New Alternative to BMI for Measuring Body Fat

March 3, 2011 -- A new way to measure body fat that relies only on measurements of your height and your hip circumference is an improvement on the commonly used measure known as body mass index (BMI), according to the developers of the new method.

"The body mass index (BMI) does not accurately represent the amount of [body] fat," researcher Richard N. Bergman, Keck Professor of Medicine at the University of California's Keck School of Medicine, tells WebMD.

The new measure, called body adiposity index (BAI), does, he says. So far, he has validated the new measurement in Hispanic and African-American populations, and says more research is required to confirm how well it works in whites and other ethnic groups.

With BMI, he says, ''you get a relative number" assessing body fat. With the new BAI, ''you get a number which is the percent fat." The new method, he says, is more accurate.

While an improvement on the BMI method is merited, the new method appears to have its limitations, too, says Fabio Comana, an exercise physiologist at the American Council on Exercise, who reviewed the new research for WebMD.

The new research is published online in the journal Obesity.

Using BMI to Assess Body Fat

Besides considering body weight, BMI is the most common way doctors and others assess whether a person has too much body fat and thus at risk for health problems. It's a measure of fat based on height and weight, used for both men and women.

''The BMI has been around since the 1840s," Bergman says. A BMI of under 25, for instance, is deemed healthy, while those of 30 and above are considered obese. (A person 5 feet 10 inches tall has a BMI of 24.4 at 170 pounds and a BMI of 30.1 at 210 pounds.)

BMI is “OK in general for groups," Bergman says. But on an individual basis, it isn't so accurate in assessing body fat, especially for people who are very muscular.

For instance, he says, ''if you had a BMI of 30, you might have 25% fat [as measured on tests that directly assess body fat] if you are a man, but 35% if you are a woman." Someone very muscular may have a BMI over 25 but have little body fat, he says.

Body fat percentage ''norms" are different for men and women, according to the American Council on Exercise (ACE). For instance, it classifies a body fat percentage of 25% to 31% for women and 18% to 24% for men as ''acceptable'' ranges, with athletes and fit people having lower percentages.

Body fat percentages over 32% for women and over 25% for men are termed obese by the ACE.