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.