BMI Not Enough to Screen Kids for Weight Problems

An explosion of "super-sized" kids has American health professionals clamoring for improved obesity-screening tools.

But according to preliminary findings from a large-scale study, the body mass index (BMI), an obesity-screening tool being considered for use with kids, can produce inaccurate results if factors such as age, sex, maturity level, ethnic background and physical activity are not considered.

"One out of six children in our study who had a BMI in the normal range had an unhealthy level of body fat," says Dr. Kenneth Ellis, a Baylor College of Medicine professor of pediatrics who studies growth and body composition at the USDA/ARS Children's Nutrition Research Center. "And one out of four with a BMI in the at-risk to obese range had a body-fat level that was normal."

The study compared the results of two obesity indexes, the BMI and the percent body fat (%Fat), in an ethnically diverse population of 979 boys and girls between ages 3 and 18. The BMI values were calculated using the standard ratio of body weight in kilograms to the square of height in meters (wt/ht²). The %Fat was determined with a special 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, Dr. Ellis says. One assumption is that individuals who have a BMI within the normal range have an average amount of body fat. The other is that every ounce of body weight over the standard weight for height is fat.

These assumptions generate the most classification errors for children with BMI values in the gray area between normal weight and overweight, which is a BMI between 18 and 20 for most ages, Dr. Ellis says. The study found that body fat in children in this BMI range varied from 10 to 40 percent. Males with body-fat levels over 25 percent and females with levels over 30 percent are generally considered obese.

Few people think that normal-weight kids can have too much body fat. "If we rely on BMI alone, we risk allowing kids who probably need some type of intervention to improve their physical activity and eating habits to fall through the cracks," Dr. Ellis says.

Of equal concern is the risk of mislabeling 25 percent of high-BMI children as at-risk or overweight, despite their normal body-fat percentage. "Children are very sensitive to labeling," Dr. Ellis says. 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.

"We find amazing variations in body composition and percent body fat in different ethnic groups in this country and among citizens of different countries," Dr. Ellis says. For example, at any given BMI value, African-American children have more bone and muscle mass and less body fat than their European-American counterparts.

"If the concern is that excess body fat in childhood increases the risk of chronic illness later in life, health professionals must consider ethnicity, maturation, diet and physical activity, in addition to gender and age, when evaluating a child's BMI," Dr. Ellis says.

- JOAN CARTER, R.D./L.D.

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©2006 Texas Medical Center

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