Reliance on BMI understates the true obesity crisis, experts say
As if the nation’s weight problems were not daunting enough, a new study has found 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% 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 Commissioner of Public Health Nirav R. Shah, uses a patient’s ratio of fat-to-lean muscle mass as the “gold standard” for detecting obesity and suggests that 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 measure in the study uses a costly diagnostic test called dual-energy X-ray absorptiometry (DEXA), which already is in wide use as a means of evaluating bone density.
To recalculate their subjects’ level of obesity, the authors then applied fat-composition standards used by the American Society for Bariatric Physicians.
The new research, by Dr. Shah and New York internist Dr. Eric Braverman, 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 recategorized as normal and healthy when the DEXA standard was used.
But while men fared better than women 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.
In an interview with The Times, Braverman derided BMI as “the baloney-mass index,” and said that its 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, said Braverman. If medical interventions sought to shift patients’ body composition more toward lean muscle mass, he said—encouraging more exercise and more sleep as well as more healthful eating-- they would be more successful.
The article comes as the nation’s obesity experts are casting about for better ways to measure the nation’s state of health and to judge the success or failure of treatment programs. In the past year alone, researchers have proposed a wide range of alternatives to the BMI, and increasingly used 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 BMI has mounted.
The latest effort to calculate obesity drew praise from experts who have argued that BMI is a poor measure of an individual’s health prospects. At the same time, several independent experts said that DEXA may prove too expensive for widespread use and worried that the new study may substitute one imperfect standard for another.
“This paper is important: It’s important to point out the weakness of the BMI,” said Dr. Richard N. Bergman, director of Cedars-Sinai’s Obesity and Diabetes Research Institute. “It argues that BMI is a highly flawed measure, particularly for individuals. It’s a poor measure of fatness and we do need better measures,” he added.
But Bergman cautioned that the study’s authors “have chosen an arbitrary measure of obesity, particularly from the point of view of risk.” While the BMI has been linked to Type 2 diabetes, cardiovascular disease, certain cancers and other ills by decades of research, Bergman said the level at which body fatness contributes to such illnesses is not yet established. Until such research is done, he said, it’s hard to know whether the higher rates of obesity suggested by the measure translate into poorer health for individuals and the larger population.
The study published Monday also found that a measure of leptin, a hormone secreted by the body’s fat deposits, generally tracks with an individual’s DEXA-measured body fatness. Testing leptin levels, the authors wrote, may be a simpler, cheaper way to reliably distinguish those with healthy levels of body fat from those carrying too much.