A study suggests that people are being misdiagnosed as having obesity with a measurement known as body mass index (BMI).Credit: Andrew Caballero-Reynolds/AFP/Getty
Clinicians have debated for years whether body mass index (BMI) – a measurement derived from a person’s height and weight – is the best way to define and diagnose obesity. Now, an approach that measures waist circumference and focuses more narrowly on excess body fat has been shown to be better than BMI at predicting a person’s risk of death.
New definition of obesity moves away from BMI to focus on health
An analysis of data collected from more than 800,000 people found that using BMI to diagnose obesity failed to identify a considerable proportion of people with elevated health risks caused by excess body fat. The study was published today in Nature1.
Doctors know that BMI – calculated by dividing a person’s weight by the square of their height – does not accurately reflect body composition. For example, a muscular person may have a high BMI but little body fat. The latest study “reinforces that BMI is not a good measure of obesity at the individual level and is not effective in predicting who will develop cardiometabolic disease in the long term,” says Cátia Martins, an obesity researcher at the University of Alabama at Birmingham.
But given the simplicity of calculating BMI, it remains the most widely used tool for classifying obesity.
Size matters
A person is usually diagnosed with obesity if they have a BMI of 30 or higher. The approach presented in the study – called the combined anthropometric definition of obesity, or CAD-obesity – goes beyond BMI, combining it with measurements, such as waist circumference, linked to excess body fat. Doctors know that when adipose tissue is stored preferentially in the abdominal region, it is associated with more negative health consequences than when it is stored elsewhere, Martins says.
Why BMI is flawed – and how to redefine obesity
As part of the CAD-obesity system2First proposed last year, obese people who also have at least one of 18 specific health conditions, including heart failure and sleep apnea, are classified as having “clinical obesity.” This indicates that excess fat is harming their organs and tissues. People who are obese, but without any of these health problems, are classified as having “preclinical obesity,” in which a person has excess fat but their organs function normally.
“We didn’t know until now whether this new framework had real-world implications and prognostic value,” says Francesco Rubino, a weight-loss surgeon at King’s College London who led the effort to create the new system and is a co-author of the study. Nature paper.
But Rubino and his colleagues proved the system’s value by examining the medical records of four large groups of people in China, Spain, the United Kingdom and the United States. They used data from three such cohorts, in which more than 50,000 deaths occurred over a period of up to 15 years, to determine whether BMI or the new system was a better indicator of high mortality risk.
Gn Health
