No longer just BMI: This is how obesity could be better defined

No longer just BMI This is how obesity could be better defined As of: November 11, 2026 • 5:01 p.m

How heavy and tall you are alone does not say enough about your health. That's why the body mass index (BMI) has been criticized for years. A new study shows the benefits a more accurate diagnosis would bring.

The number of people with obesity is increasing - in Germany and worldwide. But not all people with a pathologically increased weight have the same risk of becoming seriously ill or therefore dying earlier.

In recent years, various groups of experts have therefore developed suggestions on how to better identify who is at risk and who is less so. This has now been examined in a new study.

What is important about the new diagnostic criteria: According to the experts, who is considered obese and who is not should no longer be defined solely based on the body mass index. The index (BMI for short) is calculated from the ratio of body weight and height. Currently, if a person's BMI is over 25, they are overweight. From a value of 30 you are considered obese, i.e. morbidly overweight.

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"The main problem with obesity is that the actual reason for the diagnosis, namely the increased fat mass or the incorrectly distributed fat mass, is not sufficiently well captured by the body mass index," says Matthias Blüher, director of the Helmholtz Institute for Metabolism, Obesity and Vascular Research in Leipzig.

So people with a lot of muscle could be heavy enough to have a BMI of over 30. Or the BMI of people with a lot of fatty tissue could be lower, even though they have an increased health risk. Specifically, in a recommendation published in 2025, experts advocate including data on people's body fat and its distribution in addition to BMI.

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The effect this would have on the assessment of people with obesity was examined in the study, which has now been published in the renowned journal Nature. To do this, the international research team analyzed medical data from over 844,000 people. In addition to the BMI, they also included the waist circumference and the waist-to-height ratio in the assessment.

The result: As expected, there were people who were no longer considered obese with the new definition. Others were new and had a lower BMI but an unhealthy fat distribution. The researchers also divided the disease into two groups: preclinical and clinical obesity.

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Preclinical obesity here means that a person is significantly overweight, but has no other illnesses that can be attributed to their high weight. In the case of clinical obesity, on the other hand, those affected have other illnesses, such as cardiovascular problems, diabetes or certain types of cancer.

In the study, the researchers showed that people with clinical obesity had a significantly increased risk of dying early. People with preclinical obesity only have a slightly increased risk. Such a classification could be helpful in clinical practice, says Blüher: "We cannot treat every person with obesity with the same intensity. That's why we need criteria in clinical practice that help us understand which person benefits most from our therapy recommendations."

In practice, however, clear separation between the groups can be difficult, says Hans Hauner. He is senior professor at the Else Kröner Fresenius Center for Nutritional Medicine at the Technical University of Munich.

In its proposal on which the new study is based, the expert commission developed 18 different criteria that could lead to the diagnosis of clinical obesity. For example, cardiovascular problems or type 2 diabetes. However, these diseases must also have been triggered by the high fat content. Judging this is not always easy.

The authors of the new study also admit: The new concept is likely to increase the effort involved in making an obesity diagnosis. But this is no different than with other diseases, says Francesco Rubiono from King's College in London, who was involved in the study: "If you look at other disciplines, such as neurology, rheumatology or any other discipline, then you see: making a diagnosis in a single person takes time." Those affected would also expect that. "You don't want a doctor who can make a diagnosis in two seconds."

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However, Professor of Clinical Obesity Research Matthias Blüher could imagine that the attitude towards further examinations would change if the new, more complex type of diagnosis also had a consequence. For example, if this would change the treatment of those affected.

Currently, people with a BMI over 30 can be prescribed medication such as the so-called weight loss injection or pill. With a BMI over 25 and certain previous illnesses, too. However, the medication may not be paid for by health insurance companies.

Hans Hauner from the Technical University of Munich also considers the current data to be a good argument to show the increased mortality risk, especially in people with clinical obesity. "That illustrates the urgency of treatment. If it were possible to be able to offer treatment to at least these ten percent, that would be a great step forward. I am skeptical at the moment as to whether this hope has a chance in the current health policy situation."

This topic in the program: tagesschau.de | Bye bye BMI? This could provide a new measure of obesity | 11/11/2026 | 5:00 p.m