BMI May Miss 25% of Kids with Obesity

Tape measure
(Image credit: Laborant | Shutterstock)

Some children who have a normal body mass index (BMI) might actually be obese, because they have extra body fat that's not picked up by the measurement, a new study says.

As a result, some parents may have "a false sense of reassurance" that their child is not obese, and that they do not need to focus on their child's weight, said study researcher Dr. Francisco Lopez-Jimenez, director of preventive cardiology at the Mayo Clinic in Rochester, Minnesota.

The researchers examined how well BMI — a ratio of weight to height — could identify children who are obese. BMI is a very common method for diagnosing obesity, but it has shortcomings: It cannot distinguish between the lean mass and fat mass within the body. Children are typically considered obese if their BMI falls in the 95th percentile of kids their age. [10 Ways to Promote Kids' Healthy Eating Habits]

The new study compared BMI measurements to other ways of identifying obesity that are based on body fat content, including skin-fold thickness measurements and an imaging technique called dual-energy X-ray absorptiometry (DXA).

Results showed that children who had a high BMI were almost always also considered obese based on their body fat content. But among kids who were not labeled as obese based on their BMI, 25 percent actually were obese based on their body fat content.

In other words, a diagnosis of obesity may have been missed in those children.

"[BMI] fails to identify over a quarter of children with excess body fat percentage," the researchers wrote in a paper published online June 24 in the journal Pediatric Obesity.

The findings are concerning, because a failure to recognize childhood obesity may translate "into missed opportunities to institute appropriate lifestyle interventions to mitigate future health risks," the researchers said.

Previously, the same group of researchers identified what they called "normal-weight obesity" in adults, which they applied to people who have a normal BMI but a high percentage of body fat. Just like obesity, normal-weight obesity may increase the risk of heart disease, diabetes, high blood pressure and other health conditions in adults.

It remains to be seen just what the health risks are for children who are misdiagnosed based on their BMI, the researchers said. And it's too soon to recommend that doctors routinely measure kids' body fat percentage to determine whether they are obese. But Lopez-Jimenez said that at the very least, he recommends measuring a child's waist circumference to see if it is typical for his or her age.

Follow Rachael Rettner @RachaelRettner. Follow Live Science @livescience, Facebook & Google+. Original article on Live Science.

Rachael Rettner

Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.