Obese or not? It’s time to rethink BMI, researchers argue

Livescience.com: Do you know what obesity looks like? Medical professionals and the public alike currently judge whether someone is obese based on his or her size, but a change quietly gathering steam among doctors would ensure that no one is gauged as obese by just one metric.

Doctors, researchers and policymakers need to ditch the concept of using body mass index, or BMI, to diagnose obesity, according to an opinion piece published on 28 March in the journal Obesity.

“BMI is not a good criteria to be diagnosing a disease,” said Arya Sharma, one of the authors of the piece and an obesity researcher at the University of Alberta in Canada.

BMI’s limits

Body mass index is a measure of body size based on a person’s weight and height. It is useful for tracking population trends in body weight, but it has well-known limitations when it’s applied to individuals, Sharma told Live Science. For example, many professional athletes qualify as overweight or obese based on only their BMI, simply because they have so much muscle mass, which weighs proportionally more than fat.

For the average person, other problems apply. For example, people with a BMI of 30 or above are typically considered obese. But some people with BMIs over 30 are perfectly healthy, metabolically speaking, Sharma said: Their blood pressure is good, their livers are functioning well and their cholesterol levels are within normal limits. On the other hand, some people with BMIs below 30 have health problems that could improve if they were to lose weight.

The American Medical Association classifies obesity as a disease, but size alone isn’t a disease, Sharma said.

“BMI is a clothing size. … It tells me how big you are,” he said. “It doesn’t tell me how sick you are.”

Beyond BMI

Nevertheless, BMI has become an easy benchmark in medicine. Drug and medical device companies use BMI cutoffs to decide who to include in their studies of obesity treatments; insurance companies use BMI to determine coverage. In many workplace wellness programs, the rewards and benefits that people can earn are based on BMI.

Even for many primary care doctors who don’t have extensive training in obesity treatment, BMI is a quick and objective number to cite to tell patients whether they have a weight problem, the researchers said.

“What any obesity specialist should do is use BMI as a first step, and then go beyond BMI as needed,” said Dr. Scott Kahan, director of the National Center for Weight and Wellness and a researcher at The George Washington University Milken Institute School of Public Health in Washington, D.C. Professional guidelines for obesity treatment already call for this approach, said Kahan, who was not involved in the new opinion piece.

“The concern is in primary care, where inevitably a lot of weight management has to happen,” Kahan told Live Science.

A shift from a BMI-based diagnosis of obesity to a more holistic, health-based diagnosis of obesity would involve taking a patient’s physical, mental and functional health into account, Sharma said. He and his colleagues have developed a scale called the Edmonton Obesity Staging System that includes all three.

In this view, a diagnosis of obesity would involve a check for physical health problems, such as diabetes or joint problems, plus a look at mental health issues, such as depression or social anxiety, that might arise from a person’s body image. It would also involve looking at a person’s “functional health,” meaning the ability to move around and take care of day-to-day activities.

A major shift

Some doctors have suggested drop-kicking BMI from the obesity diagnostic criteria entirely, Kahan said. But he argued that the number still has a place in the clinic. For example, if a patient has high blood pressure and high cholesterol but a BMI of 21 — at the low end of normal — it would be unreasonable for a doctor to expect that person to lose weight to solve those problems. In that case, it would make the most sense to treat the high blood pressure and high cholesterol directly, Kahan said. If, on the other hand, a patient came in with identical blood pressure and cholesterol numbers but a BMI of 31, weight-loss treatments would probably be part of the picture.

Putting BMI in the background would drastically alter the way obesity is handled within the medical system, the researchers said. Right now, a BMI of 40 marks the lower cutoff for recommending a patient for bariatric surgery, which decreases the size of the stomach, Sharma said. The surgery is also recommended for people with a BMI of 35 if they have serious weight-related health issues. But a more holistic approach to diagnosing obesity could mean that people with lower BMIs could also qualify for the surgery based on other health factors, while those with higher BMIs but good overall health numbers could avoid the risk of the surgery, since they have less to gain from undergoing the procedure.

Drug companies and regulatory agencies like the Food and Drug Administration would have to rethink who is included in clinical trials; professional organizations that set quality measures for doctors’ care would have to expand their definition of the disease beyond BMI alone.

In many ways, this is not that unusual, Sharma said. Disease classifications change all the time. Obesity is just more fraught given the cultural focus on weight and body size.

“Definitions of diseases have to grow with our understanding of what the disease actually is,” Sharma said.