Most Americans would be considered obese under new guidelines

Almost 70 per cent of US adults would be classified as having obesity under a new definition proposed by medical experts — a major increase from 43 per cent using current BMI standards.

The proposed obesity definition expands beyond body mass index (BMI) to include waist measurements and body fat scans, identifying many more people at risk of obesity-related illnesses such as diabetes and heart disease.

Researchers from Mass General Brigham applied the new criteria to data from more than 300,000 Americans aged 18 to 80, collected between 2017 and 2023. They found that obesity prevalence would rise by nearly 60 per cent overall, with the greatest relative increase among Asian participants and a larger rise in men than in women.

BMI, which is calculated using height and weight, has long been criticised for failing to distinguish between fat and muscle. For instance, athletes may have high BMIs due to muscle mass, while others with “normal” BMIs may carry unhealthy amounts of abdominal fat.

In January, an international group of experts proposed a broader definition of obesity — including those with a BMI over 40, those with high BMI and raised waist measurements, or those with excess body fat identified through scans, even if their BMI appears normal.

They also suggested separating obesity into “clinical obesity”, where illness is present, and “pre-clinical obesity”, where excess fat exists without symptoms.

“Recognising people with this type of obesity can lead to more accurate health risk stratification,” said Dr Lindsay Fourman, the study’s first author.

“For example, someone with BMI 23 but excess abdominal fat could benefit from lifestyle changes such as improving diet and increasing physical activity, even though their BMI is in the ‘normal’ range. Their physician might also more closely monitor for obesity-related complications such as pre-diabetes or fatty liver.”

The researchers found that 78,047 participants who would not previously have been classified as obese based on BMI alone would now be included under the new definition. Almost all those considered obese under current criteria would remain so.

Among those newly identified, 48 per cent already showed signs of obesity-related illness, while 56 per cent of those who met both BMI and other measures had clinical obesity.

For adults aged 70 and above, prevalence reached 78 per cent, suggesting BMI alone may underestimate obesity-related risks in older adults. As people age, muscle mass typically declines while fat increases, meaning BMI can fail to reflect true metabolic risk.

“Our data show that these individuals have a heightened risk of organ dysfunction and long-term health complications, suggesting that their classification as having obesity is clinically appropriate,” said Dr Fourman.

While overall obesity prevalence was similar between men and women, men showed the greater increase under the new criteria. Prevalence rose by similar margins across racial groups, but the largest relative increase was seen among Asian participants, who often develop metabolic problems at lower BMIs than other ethnicities.

Naveed Sattar, professor of metabolic medicine at the University of Glasgow, who was not involved in the study, described the research as “thought-provoking” but said the new definition has yet to be officially adopted.

“Regardless of definitions, it’s clear that current obesity levels pose major challenges, especially in high-income countries like the UK, where rising weight levels highlight the urgent need for better strategies to support healthier living,” he said.

The findings suggest millions more Americans could be considered at risk of obesity-related health problems under the revised definition — a change that could have wide implications for healthcare screening, insurance coverage and public health policy if implemented.