Don't Rely On BMI To Indicate Weight Problem In Children

According to the research, a better way to determine a child's body fat percentage is with bioelectrical impedance analysis (BIA) – a method that sends a harmless electrical current through the body in order to measure body fat percentage.

Using BMI measurements, the Centers for Disease Control and Prevention in 2000 estimated that 15 percent of U.S. children aged 6 to 11 were overweight. BMI measurements are the customary standard for determining whether or not a person has too much body fat.

"But there are a number of children who are at risk for becoming overweight that may not be included in the CDC estimate," said Theresa Skybo, the study's lead author and an assistant professor of nursing at Ohio State University.

The study appears in a recent issue of The Journal of School Nursing.

She and Nancy Ryan-Wenger, a professor of nursing at Ohio State, used BIA, BMI and skin-fold measurements to assess the body fat percentages of 58 third-grade children. The children attended two urban public schools, and nearly half of the children were African American.

Each child was weighed on a scale equipped with bioelectrical impedance analysis. The researchers used each child's height and weight to calculate BMI. Each child also underwent a skin-fold test – a caliper was used to measure the thickness of the skin and subcutaneous fat on the back of the upper arm.

When the researchers compared the results of the three methods, they found that one out of three children risked developing a weight problem. But BIA measurements alone gave the most telling results: nine out of 10 girls in the study were either overweight or at risk for becoming overweight, as were nearly half of the boys.

A child is considered at risk for developing a weight problem if he falls between the 85th and 95th percentile for weight in his age category (height is also taken into consideration.)

BMI results indicated that about a third of the boys and two out of five girls in the study risked developing a weight problem; however, according to the BMI results, none of the children were truly overweight. Results from the skin-fold test were similar – about half of the boys were deemed at risk for developing a weight problem, as were a third of the girls.

"There isn't a clear consensus on the definition of what 'overweight' means in children and teens, nor is there a consensus on the best method to use for determining whether or not a young person is overweight," said Skybo. "But when compared to skin-fold or BMI measurements, the BIA results gave a more reliable estimate of a child's chances of developing a weight problem."

Hydrostatic weighing, which involves submerging a person in water, is the gold standard for determining body fat percentage, but the equipment is costly and the procedure is time consuming. Also, BIA results are usually accurate within 2 to 3 percent of hydrostatic measurements, Ryan-Wenger said.

Skybo cautions those who rely solely on BMI measurements to determine children's body fat percentages.

"Body composition changes at different rates and at different times throughout youth, and varying growth rates may distort BMI measurements," she said. "For example, boys are usually leaner during growth spurts, while girls gain body fat during puberty.

She also said that while it's possible BIA measurements could overestimate the number of children truly at risk for becoming overweight, the long-term health risks of unidentified and untreated weight problems have enormous implications for a child's physical, social and emotional well-being.

Mounting evidence suggests a link between weight problems in early childhood and chronic health problems in adulthood such as obesity, diabetes and high cholesterol, all of which can contribute to heart disease.

"The likelihood that an overweight child will be an overweight or obese adult increases as the child ages," said Skybo, adding that after age 6, overweight children have a one-in-two chance of being overweight or obese as adults, compared with a one-in-ten chance for normal-weight boys and girls.

Knowing a child's body fat percentage could help nurses, physicians and parents lower a child's chances of developing these risk factors later in life.

"The leading risk factors for the development of heart disease begin in childhood and include obesity, poor nutrition and high cholesterol levels," said Skybo. "Overweight children are 2.4 times more likely to have elevated cholesterol levels than are normal-weight children."

The researchers received support for this study from the American Heart Association, the Ohio Chapter of the National Association of Pediatric Nurse Practitioners and Ohio State University.