental factors that increase obesity risk, including limited access to healthy foods, fewer safe spaces for exercise, and higher stress levels.
A 2023 CDC report found that black adults had the highest obesity rates in 38 states and territories. Public health experts say that expanding the medical definition could help identify those at risk earlier and improve access to care.
Dr. Armando Castro-Tie, chair of surgery at South Shore University Hospital Northwell Health, told Newsweek the change is overdue.
“The key takeaway is we’re really trying to look at what obesity means and address those individuals who are clinically obese and would be considered so by using more than just BMI,” Castro said.
He added that the current approach has long been too narrow. “It doesn’t allow individuals to get the care that we know can be of benefit to them,” he said. “Certain medications, certain access to different kinds of treatment, things like that.”
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How Black Americans can lower risk
While researchers continue to debate how obesity should be defined, doctors agree that prevention starts with everyday habits. Choosing whole foods like fruits, vegetables, and lean proteins over processed meals can make a major difference.
Staying active through regular walking, dancing, or even household chores can help maintain a healthy weight and improve heart function.
Getting enough sleep and managing stress are also important. Chronic stress can raise cortisol levels, which may lead to fat storage around the waist. Limiting sugary drinks and monitoring portion sizes can help reduce calorie intake without strict dieting.
For those already diagnosed with obesity or related conditions, regular checkups with a healthcare provider are key. Early screening for high blood pressure, diabetes, and cholesterol can catch problems before they become severe.
