MONDAY, April 2 (HealthDay News) -- The way that obesity is currently measured greatly underestimates the actual number of women who are obese, a new study suggests.

Almost half of women currently labeled as not obese by virtue of their body mass index (BMI) turned out to be obese when measured by a newer method focusing on their percentage of body fat by weight, the research found.

Researchers Dr. Eric Braverman, president of the Path Foundation in New York City, and Dr. Nirav Shah, the current New York state health commissioner, say that an accurate measurement of obesity should include percentage of body fat as well as the ratio of height and weight known as BMI.

"If you're counting on looking at your body fat based on body mass index, it's virtually completely unreliable," Braverman said.

Based on BMI alone, "roughly 30 percent of Americans are obese, but when you use other methods, closer to 60 percent are obese," he said. "We call BMI the 'baloney mass index,'" Braverman noted.

"We are fatter than we realize; it's the percent of body fat, not BMI, that makes you obese," he explained.

The problem is especially seen among women, because "as women age, they tend to lose bone and replace muscle with fat," Braverman said.

The report was published online April 2 in the journal PLoS ONE.

Braverman and Shah found that when women had a special scan called a dual-energy X-ray absorptiometry (DEXA) scan, which measures body fat, muscle mass and bone density, obesity measured by BMI alone underestimated obesity.

Among the more than 1,300 people who underwent DEXA in the study, almost half of women (48 percent) were misclassified as not obese by BMI, but were found to be obese by percent body fat on DEXA.

In contrast, 25 percent of men were misclassified as being obese by BMI, but were in fact not obese by percent body fat.

In fact, the researchers said, all of the study participants who were found to be obese by DEXA were women.

Braverman noted that a DEXA scan is expensive, so it wouldn't be practical for routine assessment. However, a simple blood test that measures leptin levels can serve the same purpose, he said. Leptin is a hormone involved in regulating appetite and metabolism.

In the study, levels of leptin correlated to body fat. The researchers said leptin levels can be used along with BMI as a more accurate measure of obesity.

"Leptin is a better marker of obesity in women," Braverman stressed, adding that successful weight loss depends on lowering leptin levels. However, he said, women can develop leptin resistance -- a metabolic disorder most often seen after menopause -- which makes dieting ineffective.

People with leptin levels below 5 nanograms per milliliter (ng/mL) are considered thin and levels up to 10 are considered normal weight. Leptin levels of 10 to 30 ng/mL are correctable through diet and exercise, Braverman said, but extremely high levels are hard to reduce.

The effect of leptin is not as powerful in men, he said. But men with low leptin levels are very fit, he added.

Braverman believes that, eventually, leptin tests will become a regular part of a physical exam and people with high levels will be treated with various drugs and diets designed to reduce leptin levels.

"Everyone is going to get a leptin level [reading] just as they do cholesterol [level reading] today," he said.

Commenting on the study, Dr. William O'Neill, a professor of cardiology and executive dean for research at the University of Miami Miller School of Medicine, said that "it's a little bit alarming."

"Traditionally we have used BMI. But this study tells you BMI is relatively accurate for men, but for women it really underestimates how many women are obese," he said.

There aren't a lot of data yet on the health benefits of lowering leptin, O'Neill pointed out. "We will have to see if elevated leptin levels are a cause of a bad outcome. This hasn't been done yet," he noted.

"It certainly sensitizes me to the possibility that ordering leptin levels in women who might be obese may be worthwhile," O'Neill added.

Whether lowering leptin levels alone will reduce obesity isn't known. "We don't know if leptin can be a primary target -- it may just be a marker of body fat," he suggested.

But people with high leptin levels should lose weight, O'Neill said.

More information

For more on obesity, visit the U.S. National Library of Medicine.