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Last updated on April 25, 2014 at 5:25 EDT

BMI Not An Accurate Obesity Measurement

April 6, 2009

Forthcoming article in British Journal of Nutrition claims certain ethnic groups may not be getting accurate estimates of disease risk

Published online today in British Journal of Nutrition (DOI 10.1017/S0007114509325738) the new study shows “that the number used to indicate weight category does not reflect the same amount of body fat for some races compared to others,” according to senior authors Dr. Andrew Jackson, Emeritus Professor of Health and Human Performance, University of Houston and Dr. Molly Bray, Associate Professor of Pediatrics – Nutrition at the USDA/ARS Children’s Nutrition Research Center, Baylor College of Medicine and Texas Children’s Hospital. “The results are consistent with other studies that say BMI is inexact and should be tailored to help target those at risk.”

BMI is a formula that estimates a person’s body fat using only his/her weight and height. The result is then used to determine weight categories: 18.5 and below is considered underweight; 18.6 – 24.9 healthy; 25 – 29 overweight and 30+ obese.

“This 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, rather than using other potentially biased methods employed in the past as “gold standards” to examine body composition, 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.

“These results demonstrate that body composition measures obtained from DXA can be used to more accurately define risk factors and provide better recommendations for the treatment and care for patients,” explains Bray.

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