As if the nation’s weight problems were not daunting enough, a new study has found that the body mass index, the 180-year-old formula used to distinguish between healthy and unhealthy weight, may be incorrectly classifying about half of women and just over 20% of men as being the picture of health when their body-fat composition suggests they are obese.

The study, published Monday in the journal PLoS One, uses a patient’s ratio of fat to lean muscle mass as the “gold standard” for detecting obesity and suggests that it could be a better bellwether of an individual’s risk for health problems.

The researchers suggested that body fat would predict individuals’ health risks better than the BMI. To measure fatness, they used a costly diagnostic test called dual-energy X-ray absorptiometry, or DXA, and calculated subjects’ level of obesity based on fat-composition standards used by the American Society of Bariatric Physicians.

The results also suggest that the BMI is a poor measure of fatness in men — but not always in a way that underestimates their obesity. In all, 20% of the study’s men shifted from normal and healthy into the obese column under the new measure. But far more frequently than was the case among women, men who were obese by the BMI standard were reclassified as normal and healthy when they were measured with the DXA.

Though men fared better than women under the proposed new standard, the resulting picture is uniformly grim, according to the study’s authors, Dr. Nirav R. Shah, New York’s state commissioner of health, and Dr. Eric Braverman, a New York City internist in private practice.

“We may be much further behind than we thought” in addressing the nation’s crisis of obesity, the authors wrote.

In an interview, Braverman derided the BMI as “the baloney mass index” and said that its widespread use was “feeding the failure” of public health policies and treatments aimed at fighting obesity. The 1,393 patients in the study were from his Manhattan practice.

Efforts to get patients to shed extra pounds have produced weight loss in the short term but fatter patients in the long run as weight is regained, Braverman said. Medical interventions would be more successful if, instead of focusing on weight, they encouraged patients to shift their body composition toward lean muscle mass by recommending more exercise, more sleep and more healthful eating, he said.

The nation’s obesity experts are searching for better ways to measure the nation’s state of health and to judge the success or failure of treatment programs.

“We’re kind of stuck right now,” said Dr. James Hebert, an expert on obesity-related diseases at the University of South Carolina who was not involved in the study. “There’s a casting about for better ways to address the public health problem of obesity. And to do so, you have to measure the problems and their public health consequences. This is a technique or method that holds promise.”

In the last two years, researchers have proposed a wide range of alternatives to the BMI; increasingly, they are using them to measure the effectiveness of interventions such as weight-loss counseling, exercise regimens and drug therapies.

Simple measures such as waist circumference, hip circumference and waist-to-hip ratios have gained new adherents as criticism of the BMI has mounted. At the University of Alabama, researchers are set to explore whether a computer program can analyze a series of photographs of a patient and, based on known patterns of fat deposition and musculature, recognize dangerous fat better than the BMI alone.

The latest effort to calculate obesity drew praise from experts who have argued that the BMI is a poor measure of an individual’s health prospects. At the same time, many said that DXA scans — currently used to evaluate bone density — may prove too expensive for widespread use.

There was also concern that the new study may substitute one imperfect standard for another.

“It’s important to point out the weakness of the BMI,” said Richard N. Bergman, director of Cedars-Sinai’s Obesity and Diabetes Research Institute in Los Angeles. “It’s a poor measure of fatness, and we do need better measures.”

But Bergman cautioned that the study’s authors “have chosen an arbitrary measure of obesity, particularly from the point of view of risk.” For all its shortcomings, the link between the BMI and Type 2 diabetes, cardiovascular disease, certain cancers and other ills has been established by decades of research, Bergman said. But the precise level at which body fatness, as measured by DXA, contributes to such illnesses is not yet established.

In the meantime, he said, it’s hard to know whether the higher rates of obesity suggested in the current study actually translate into poorer health.

The study also found that a measure of leptin, an appetite-boosting hormone secreted by the body’s fat deposits, generally tracks with an individual’s level of fatness as measured by DXA. Testing leptin levels may be a simpler, cheaper way to reliably distinguish people with healthy levels of body fat from those carrying too much, the researchers wrote.

Braverman said that the cost of measuring blood concentrations of leptin could someday fall to as little as $1 — a far cry from about $300 for a DXA scan, which is usually done to assess osteoporosis risk. At a minimum, patients who get a DXA test to measure bone loss should have their body fat assessed as well, he said.

Physicians should also consider taking routine measures of patients’ leptin levels as a useful indicator of body composition and possibly a predictor of obesity-related disease, he said. Leptin could become as valuable as cholesterol and blood pressure are as harbingers of disease risk, he said.

melissa.healy@latimes.com