A study suggests that people are being misdiagnosed as having obesity with a metric known as the body mass index (BMI).Credit: Andrew Caballero-Reynolds/AFP/Getty
Clinicians have debated for years whether body mass index (BMI) — a metric 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 closely on excess body fat has proved to be better than BMI at predicting a person’s risk of death.
New obesity definition sidelines BMI to focus on health
An analysis of data collected from more than 800,000 individuals 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.
Physicians know that BMI — calculated by dividing a person’s weight by the square of their height — doesn’t accurately reflect body composition. For example, a muscular person might have an elevated BMI but have little body fat. The latest study “reinforces that BMI is not a good measure of obesity at the individual level and is not good in terms of predicting who is going to develop cardiometabolic disease in the long term,” says Cátia Martins, an obesity researcher at the University of Alabama at Birmingham.
But given how simple it is to calculate BMI, it remains the most widely used tool to classify obesity.
Waist 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 measures, such as waist circumference, that are linked to excess body fat. Physicians know that when fat tissue is preferentially stored in the abdominal area, it is associated with more negative health consequences than when it’s stored elsewhere, Martins says.
Why BMI is flawed — and how to redefine obesity
Under the CAD-obesity system2, first proposed last year, people with obesity who also have at least one of 18 specific health problems — including heart failure and sleep apnoea — are classified as having ‘clinical obesity’. This indicates that excess fat is harming their organs and tissues. Those with obesity, but without any of these health problems, are classified as having ‘preclinical obesity’, in which a person has excess fat but their organs are functioning normally.
“We did not know until now whether or not this new framework had implications in the real world and a 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 Nature paper.
But Rubino and his colleagues proved the system’s worth by examining medical records from four large groups of people in China, Spain, the United Kingdom and the United States. They used data from three of those cohorts, in which more than 50,000 deaths occurred over up to 15 years, to determine whether BMI or the new system was a better indicator of elevated mortality risk.