First introduced in the 19th century, body mass index has been used to define obesity in the U.S. for decades. Now, a new study questions the centuries-old standard.
Two in five U.S. adults are living with obesity. Obesity can increase the risk of cardiovascular disease, diabetes, cancer, and other chronic health conditions. Also known as “BMI,” it is a calculated measure of a person’s body weight divided by their height to determine obesity.
BMI has recently come under heightened scrutiny regarding its methodology, history and what it says about patients and their health. During the Summer Olympics last year, U.S. rugby star Ilona Maher made waves when she said that her BMI is technically listed as overweight, despite being one of the world’s top athletes.
“I am considered overweight. But alas, I’m going to the Olympics, and you’re not,” the Bronze medalist posted on TikTok.
A new study draws the effectiveness of the calculation into question, with researchers at University of Florida Health suggesting it is “deeply flawed” and fails to predict the risk of future death.
“This study is a game-changer,” Dr. Arch Mainous, a professor and vice chair of research in the University of Florida College of Medicine’s Department of Community Health and Family Medicine, said in a statement. “This is the ultimate Coke versus Pepsi test. And BMI failed.”
Mainous was the lead author of the study, which was recently published in the journal Annals of Family Medicine.
Instead, they concluded that a direct measurement of body fat using a fairly inexpensive device that picks up on the resistance of body tissue to a small electrical current is far more accurate. The method is known as “bioelectrical impedance analysis,” and many models of the devices are under $300. They are found in some primary care practices.
While doctors would like to use direct measurements such as DEXA scans, they are not widely available and machines cost tens of thousands of dollars. DEXA stands for dual-energy X-ray absorptiometry and is “considered the gold standard to measure body fat.”
“But it’s never going to be viable in a doctor’s office or family practice,” Mainous said.
Using data from the National Health and Nutrition Examination Survey, which is linked to the national death index, Mainous and his co-authors looked at the health of 4,252 people.
They determined that people with high body fat, as measured by bioelectrical impedance, were 78 percent more likely to die of any cause than those with healthy body fat levels during the 15 years they were tracked. Those individuals were also three-and-a-half times more likely to die of heart disease.
In contrast, there was no statistically significant association with 15-year mortality risk from any cause, including heart disease, when BMI is used.
The researchers said the two methods were analyzed in a way that removes the effects of age, race and income.
They pointed out that BMI can classify some people with a “normal” BMI, even when they have a high percentage of body fat. A normal weight is a BMI between 18.5 to 24.9, overweight is between 25 and 29.9, and obese is any value 30 and above.
Furthermore, senior author Dr. Frank Orlando, medical director of University of Florida Health Family Medicine – Springhill, noted that BMI cannot distinguish between muscle and fat mass, only providing an indirect indication.
“For example, people who are bodybuilders can really elevate their body mass index,” Orlando said. “But they’re healthy even with a BMI indicating they’re obese.”
Notably, the centuries-old practice does not account for how race, ethnicity, and gender affect variations in body size and fat percentage.
“BMI is just so ingrained in how we think about body fat,” Mainous added. “I think the study shows it’s time to go to an alternative that is now proven to be far better at the job.”
