Much derided BMI is useful for assessing fat levels in kids, study suggests


Children checking their weight.
Enlarge / Children checking their weight.

Kids and teens with a high body mass index (BMI) were 29 times more likely to have a high fat mass index compared with youths who had lower BMIs, making the controversial metric a “very good screening tool,” according to a study published Monday in the journal Pediatrics.

The study adds to the long-standing debate about the use of BMI, which has always been an imperfect proxy for assessing a person’s body fat, aka adiposity. Last year, the American Medical Association adopted a strongly worded policy calling out the calculation’s “significant limitations” and “historical harms,” including “racist exclusion,” because its use is largely based on white populations. But other experts have pushed back, arguing that the maligned metric is still a quick, low-cost, tool for assessing the health and risks of patients.

BMI, calculated by dividing a person’s weight in kilograms by their height in meters squared, has long been used as a shorthand way of assessing a person’s adiposity. In children, high BMIs are associated with cardiovascular disease risks later in life. But, the metric can also be misleading. Because it is based solely on weight and height, it does not distinguish between fat and lean mass. As such, athletes with relatively large amounts of lean muscle mass can easily have BMIs that put them in the categories of having overweight or obesity, while those with little lean mass but high fat mass can still have BMIs in a normal range. BMI also does not tell clinicians anything about body fat distribution, which can be important for health risks. And research has found that the relationship between BMI and adiposity can differ by race and ethnicity. For instance, at the same BMI, Black children tend to have less adiposity than white children.

Amid the continued debate about BMI, David Freedman, a retired scientist from the Centers for Disease Control and Prevention, along with colleagues, looked at the relationship between BMI and adiposity in 6,928 kids and teens (ages 8 to 19 years old) in the US. For the study, the researchers also had dual-energy X-ray absorptiometry measurements, a gold standard for body composition that measured fat mass and lean mass in each of the youths. With that data, the researchers not only knew the kids’ BMIs but also their fat percentage (fat mass divided by weight), their fat mass index (fat mass divided by height in meters squared), and their lean mass index (lean mass minus bone mineral, divided by height in meters squared).

For the comparisons, the researchers considered “high BMI” to be at or above the 95th percentile of the CDC growth charts, which is also often used as a cutoff for obesity. Based on those growth curves, which were first developed in 1977, about 20 percent of children in the study had a high BMI.

The researchers found that having a high BMI was highly predictive of having a high fat mass index, while it was less predictive of fat percentage and lean mass index. Those with a high BMI were 29 times more likely to have a high fat mass index (FMI), while they were only 15 times more likely to have a high lean mass index (LMI). “[B]ecause a high BMI is more strongly related to high levels of FMI than LMI, it is a good screening tool for high adiposity,” Freedman and colleagues wrote.

In an accompanying commentary by Jaime Moore and Stephen Daniels, both of Children’s Hospital Colorado and the University of Colorado in Aurora, the outside experts supported the continued use of BMI in pediatrics. “Overall, on the basis of these results, pediatricians can feel confident that identification of elevated BMI (≥95th percentile) continues to be an effective way to screen for increased adiposity,” they wrote. However, they note that calculating BMI is just a “first step.”

Assessing what a BMI means for each patient should involve a “person-centered” approach that accounts for health history and other testing, they write. “Further, treatment goals should be focused on health and quality-of-life outcomes, as opposed to a number or percentile on a growth chart.”

The results of the pediatric study are similar to a study on adults published last year in Scientific Reports. In that study of over 18,000 adults, researchers also looked at BMI and dual-energy X-ray absorptiometry measurements. They found, generally, that BMI is a good indicator for adiposity assessed by the gold-standard measurement, but the relationship was stronger for women than men, younger adults than older adults, and for white, Black, and Hispanic patients than Asian patients.



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