BMI May Miss Dangerous Body Fat, UK Biobank Study Finds

Most people are familiar with the process: step onto a scale, receive a BMI score, and be classified as underweight, normal, overweight or obese.

For decades, clinicians have used BMI to assess weight-related health risks. However, emerging research indicates that it may overlook many people carrying hazardous amounts of body fat.

A study involving almost half a million people has found that body fat percentage and waist circumference could forecast health risks more precisely than BMI on its own.

BMI cannot detect harmful fat

BMI has a fundamental limitation: it does not directly measure body fat.

Belgian statistician Adolphe Quetelet developed the formula in the 1830s. It was designed to describe average body size in populations rather than to diagnose illness in individual people.

Nevertheless, it eventually became one of the most commonly used measures in medicine.

The difficulty is that BMI counts all body weight alike. It merges muscle and fat into one figure, despite the very different effects these tissues have on the body.

A muscular athlete and an inactive adult can have identical BMI values but entirely different health risks.

Nor can BMI indicate where fat is held in the body, even though this detail is more important than many people appreciate.

Doctors want better measures

In January 2025, The Lancet Diabetes & Endocrinology Commission urged adoption of a new obesity definition centred on excess body fat, rather than BMI alone.

The report said doctors should use BMI alongside further measures, including waist circumference and body-composition assessments.

The aim is to recognise people whose health is endangered by harmful fat accumulation, including those whose BMI seems normal. The new UK Biobank study examined this proposal.

“Obesity is increasingly recognized as a disease, but BMI is often used alone when diagnosing obesity without considering broader health,” says Sophie Gunnarsson from Lund University.

“The method has several limitations, and our study provides new evidence that integrating body fat percentage and waist circumference captures risk dimensions missed by BMI alone.”

Body fat reveals risk

The researchers examined health information from 489,311 adults. As well as BMI, they applied two extra measurements.

The first, body fat percentage, estimates the proportion of an individual’s body made up of fat tissue.

The second, waist circumference, offers an indication of the amount of fat surrounding internal organs.

Participants were assigned to groups using sex-specific thresholds. Women with body fat above 35 percent and men with body fat above 25 percent entered higher-adiposity groups.

Waist measurements over 88 centimetres for women and 102 centimetres for men indicated increased abdominal fat.

From these measures, the researchers established five risk categories, spanning low to very high risk.

Body fat and disease risk

Using electronic health records, the team tracked participants for more than 13 years.

Over that period, more than 24,000 people had major cardiovascular events. More than 30,000 developed type 2 diabetes, and almost 15,000 received a diagnosis of chronic kidney disease.

The link between body fat measures and disease risk was evident. The chances of illness rose as body fat percentage and waist circumference increased.

Those in the very highest risk category had more than nine times the risk of developing type 2 diabetes compared with people in the lowest category.

Their kidney-disease risk was more than twice as high, while their cardiovascular risk rose by 63 percent.

Normal BMI can mislead

Among the study’s most notable results was the finding concerning people with apparently normal BMI.

Roughly one third of those in the highest-risk group were actually within the normal-to-overweight BMI range.

Almost five percent of people in the top two risk groups had a fully normal BMI, despite having dangerous fat distribution.

This indicates that people deemed healthy on the basis of BMI alone may nevertheless face substantial health risks.

Compared with people with healthier fat profiles, those with normal BMI but high adiposity had a 45 percent greater risk of cardiovascular disease, a 58 percent greater risk of kidney disease, and more than four times the risk of type 2 diabetes.

This is commonly known as normal-weight obesity. People with the condition may appear slim while carrying metabolically harmful fat deep within their bodies.

BMI tells a partial story

The research does not call for BMI to be discarded completely.

The researchers found that BMI remained informative when used alongside body fat percentage and waist circumference.

Within every adiposity group, higher BMI values were still associated with increased overall disease risk.

The point is not that BMI has no value, but that it should not be treated as the sole measure that matters.

Combining several assessment tools provides a far clearer view of health risk.

Simple tests detect danger

A key benefit of this method is that it is practical.

Body fat percentage can be estimated with bioelectrical impedance devices, which are already available in many clinics and on home scales. Measuring waist circumference requires nothing more than a tape measure.

Both approaches are quick, inexpensive and broadly accessible.

The researchers also observed that bioelectrical impedance generally underestimates body fat when compared with advanced imaging scans.

As a result, the actual number of people at risk may be greater than the study indicates.

Obesity treatment may change

The findings could also affect how obesity treatments are allocated.

New anti-obesity medicines, including GLP-1 receptor agonists, have grown in popularity but remain costly. Access to them often relies heavily on BMI cut-offs.

The study indicates that this approach could overlook high-risk people with normal BMI, while steering treatment towards lower-risk people whose body weight reflects factors other than harmful fat accumulation.

“Our analyses show that combining body fat percentage and waist circumference when screening for obesity can help us identify individuals at high risk of developing obesity-related diseases that may be missed by using BMI alone,” noted Gunnarsson.

“The findings may help improve risk stratification as well as prioritization for lifestyle interventions, anti-obesity therapies, and weight loss surgery.”

Personalised obesity care

The researchers think the work may help shift obesity care towards more personalised medicine.

“I think that our new study is a fantastic example of how researchers in academia and industry can collaborate and hopefully contribute with new knowledge that may help identify individuals who are at elevated risk of obesity-related diseases,” said Rashmi Prasad, co-author of the study.

“We are already planning to carry out studies where we investigate whether the classification of individuals with obesity can be applied on other population groups.”

“Long-term, we hope that our research will lead to individualized treatment of obesity and prevent related diseases in high-risk individuals.”

For almost two centuries, BMI has influenced how society understands body weight and health. Yet accumulating evidence suggests that the future of obesity care may focus less on total weight and more on fat location and its effects on the body.