Millions of obese women may be falling through the cracks of current weight guidelines, US doctors say.

In a new study of women aged 20 to 33 years, they found that more than a quarter weren’t labeled as obese according to widely used standards, even though more accurate measurements put them in that category.

Hispanic women stood out in particular, with more than two-thirds landing in the obese range when doctors measured their body fat. If confirmed, that number would place them ahead of African American women, who have the highest obesity rates according to current US standards.

“We are missing a lot of the women who need assistance,” Dr. Abbey Berenson, who worked on the study, told Reuters Health. “We’re not counseling them accurately.”

Obesity is a risk factor for high blood pressure, diabetes and heart disease. “There are probably effects on almost any organ in the body,” said Berenson, an expert in women’s health at the University of Texas Medical Branch at Galveston.

As a proxy for body fat, doctors use the body mass index (BMI), a measure of weight in relation to height, to gauge overweight and obesity. The US National Institutes of Health defines obesity as a BMI of 30 or greater, regardless of sex and ethnicity.

But researchers have criticized this ‘one-size fits all’ cutoff value, saying that it may miss more than half of obese men and women in the US.

“This is the first study in reproductive-age women,” said Dr. Mahbubur Rahman, also of the University of Texas, and an author of the new report, published in the medical journal Obstetrics and Gynecology.

Rahman and Berenson looked at data for more than 500 women, who had had their body composition examined with X-rays in an earlier study. They compared each woman’s BMI with her percent of body fat, which BMI is supposed to reflect.

Many women had a BMI below 30, but still had more than 35 percent body fat, which indicates obesity according to World Health Organization criteria.

Overall, 145 women were labeled obese according to the WHO criteria, but not according to their BMI value. BMI turned out to work particularly poorly in Hispanic and Caucasian women, who have more body fat per BMI than African Americans.

The researchers suggest lowering the BMI obesity threshold and tailoring it to different ethnic groups. The exact values are still unclear, but their results suggest the dip could be close to five BMI points for Caucasian women, approaching the current threshold for overweight.

“What we’re recommending public health-wise is a lowering of the BMI cutoff for Hispanic and white women,” said Berenson. “This is an immediate issue; it should not be put on the back burner.”

Catherine Loria, of the NIH’s National Heart, Lung and Blood Institute in Bethesda, Maryland, said the Institute was already working on updating its BMI thresholds.

“The cut points that are used to define overweight and obesity were based on all the available evidence at the time the guidelines were written in 1998,” she told Reuters Health.

“We recognize that more studies have become available since that time, and we are right now in the process of updating the guidelines,” she added.

Berenson said classifying overweight and obese women accurately was important in order to provide optimal weight-loss counseling.

“While it’s difficult to say no to delicious food, it’s probably easier than giving yourself daily insulin injections,” she said.

SOURCE: Obstetrics and Gynecology, online April 22, 2010.