Body mass index, or BMI, is a quick, inexpensive tool designed to determine how healthy a person’s weight is for their height and help determine risk for obesity related health issues. BMI is calculated by dividing weight in kilograms by height in meters squared. A healthy BMI is 18.5-24.9, 25-29.9 is considered overweight, and over 30 is considered obese. BMI has long been criticized as an inaccurate assessment as it does not distinguish between body weight due to muscle (which weighs more) versus fat. Percentage body fat a more accurate assessment but cannot be easily measured in the office. The American Society of Bariatric Physicians defines obesity in women as >30% body fat. A recent study published in PLoS One, an on-line scientific journal, compared rates of obesity based on BMI and percent body fat with some surprising and concerning conclusions, particularly for their female participants.

Dr. Eric Braverman and co-author Nirav Shah, New York State’s health commissioner, compared a person’s BMI with body fat percent as determined by DEXA scan (Dual-energy X-ray absorptiometry) which has the ability to differentiate between bone, fat, and muscle tissue. (The usual DEXA scans used to screen for osteoporosis do not routinely measure fat and muscle tissue as was done in the study). They found that nearly half of women categorized as overweight by BMI were found to have >30% body fat, consistent with obesity. This misclassification in women increased with advancing age.

Obesity versus overweight; this is not simply an issue of semantics. The larger issue raised by this study is there is a large group of women where BMI, the most commonly used assessment of obesity, is underestimating their risk of future disease. These so-called “normal weight obese” may have an increased risk for conditions such as high cholesterol, coronary artery disease, high blood pressure, and diabetes but may not be getting the appropriate focused counseling to improve their health.

Does this mean everyone should go out and get a DEXA scan? Cost and inconvenience make this less than ideal. Though we are still in search of a perfect screening test for obesity it is important to remember that weight is only one factor related to risk for disease. Healthy lifestyle habits, such as regular exercise and a diet rich in fruits and vegetables, are important for everyone regardless of what the scale says.