“This [Body-Mass Index] scale was created years ago and is based on Caucasian men and women,” says Bray, “It doesn’t take into account differences in body composition between genders, race/ethnicity groups, and across the lifespan.”
In the current study, ... researchers used dual-energy x-ray absorptiometry, which is a low dose x-ray known as DXA, to determine percent fat. DXA can be used to estimate bone density, lean mass and fat mass.
When the two results were compared, researchers found that the DXA estimate of percent fat of African American women was 1.76 percent lower for the same BMI compared to non-Hispanic white women. Since BMI is assumed to represent body fatness, an African American woman would not be considered overweight or obese until she reached a higher number than what is indicated by the current BMI standards. The opposite is the case for Hispanic, Asian and Asian-Indian woman. Their percent fat is higher by 1.65 percent, 2.65 percent and 5.98 percent, respectively. So they would be considered overweight or obese at amounts lower than what the BMI standards indicates. The results for men were similar.
“Right now non-Hispanic white women are not considered obese until they have a BMI of 30 or above. Based on our data in young adults, for Hispanic women the number would be around 28,” says Bray. “For African American women the number to cross is around 32."
Bone mineral content, hydration state, and the density of lean mass found in different ethnic groups are some factors that account for the differences.
I pointed all this out in "Is Love Colorblind" in 1997.