The obesity epidemic could be far worse than previously thought. BMI, an estimate of body fat, often isn't accurate enough to categorize people as being at-risk. NBC's Dr. Nancy Snyderman reports.
By Joyce Ho and Dr. Nancy Snyderman
The nation’s ever-growing obesity epidemic may be far worse than originally thought. New research demonstrates that even people with a healthy Body Mass Index, a commonly used scale to measure body fat, could actually be obese and at risk for a host of complications.
A study published Monday in the journal PloS One found that using BMI as an indicator of obesity actually misclassifies 39 percent of Americans as “overweight” rather than “obese.” And because BMI doesn’t distinguish between fat and muscle, some people with normal BMIs may have dangerously high amounts of fat in their bodies.
Without an accurate measurement of body fat, the researchers say, millions of people don’t know they are at high risk for a number of obesity-related diseases.
“The fat is what causes heart disease, cancer, menstrual problems, depression, anxiety, and a host of medical problems,” said Dr. Eric Braverman, a professor at Weill Cornell Medical School and co-author of the study. “So if you want to save society from a lot of illnesses … you have to identify how much fat they have.”
More than one in three adults in the U.S. are obese, as defined by a BMI of 30 or higher, according to the Centers for Disease Control. Obesity measurement, however, has been a controversial topic for years, and the widely-used BMI calculation has been called outdated by experts.
BMI is calculated through a simple formula: weight divided by height squared. The ease of calculation made this formula popular, even though it’s nearly 200 years old. In Braverman’s study, researchers compared the BMI with a different measurement, the Dual Energy X-ray Absorptiometry (DXA) scan. DXA scans, commonly done in women to check for osteoporosis, measure percentage of body fat, muscle mass, and bone density.
Of the 1,393 people studied, 26 percent were classified as obese when body fat was measured with BMI, whereas 64 percent of them were considered obese when measured with DXA. The misclassification was observed more often in women and increased with advancing age: 48 percent more women between the ages of 50 to 59 were classified as obese when measured with DXA instead of BMI, and among women ages 70 and above, 59 percent more were considered obese after getting a DXA scan.
According to the authors, BMI is an inaccurate measure for obesity – but especially in this demographic, because as women age they lose more muscle to fat than men. BMI, which does not distinguish between muscle or fat, does not reflect this bodily change.
“BMI doesn't tell you how much fat … you have,” said Braverman. “So without knowing how much fat you have, you can't really save people from illness. It is the number one predictor of who's going to live or die.”
Researchers also tracked blood levels of leptin, a hormone produced by fat cells that regulates hunger and energy expenditure. Increased blood leptin levels correlated well with DXA scan results, highlighting the potential for a simple leptin blood test to be a measurement for obesity.
Based on these findings, Braverman and his co-author Dr. Nirav Shah, the current New York state health commissioner, suggest lowering the BMI definition of obesity from 30 to 24 in women and down to 28 in males. Under these suggested guidelines, a woman who is 5’ 6” and 150 pounds would be considered obese. Under the current BMI standards, the same woman would be considered healthy.
“Fat is costing the country a fortune, by not measuring it,” said Braverman. “A dollar blood test and doing our bone density scans with body fat scans at the same time is going to save us an enormous medical cost in the end.”
NBC’s Stacey Naggiar, Chiara Sottile and Joo Lee contributed to this report.
- America’s fattest cities
- Size matters for obese cancer patients’ drug dose
- U.S. advisers back obesity pill