Body-mass formula may underestimate obesity

WASHINGTON -- As if the nation's weight problems were not daunting enough, a new study finds that the body-mass index, the 200-year-old formula used to distinguish between healthy and unhealthy weight, may be misclassifying roughly half of women and just over 20 percent of men as healthy when their body-fat composition suggests they are obese.

The study, published in the journal PLoS One and co-authored by New York City Health Commissioner Nirav R. Shah, uses a patient's ratio of fat-to-lean muscle mass as the "gold standard" for detecting obesity and suggests it may be a bellwether of an individual's risk for health problems.

The study finds that for women over 50, especially, many whose BMIs suggest they are the picture of health are, in fact, dangerously fat. The study uses a costly diagnostic test called dual-energy X-ray absorptiometry.

To recalculate their subjects' level of obesity, the authors applied fat-composition standards used by the American Society for Bariatric Physicians.

The new research also suggests that BMI is a poor measure of fatness in men, but not always in a way that underestimates their obesity. Far more frequently than for women, men who were obese by the BMI standard were re-categorized as normal and healthy.

But while men fared better under the proposed new standard, the resulting picture is uniformly grim.

"We may be much further behind than we thought" in addressing the nation's crisis of obesity, say the authors, New York physician Eric Braverman and Shah, who co-authored the article before assuming his current position.

Braverman said the BMI's widespread use "is feeding the failure" of measures aimed at fighting obesity. Efforts to get patients to lose weight have produced short-term weight loss and often, as weight is regained, fatter patients, he said.

If medical interventions sought to shift patients' body composition more toward lean muscle mass, he said -- and encouraged more exercise and more sleep as well as more healthful eating -- they would be more successful.