Atlanta (CBS Atlanta) – According to a new study, obesity is undercounted in children because the commonly used body mass index may leave out up to 25 percent of children that have excess body fat.

“BMI is not capturing everybody who needs to be labeled as obese,” Francisco Lopez-Jimenez, director of preventive cardiology at the May Clinic in Rochester, Minnesota, and a researcher in the study, told the Wall Street Journal.

Measuring body-mass index is not only the most common way to screen for obesity; it is also relatively easy and inexpensive. BMI is calculated based on a person’s height and weight.  This technique is not best suited for children because their height and weight do not proportionally increase as they grow.

“It doesn’t mean that we cannot use BMI in childhood but it requires extra caution,” Ruth Loos, a professor of preventive medicine at the Icahn School of Medicine at Mount Sinai in New York, who was not involved in the study, said.

Both the Centers for Disease Control and Prevention and the American Academy of Pediatrics recommend using BMI to screen for obesity in children starting when they are 2 years old.  It is also most commonly used by school nurses.

According to the CDC, the rate of childhood obesity has more than doubled in the last 30 years.

In their most recent study, the CDC reported that 8.4 percent of preschool-age children were obese in 2011 to 2012.

“BMI is helpful in showing broad trends of obesity in the overall population but isn’t a reliable gauge of health risk in individual children,” Lopez-Jimenez said.

Dr. Lopez-Jimenez suggests more research be done in order to analyze the different degrees of fatness that may be associated with health risks for children as they grow.

“We need to get more data and not to accept BMI as the gold standard to measure fatness because it’s not,” he said.

Sandra Hassink, medical director of the American Academy of Pediatrics; Institute for Healthy Childhood Weight, says if you use BMI to classify children for obesity and overweight, it must be followed by a set of questions about eating and physical activity to determine if a child faces health risks.

“The take home is that BMI is an initial first step to assess risk,” Hassink said. “I think we need another measure that really may speak in a more refined way to the relative amount of adiposity versus muscle.”

Other ways to measure body fat is with bioelectric-impedance devices, such as specialized weigh scales and small gadgets, which send a small amount of electrical current across the body to estimate body fatness.

More expensive options are the Bod Pod, which measures body volume and then calculates density; or the DXA, which is used for bone-density tests but can also measure body fatness.

The Mayo Clinic says that additional data is needed before they can suggest an alternative way to measure BMI.

The research is set to be published in the journal Pediatric Obesity next week.


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