A new definition of obesity would classify more than three-quarters of US adults as having the condition, according to a recent study.
Using the traditional measure – a body mass index (BMI) of 30 or higher – around 43 per cent of Americans are currently considered obese. But when researchers applied a broader definition that also includes waist-to-height and waist-to-hip ratios, the estimated prevalence jumped to 75.2 per cent.
The analysis, led by scientists from Yale School of Medicine and Harvard University, suggests that relying on BMI alone may substantially underestimate how many people have excess body fat.
“BMI is a simple calculation based on weight and height,” study lead Dr Nora Al-Roub from the Yale School of Public Health told BBC Science Focus. “Although it’s easy to calculate, it doesn’t tell us where fat is stored in the body or how much of a person’s weight is fat versus muscle.”
Under the revised definition, all adults with a BMI of 30 or above were classed as having obesity. But so were 38.5 per cent of those with a BMI below 25, which is typically considered healthy.
“The increase highlights how many people with a normal or slightly elevated BMI still carry unhealthy levels of body fat,” Al-Roub said.
The study highlighted how these newly reclassified obese people might not need immediate medical attention, but do have an elevated risk of other issues, such as diabetes or heart problems. As such, they could benefit from early preventative measures.
The guidelines used were first proposed in January 2025 by the Lancet Diabetes & Endocrinology Commission, led by Prof Francesco Rubino from King's College London. More than 70 medical organisations have since endorsed the framework.
According to Rubino, however, the recent analysis slightly misconstrues the guidelines. The extra measurements, he said, were designed to identify people who "can have obesity-level adiposity despite a BMI just below the obesity threshold.”
He points out that while thresholds can vary by sex and ethnicity, a BMI of 25 is well below what would usually be considered obese.
However, Al-Roub said that the study was not an attempt to rewrite the definition. “Our goal was not to reinterpret the guidance,” she told BBC Science Focus, “but to understand what happens to population-level obesity estimates when these criteria are applied uniformly in a real-world dataset.”
What are the new guidelines?
The new guidelines recommend using BMI as a screening tool, to establish who might be at risk, then applying several additional measurements. The exact thresholds depend on age, sex and ethnicity, but some commonly used cutoffs are:
- Waist circumference over 102cm for men, and 88cm for women
- Waist-to-hip ratio over 0.9 for men and 0.85 for women
- A waist-to-height ratio over 0.5
To be classed as obese, a person would then need to meet one of the following criteria:
- BMI over 40
- BMIs between 39 and 30 (or an ethnic specific threshold) and one of the additional measurements
- BMI below, but near, 30 (or ethnic specific threshold) and at least two of the additional measurements.
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