BMI not enough to screen kids for weight problems
HOUSTON(March 27, 2000)An explosion of "super-sized"
kids has American health professionals clamoring for improved obesity-screening
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," said 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 in Houston. "And one out of four
with a BMI in the at-risk-to-obese range had a body-fat level that
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,
Ellis said. 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
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, Ellis said. 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," Ellis said.
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,"
Ellis said. 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," Ellis said. 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,"
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