Weigh BMI limitations when evaluating children

An explosion of "super-sized" kids has American health professionals greeting the body mass index (BMI) with open arms. But preliminary findings from a large-scale CNRC study suggest tempering enthusiasm with an understanding of the BMI's limitations. The BMI is an additional obesity-screening tool released with the new pediatric growth charts in May.

"One out of six children whose BMI value was in the normal range was found to have an unhealthy level of body fat. And one out of four with a BMI in the at-risk to obese range actually had a body-fat percentage in the normal range," said Dr. Kenneth Ellis, a body composition expert with the CNRC and Baylor professor of pediatrics.

Ellis compared calculated BMI values for an ethnically diverse group of 979 boys and girls, ages 3 to 18, to scientific measurements of their body fat. BMI values were calculated using the children's height and weight, while their actual body fat was measured using a sensitive instrument called DXA.

"Two basic assumptions regarding body composition lead to inaccuracies when the BMI is used as a one-size-fits-all screening tool for fatness," Ellis said. One is that every individual whose BMI is within the normal range has a normal amount of body fat. The other is that every ounce of body weight over the standard weight-for-height is fat.

Few people think that normal-weight kids can have unhealthy levels of body fat.  Yet, the DXA measurements of children with calculated BMI values between 18 and 20 showed body-fat percentages ranging from 10 to 40 percent.  Body-fat levels over 30 percent are generally considered unhealthy in children over 3 years of age.

"Clearly, if we rely solely on the BMI, kids who probably need some type of intervention could fall through the cracks," Ellis said.

There is also the risk of labeling one out of four children with a high BMI as at-risk or overweight when they actually have normal amounts of body fat. High levels of physical activity, early maturation, genetics and ethnicity can all contribute to a child having a high BMI but a healthy amount of body fat.

"If our concern is that excess body fat increases the risk for cardiovascular disease, Type 2 diabetes and other illnesses in children now and later in life, health professionals must consider other factors in addition to BMI values when evaluating children," Ellis said.

When assessing your child's BMI, remember:

• BMI is only a guideline. It does not measure bone, fat or muscle.
  
• BMI values considered "appropriate" increase with age.
  
• There is no "right" BMI value for any single child at any age.
  
• BMI values should not be compared between children.
  
• A high or low BMI value isn't always cause for concern.

If you have concerns about your child's weight, see a physician.

Note:  The new pediatric growth charts, including BMI charts and a BMI calculator, are available on the Centers for Disease Control (CDC) website