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

Enlarge / Children checking their weight. (credit: Getty | BSIP)

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.