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Ethnic BMI values

Female BMI cut off values do not account for ethnic differences in body composition

BMI cut-off values for obesity are too high
Mahbubur Rahman

According to a new study from the University of Texas Medical Branch (UTMB), current National Institutes of Health BMI cut off values fail to identify many women as obese, as it does not take ethnic differences in body composition into consideration.

The research claims that the BMI cut-off values for obesity are too high for many reproductive-age women in the US and should be adjusted to produce proper diagnosis of obesity. The findings have been published in Obstetrics and Gynecology.

Researchers found that the current BMI standards misclassify as ‘not obese’ nearly half of reproductive-age women who are defined as obese by the World Health Organization (WHO), which uses actual body fat percentage to determine obesity.

Of particular concern is the finding that more than half of white women and more than two-thirds of Hispanic women are obese by the WHO standard.

Study design

The study is the first to look at obesity classifications among ethnically diverse reproductive-age women, and it finds that a single BMI value is not appropriate for a diverse population and that racial and ethnic-specific BMI cut-off values are needed.

The obesity rate was highest among Hispanic women at 69.1%

“It is especially important to accurately assess obesity in reproductive-age women, as they are more likely to be obese than similarly aged men,” said lead author, Dr Mahbubur Rahman, Assistant Professor in the UTMB Department of Obstetrics and Gynaecology and Center for Interdisciplinary Research in Women's Health. “These women are at risk for cardiovascular disease, diabetes and other obesity-related health conditions and may forgo or be overlooked for needed tests and treatments.”

Over the past 20 years, the US has seen a dramatic increase in obesity, which the National Institutes of Health (NIH) defines as having a BMI>30. Comparatively, the WHO defines obesity as greater than 25% body fat in men and greater than 35% in women. While this criterion is an accurate gauge, it is expensive and difficult to measure.

The study analysed 555 women between 20 and 33 based on age, height, weight, BMI, and body composition. The subjects included 189 white, 159 black and 207 Hispanic women.

The researchers assessed subjects' obesity rates and BMI accuracy using three measurements: the current NIH BMI, the WHO guidelines and the researchers own ethnic-specific BMI cut-off values.

Using the current NIH BMI, 205 women were classified a obese. The obesity rate in black and Hispanic women, 46.5% and 37.7%, respectively, was significantly higher than in white women (28%).

However, the WHO guidelines classified 350 women as obese, 63.1% of the total sample. The obesity rate was highest among Hispanic women at 69.1%. The rates were similar in white and black women, with 58.7% and 60.4%, respectively, classified as obese.

Finally, researchers applied their own ethnic-specific BMI cut-off values and found that 311 women were obese.These differing cut-off values were determined through statistical procedures that identified BMI values corresponding to the WHO's definition of obesity.

The values also take into account the fact that white and Hispanic women have approximately three percent higher body fat than black women for a given BMI. Currently, NIH BMI obesity rate calculations show that black women have the highest obesity rate.


However, these study findings suggest that Hispanic women have the highest rates, based on body fat percentage, and that obesity prevention programmes should place special emphasis on Hispanic women.

“BMI is not 'one size fits all'. Inaccurate classification can decrease the impact of obesity prevention programs that will result in many women not receiving the help they need,” said senior author Dr Abbey Berenson, Professor in the Department of Obstetrics and Gynaecology and Director of the UTMB Center for Interdisciplinary Research in Women's Health.

The researchers recommend that women whose BMI25 and 29.9 receive additional counselling to reduce their body weight and avoid obesity related morbidity.

Berenson added that lowering the current NIH cut-off values may result in labelling a few women as obese who are not, but that this would be far offset by the benefit of reaching millions of women in need of body weight and health interventions. The researchers believe these findings are applicable to men and other age groups and anticipate follow-up studies.

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