Nature study finds BMI alone misdiagnoses obesity and urges waist measurement as well

What the study found

Researchers led from King's College London, with a team that includes José Manuel Fernández-Real of the Girona biomedical research institute, analysed 844,108 adults in China, the United Kingdom, the United States and Spain. Their central finding is that BMI gets obesity wrong in both directions when a waist measurement is added to the calculation. According to The Independent, BMI missed obesity-level body fat in up to 8.6% of participants and labelled up to 3.1% as obese when their fat levels were normal. The Guardian reports a wider missed range of 5-9% of adults and says BMI wrongly diagnoses obesity in as many as 3%. For the UK alone, the study suggests 2.4 million people could carry dangerous levels of body fat that BMI does not pick up, while 1.3 million people classed as obese may have no excess fat or excess risk.

Mortality data add weight to the case. ABC reports that excess weight was linked to a higher risk of death only when waist size was measured alongside BMI, never with BMI on its own. La Vanguardia reports that ten-year mortality is 12% lower among people who are overweight by BMI but lack visceral obesity than among people of normal or low weight.

Hidden obesity missed by BMI (up to)8.6%

Overdiagnosed as obese by BMI (up to)3.1%

Hidden obesity missed by BMI (up to)

8.6 %

Overdiagnosed as obese by BMI (up to)

3.1 %

Data from the article text

How the new method works

The approach starts with BMI but adds a second measurement. A person with a BMI above 40 counts as obese. A BMI of 30 or more counts as obese when the waist is raised, and so does a BMI of 25 to 29.9 when both waist size and waist-to-height ratio are high. The thresholds cited for waist size are more than 102cm for men and 88cm for women, with a waist-to-height ratio of about 0.57. A Lancet Commission of more than 50 international experts put the idea forward in January 2025, while the European Association for the Study of Obesity had proposed a new diagnostic framework for adults in 2024. A second step separates clinical obesity, where excess fat already impairs organ function, from pre-clinical obesity, where the fat raises risk but organs still work normally. ABC reports that clinical obesity accounts for 23% to 31% of obesity diagnoses, a prevalence of 5.6% to 10.9% in the general population.

Why researchers say BMI falls short

Francesco Rubino, a professor at King's College London and one of the lead authors, made the core objection at a briefing ahead of publication.

“BMI doesn't tell you anything about body fat content.”

He argued that the same BMI can describe very different health situations, because it does not show whether a person's organs are working or whether the individual is ill. He also said BMI thresholds are used to decide who receives treatment and who gets coverage for it, even though they do not accurately measure individual health.

Spain's figures

EL PAÍS reports that in Spain the waist-based measure increases the number of people classed as obese by almost 40% compared with the BMI method. La Vanguardia breaks the picture down further: more than three million people in Spain carry excess abdominal fat without excess weight and so go unidentified as obese, while nearly half a million are labelled obese by weight without excess abdominal fat. In Catalonia those groups number more than 500,000 and more than 70,000. The Catalan sample was drawn from patients in primary care, and José Manuel Fernández-Real, co-director of the international study, warns that it is not perfectly representative of the whole population.

What happens next

The study's authors argue that more precise identification could improve patient outcomes and help shape health policy, according to El Mundo. Fernández-Real told La Vanguardia that a home waist measurement is a simple and useful screening tool, but that a health professional should confirm any diagnosis because measurement errors can occur. A clinician would also need to establish whether excess fat is already affecting a patient's health, which requires a fuller assessment.