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.
Developing the BMI Alternative
Bergman's team evaluated 1,733 Mexican-Americans and 223 African-Americans using the new BAI formula and compared how it correlated with their percent body fat as measured by DXA (dual-energy X-ray absorptiometry). DXA scans measure body fat and muscle and bone mineral; it’s viewed as the gold standard by experts.
The researchers chose hip circumference and height, Bergman says, because both are strongly correlated with the percent of body fat. Knowing the body weight is not necessary in the new method. The formula is:
BAI = (hip/height x the square root of height) minus 18.
Or, in other words, the hip measurement in centimeters divided by the height in meters times the square root of height minus 18.
Darko Stefanovski, PhD, an assistant professor of research at USC and a co-author of the study, offered two examples. A man 5 feet 9 inches tall who weighs 210 pounds and has 44.4-inch hips would have a BMI of 31, a BAI of 31.2, and a DXA result of 34.3% fat, so the BAI is closer to the DXA result.
A woman 5 feet 4 inches tall who weighs 127 with 37-inch hips would have a BMI of 21.7, a BAI of 27.8, and a DXA of 28.4% fat.
Besides the need for more research to see if the BAI holds for whites and other ethnic groups, Bergman say research is need to determine how well the BAI can predict health outcomes such as higher risk for heart problems and diabetes.
BMI Alternative: Will It Catch On?
''The notion of coming up with an alternative method is merited," Comana tells WebMD. BMI, he says, is limited.
For instance, he agrees that it overestimates body fat in lean people, especially male athletes with high muscle mass.
However, the new research has limitations, he says. The study population of Hispanics, for instance, includes people from ages 18 to 67 with BMIs ranging from 17.1 to over 71.
The researchers found large differences in the BAI scores between very lean Mexican-Americans and very lean African-Americans. For instance, Mexican-Americans who had less than 10% body fat on the DXA tests had, on average, a BAI score of 21.9, while the African-Americans with less than 10% body fat had an average BAI score of 18.4
The limitations with the BAI, Comana says, seem similar to those experienced with BMI.
"This study needs to be replicated with whites and Asians and all other groups, or they need a larger population, or both," he says.