Children and teenagers with a high body mass index (BMI) are 29 times more likely to have a high BMI than those with a lower BMI, making the controversial index a “very good screening tool,” the study said. The study was published Monday in the journal Pediatrics.
The study adds another chapter to a long-running debate over the use of BMI, which has always been used as an imperfect proxy for assessing a person’s body fat, and therefore obesity. Last year, the American Medical Association adopted a policy that strongly condemned the calculation of BMI, saying it contains “serious limitations” and “historical harms” and even “racial exclusion” because its use is based on a predominantly white population. But other experts have countered, arguing that the notoriously maligned index is a quick, low-cost tool for assessing a patient’s health and risk.
BMI, calculated by dividing weight (kg) by height (m) squared, has long been used as a simple way to assess a person’s obesity. In children, a high BMI is associated with a higher risk of cardiovascular disease in the future. However, this index can also be misleading. Because it is based only on weight and height, it does not distinguish between fat and lean mass. Thus, an athlete with a relatively large amount of lean muscle mass may easily have a BMI that falls into the overweight or obese category, while an athlete with less lean mass and more fat mass may have a BMI within the normal range. Also, BMI does not tell clinicians anything about the distribution of body fat, which may be important for health risk. Studies have also found that the relationship between BMI and obesity may vary by race and ethnicity. For example, black children tend to be less obese than white children for the same BMI.
As the debate over BMI continues, former Centers for Disease Control scientist David Friedman and his colleagues looked at the relationship between BMI and obesity in 6,928 U.S. children and adolescents (ages 8 to 19). For the study, the researchers also performed dual-energy X-ray absorptiometry, the gold standard for body composition, which measures fat and lean mass for each adolescent. With that data, the researchers were able to determine not only the children’s BMI, but also their fat percentage (fat mass divided by weight), fat mass index (fat mass divided by height in meters squared), and fat-free mass index (fat-free mass minus bone mineral mass divided by height in meters squared).
For their comparison, the researchers considered a “high BMI” to be above the 95th percentile on the CDC growth charts, which are also often used as a measure of obesity. About 20 percent of the children in their study had a high BMI, based on the charts, which were first developed in 1977.
The researchers found that a high BMI was highly predictive of a high fat mass index, but less so for fat percentage and lean mass index. People with a high BMI were 29 times more likely to have a high fat mass index (FMI) but only 15 times more likely to have a high lean mass index (LMI).”[B]”Because high BMI is more strongly associated with high FMI than LMI, BMI is a suitable screening tool for individuals with high obesity,” Friedman and colleagues wrote.
in Accompanying commentary In a study by Jayme Moore and Steven Daniels of Children’s Hospital Colorado and the University of Colorado at Aurora, outside experts supported the continued use of BMI in pediatrics. “Overall, based on these results, pediatricians can be confident that identifying elevated BMI (above the 95th percentile) remains an effective way to screen for increased obesity,” they wrote. But they noted that calculating BMI is only “a first step.”
Evaluating what BMI means for each patient requires a “person-centered” approach that takes into account health history and other testing, the researchers wrote. “Furthermore, treatment goals should focus on health and quality of life outcomes, not growth curve numbers or percentiles.”
The results of the pediatric study A study of adults published in Scientific Reports last yearIn this study of more than 18,000 adults, the researchers also looked at BMI and dual-energy X-ray absorptiometry. They found that while BMI is generally a better indicator of obesity as assessed by the gold standard measure, the relationship was stronger in women than in men, in younger adults than older adults, and in white, black, and Hispanic patients than in Asian patients.
