New Method of Calculating Body Fat Unlikely to Replace BMI

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If you've ever strived to achieve a healthy weight, chances are you're familiar with your body mass index (BMI) – a ratio of weight to height that's considered a reliable measure of body fatness.

While BMI is widely used and easy to calculate, it's not without flaws. For example, a lean but very muscular person might have a high BMI, designating that he or she is overweight, when in fact that's not the case.

A new study presents an alternative way to gauge how much fat we're carrying around. The study introduces "the body adiposity index," which estimates an individual's percentage of body fat.

A precise measurement of body fat requires complex and costly tools, such as a dual-energy X-ray absorptiometry scanner. But if you don't have one of those, the new index can give an estimate that simply requires knowing your hip circumference and height. A calculator wouldn't hurt either.

The body adiposity index might prevent wrongly classifying people as overweight or obese, and also catch some cases that might be overlooked. And it might be particularly useful in developing countries, where accurate body weight measurements are not easily obtained, the researchers said.

However, some experts are underwhelmed with the new index, and don't think it will catch on. While BMI is an imperfect measure, the new index has flaws as well, they said.

For one, it relies on an accurate measurement of hip circumference, which can be difficult to obtain, particularly on obese people, said Richard Atkinson, director of the Obetech Obesity Research Center in Richmond, Va., who was not involved in the study.

In addition, a substantial amount of research has linked specific BMI values with other measures of health, said Michael Schmidt, an assistant kinesiology professor at the University of Georgia. For example, one recent study found that people with a BMI over 40 were almost three times more likely to die from the H1N1 flu than other adults. But this is not the case for body fat — researchers don't yet know what body fat thresholds indicate health risks, he said. The study is published today (March 3) in the journal Obesity.

The body adiposity index

To develop the body fat index, Richard Bergman, of the University of Southern California, Los Angeles, and colleagues analyzed information from about 1,700 Mexican-Americans. They sought the characteristics — such as gender, age, height, weight, hip circumference or some combination of these traits — that best correlated with body fat as measured by dual-energy X-ray absorptiometry.

The researchers found that hip circumference and height were strongly linked with body fat. From these traits, the researchers developed an equation for the body adiposity index:

Hip circumference / (Height X √Height) – 18

(Like I said, you might need a calculator.)

The researchers validated their equation in a second study population of about 220 African-Americans.

More work is needed to determine if the index could be used in Caucasians as well, the researchers said.

Moreover, the equation cannot distinguish between belly fat — thought to be particularly bad for your health — and fat in other parts of the body.

Better tools

But to understand and treat obesity, physicians need more advanced tools than either index, Atkinson said.

"If you're an obesity physician… you really need to have something more sophisticated than BMI," or a hip circumference and height ratio, Atkinson said.

Using an index to measure body fat is like using a stethoscope to figure if a patient has a heart problem, Atkinson said.

"No cardiologist is going to try to figure out what the heart's doing by simply listening to it," Atkinson said. "They're at least going to get an EKG."

There are now less expensive ways to measure body fat, such as a bioelectrical impendence analysis, which sends an electric current through the body and can distinguish between fat and lean tissue, Atkinson said.

"We're still using the same flawed technique for a disease that affects a third of the adult population in the United States," Atkinson said. "That's unconscionable."

Pass it on: A new index provides an alternative way to calculate body fat. However, it's unlikely to replace BMI, experts say.

Follow MyHealthNewsDaily staff writer Rachael Rettner on Twitter @RachaelRettner.

This story was provided by MyHealthNewsDaily, a sister site to LiveScience.

Rachael Rettner

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.