Move aside BMI: There’s a better way to define obesity, commission finds

An international committee of scientists has proposed a change to the way obesity is defined and diagnosed. The goal is to offer a more nuanced and objective way to assess body composition, by adding more metrics, such as waist circumference, to the criteria.

For years, the method physicians have used to determine whether a person has obesity is to calculate their BMI — or body mass index. BMI is a quick and inexpensive estimate based on weight and height.

But BMI doesn’t provide adequate information about a person’s health, the commission concludes, and the reliance on BMI alone can result in a misdiagnosis.

The current BMI-based measure of obesity can both overestimate and underestimate how much body fat a person has, explains Dr. Robert Kushner, an endocrinologist at Northwestern University who is part of the Lancet Diabetes & Endocrinology Commission.

“About 40% of the adult population in America has obesity, when it’s defined solely by BMI,” Kushner says. But do all of these millions of Americans have a disease? And do they all need treatment? Kushner says the commission aims to give some clarity to the confusion.

Some people are stocky and muscular. When they step on the scale, their body weight may place them in the category of obesity, due to the weight of the muscle mass. But these folks may be healthy.

On the other side of the coin, some people have very low muscle mass, but carry excess weight around their organs. Their BMI may be in the normal range, but their level of belly fat may put them at risk of metabolic disease.

“What’s new is that we try to define obesity by excess body fat, which is what obesity actually is,” Kushner says.

The commission recommends that excess body fat should be confirmed by measuring a person’s waist circumference, waist-to-hip ratio, or waist-to-height ratio. Another option is a direct measurement of body fat from a screening, such as a DEXA scan, when this option is available and affordable.

“We’re not throwing out BMI, we are now recommending that individuals have another measurement obtained that more directly gets an estimate of body fat,” Kushner says.

The bottom line: The new measures focus on the amount of fat that people carry around their waists.

“Fat in the belly causes systemic inflammation, which then goes on and causes other metabolic problems like elevated blood sugar, elevated blood pressure and increased fats in the blood,” Kushner explains. This can set the stage for metabolic diseases including diabetes as well as heart disease.

In addition, the commission recommends two new categories of obesity, based on objective measures of illness. The first category is called ‘clinical obesity’, for people who already have a chronic disease associated with obesity. And the second category is called ‘pre-clinical obesity’ and would signify that a person has elevated risks for developing a health condition due to their level of body fat.

“Our reframing acknowledges the nuanced reality of obesity and allows for personalised care,” says Dr. Francesco Rubino, of King’s College London, who is chair of the commission.

The Commission included 56 experts with expertise in nutrition, endocrinology, internal medicine and public health. Their report is published in the Journal Lancet Diabetes & Endocrinology.

The report comes amid a significant rise in obesity around the globe. The World Health Organization estimates more than one billion people are living with obesity, and the World Obesity Federation estimates that the economic impact of the condition will rise above $4 trillion by 2035.

This story was edited by Jane Greenhalgh