Study finds BMI, despite flaws, is useful estimate of body fat in children

The body mass index has long been slammed as a blunt instrument for evaluating health, even more so with new obesity drugs changing the conversation about weight and well-being. Now a study reasserts BMI’s value as a screening tool in children to detect high levels of body fat, a measure tied to greater risk of cardiovascular disease, early atherosclerosis, and a high BMI in adulthood.

BMI is an equation that divides a person’s weight in kilograms by their height in meters squared. For children, growth curves from the Centers for Disease Control and Prevention are used to track a child’s trajectory, rather than assign them to one of the four categories familiar to adults (underweight, healthy weight, overweight, obesity). For people of all ages, BMI is a proxy for body fat, whose best measurement involves dual-energy x-ray absorptiometry (DXA), an expensive tool impractical in primary care. 

The new study recognizes the limitations of BMI because its two measures are simply height and weight, numbers that may be better suited to studying populations than to directing care for individual patients. And it notes that at the same BMI, Black children have less adiposity than white children. The American Medical Association has urged doctors to de-emphasize the use of BMI in assessing health and obesity, decrying its use for “racist exclusion” and for causing “historical harm” because it was based only on white populations. 

Critics also say those two metrics can’t distinguish fat mass from lean mass, so the researchers set out to see how well BMI compares to DXA when looking at the two kinds of body mass. They analyzed data using both measures from 6,923 young people age 8 through 19 in the National Health and Nutrition Survey, conducted from 2011 through 2018. 

Young people with a high BMI — defined as equal to or higher than the 95th percentile on the growth curve — were 29 times more likely than those with lower BMI to have a high fat mass. 

Because high BMI was more strongly linked to high levels of fat mass, its value would be greatest in screening for high adiposity, the authors said. 

Ihuoma Eneli, visiting professor of pediatrics-nutrition at the University of Colorado School of Medicine, called the study carefully done and much needed, given the controversies surrounding BMI as the core measure to define obesity. 

“We need to take criticisms seriously and see it as an opportunity to test and retest our assumptions, as this paper has done,” she told STAT via email. “It’s particularly important in pediatrics as the growth and development is one of the core areas we use to define a thriving child and features prominently in every aspect of pediatric practice/discipline. BMI use is not all about obesity alone.”

As a clinician, Eneli has had patients and parents question a high BMI number in a child who was an athlete. Sometimes she shared their doubt, but later testing would confirm excess body fat. 

“High BMI is a very good indicator of high fat mass,” senior study author David Freedman told STAT. Now retired from the CDC’s Division of Nutrition, Physical Activity, and Obesity, his work over 35 years concentrated on BMI and cardiovascular disease risk factors in children. 

“Throughout my career, there’s always been criticisms of BMI,” Freedman said. Although the study concluded BMI is not perfect, it has “utility” in research and medical care. “I wrote this study just to try to examine how good is BMI as an indicator of high adiposity,” he said.

BMI was better at its higher end in detecting high body fatness. When looking at children and adolescents at or above the 95th percentile for BMI, it’s important to remember that the CDC growth charts were drafted roughly 50 years ago. About 20% of children are now above that 95th percentile.  

The paper, posted Monday in Pediatrics, follows last year’s guidelines issued by the American Academy of Pediatrics (which publishes the scientific journal) recommending weight-loss drugs for children as young as 12 whose weight and age place them at the high end of growth charts. That policy, which acknowledged debate about BMI, drew pushback from experts concerned about focusing on weight, not health. For adults, critics have pointed to a 2016 Nature study that documented how relying on BMI alone could incorrectly classify both good and bad cardiometabolic health.

Late last year, the U.S. Preventive Services Task Force said it needed more evidence to fully understand the long-term health outcomes for weight-loss medications in children before making a recommendation. Instead, it advised clinicians to provide or refer children 6 or older who have a high BMI to intensive behavioral interventions to help them achieve a healthy weight and improve their quality of life. 

“Although BMI is not a perfect tool by any means, it is a simple tool which can be used to screen for adiposity and associated risks of adverse health outcomes,” said Sharon Weston, a senior clinical nutrition specialist at the Optimal Wellness for Life Clinic at Boston Children’s Hospital. She was not involved in the Pediatrics study. “It is important, however, to use BMI in conjunction with other parameters to measure overall health and wellness,” such as diet, sleep, and physical activity.

The new study did not make race and ethnicity comparisons, but cites other research based on the same data set that saw differences mostly at lower percentiles on the CDC BMI growth charts. Reasons for these disparities aren’t known, but the authors suggest environmental, social, behavioral, and nutritional factors may be at play. 

Eneli, who was a co-author of the American Academy of Pediatrics guidelines on obesity drugs in children, called the history of how BMI came about disconcerting. “One wonders what this study will look like analyzed using race/ethnicity,” she said.

Freedman said he has launched a study of those differences and expects to publish results next year.

Alternatives to BMI for screening include measuring waist and neck circumference, Weston said, but for their use to become standard in clinical practice, developing optimal cutoff points in children would need to be developed.

Today’s findings reinforce BMI’s usefulness for spotting children whose fat mass is rising, Jaime Moore and Stephen Daniels of Children’s Hospital Colorado wrote in a companion commentary, but it’s only a start.

“Improved standardization for the identification of pediatric obesity and severe obesity using BMI, when paired with equitable delivery of treatment, could help to counteract weight bias and reduce disparities in obesity-related health outcomes,” they said. “We remind practitioners that the standardized use of BMI to identify patients with obesity is a first step.”  

STAT’s coverage of chronic health issues is supported by a grant from Bloomberg Philanthropies. Our financial supporters are not involved in any decisions about our journalism.