Why BMI May Fall Short

Body mass index may not always be an accurate measure of health

Whether you're bummed about your BMI or encouraged by it, when you're taking stock of your overall health, you may not want to stop there.

A recent New York Times article suggests that while body mass index (BMI) is a valuable tool used to screen for weight categories that may lead to health problems, it's not necessarily an accurate depiction of the body fat percentage or overall health of all patients.

BMI is a measure of body fat based on height and weight. According to the Centers for Disease Control and Prevention (CDC), for adults ages 20 and older, a BMI below 18.5 is considered underweight, a BMI between 18.5 and 24.9 is considered normal weight or healthy, a BMI between 25 and 29.9 is considered overweight, and a BMI of 30 or above is considered obese.

These categories are the same for adult men and women of all body types and ages. According to the CDC, however, BMI doesn't take into account the proportion of bone, muscle and fat in the body. Because of this, a person with strong bones, good muscle tone and low fat may have a high BMI.

And it may work the other way, too.

A 2012 study published in PLOS One found that BMI may be incorrectly classifying up to 50 percent of women and 20 percent of men as healthy when they are actually obese.

According to the CDC, BMI is an incorrect indicator of health for about 18 percent of all US adults.

In 2005, the CDC looked at the body fat of 5,000 US adults across the country as part of the National Health and Nutrition Examination Survey. Participants' weights and heights were measured, and a density-measuring X-ray was used to calculate their body fat percentages.

A strong link was found between BMI and body fat percentage but, for 20 percent of these participants, the two numbers didn't match up.

Eleven percent of those who were considered overweight by BMI standards fell within the normal body fat range, while 31 percent of those considered of normal weight by BMI standards had excess body fat.

Also, women were more likely than men to fall within the normal range for BMI but have excess body fat.

The CDC classified more than 25 percent total body fat for men and more than 35 percent for women as excess body fat.

According to the CDC, these discrepancies may boil down to affordability. While less accurate, BMI is easier to measure than body fat percentage because more precise tools tend to be expensive and inconvenient.

In a 2013 article published in Science, Mitchell A. Lazar, MD, PhD, a professor of medicine and genetics at the University of Pennsylvania, and Rexford S. Ahima, MD, PhD, the director of the Institute for Diabetes, Obesity and Metabolism, discussed the challenges doctors may face when assessing the health of obese patients.

"There is an urgent need for accurate, practical and affordable tools to measure fat and skeletal muscle, and biomarkers that can better predict the risks of diseases and mortality," Drs. Lazar and Ahima wrote. "Advances to improve the measurement of obesity and related factors will help determine the optimal weight for an individual, taking into account factors such as age, sex, genetics, fitness, pre-existing diseases, as well novel blood markers and metabolic parameters altered by obesity."

So what's the takeaway?

The most accurate picture of a patient's health may not boil down to one simple measurement, some experts say. A closer look that involves the influences of diet, activity level, body composition and other risk factors may bring the whole picture into focus.