The standard measure used to define obesity may overestimate the prevalence of the problem among African Americans and underestimate it for some other groups, including Asians and Hispanic women, a new study suggests.

Doctors use body mass index (BMI), the ratio of weight to height, to categorize people as under-, normal- or overweight. People with a BMI of 30 or higher are considered obese.

However, BMI has its limits. Two men can weigh 200 pounds, for example, but one may have a high percentage of body fat, while the other is an athlete with a large muscle mass. Both may be technically overweight based on BMI.

Then there is the question of race. Current BMI “cut-scores” are based on older studies of white adults - which may or may not be applicable to all racial and ethnic groups.

In the new study, reported in the British Journal of Nutrition, researchers found that the standard BMI categories are not a universally good fit.

Using data from more than 1,300 young adults, they found that standard BMI cut-points put too many African Americans into the “obese”

category, while underestimating the problem among Hispanic women, Asian women and adults of Indian descent.

“Our data shows we should have different cut-scores for race for both men and women,” lead researcher Dr. Andrew S. Jackson, a professor emeritus at the University of Houston, told Reuters Health.

An obesity cut-score higher than 30 may be more accurate for African Americans, he explained, while a lower figure may be better for Hispanic and Asian women, as well as Asian-Indian adults.

The issue is that for any given BMI number, people of different races have, on average, a different percentage of body fat. Using an X-ray technique called DXA - which is currently used to measure bone density - Jackson’s team found that for any given BMI number, African Americans had, on average, a lower body fat percentage than their white counterparts.

Among men, African Americans’ body fat percentage was nearly 5 percent lower, on average, while the difference was close to 2 percent among women.

However, because they also tend to have larger bones and denser muscles, some African Americans’ weight may inaccurately push them into the “obese” BMI category. The reverse appears true for certain other groups, whose overall weight may remain within normal range despite having a relatively high body fat percentage.

DXA is a more way accurate of gauging a person’s body composition than BMI, Jackson said. But its expense and relative inconvenience means it will not be replacing BMI in routine care.

SOURCE: British Journal of Nutrition, online April 6, 2009.