What BMI Means for Kids—and Why It's Problematic

Body mass index (BMI) has long been seen as the ultimate measure of physical health, but it can be misleading. Here's what parents should know.

<p>Illustration by Michela Buttignol for Parents</p>

Illustration by Michela Buttignol for Parents

For decades, BMI, or body mass index, was the gold-standard metric for assessing a person's physical health and overall well-being. The screening method measures your weight relative to your height to help determine whether you are at risk for various health conditions, from heart disease to diabetes.

In recent years, BMI meaning and importance have been called into question. Many experts claim that BMI doesn't always give the best measure of health, since it's based on an outdated system that's noninclusive of various body types, including children's. So why do pediatricians still use BMI, and what are some better alternatives for assessing well-being? Here's everything parents need to know.

What BMI Is

BMI is calculated by dividing a person’s weight by their height, specifically weight in kilograms divided by height in meters. “High BMIs indicate that a person is carrying extra weight relative to their height, which may be because of extra body fat," says Thao-Ly Phan, MD, director of the Healthy Weight and Wellness Clinic at Nemours Children’s Health in the Delaware Valley. "But that's not always the case, particularly in children.”

According to the Centers for Disease Control and Prevention (CDC), "BMI does not measure body fat directly, but BMI is moderately correlated with more direct measures of body fat."

Related: Should Kids Be Weighed in Gym Class?

Why BMI Is Outdated

Some experts say that BMI is an outdated health screening tool. BMI was invented in the 1830s by a Belgian statistician, mathematician, sociologist, and astronomer named Lambert Adolphe Jacques Quetelet. It was known then as the Quetelet Index and used data from predominantly European men. It did not consider body-type differences in people of different sexes or races, which makes it inherently problematic for modern use.

“BMI was created based on data from a study of entirely of white, European men,” says Katherine Hill, MD, a pediatrician and eating disorders doctor. "As a result, the conclusions it creates are unreliable for those outside of that demographic."

Some researchers have called for a revision to the BMI measurement system that accounts for body type differences between various racial and ethnic groups. Others point out that the current standards used for BMI fail to consider a person's biological sex or aging process, making the screening tool misleading for wide swaths of society.

Why BMI is Noninclusive

It's worth noting that BMI is interpreted differently for children and teens, though it's calculated using the same formula as adult BMI. “Children and teen BMI need[s] to be age and sex-specific because the amount of body fat changes with age and the amount of body fat differs between girls and boys,” says the CDC. However, the measurement is still problematic.

For one, BMI does not take into account a child's muscle mass, or their bone density and overall body composition. “Since different people have a variety of bone sizes and build muscle or store fat differently, this makes the number somewhat arbitrary,” says Dr. Hill.

Some pediatricians and psychologists worry that a high BMI will cause kids to feel bad about their bodies, even if they are in great health. “BMI is not only doing a disservice to those in larger bodies, but is perpetuating disordered eating, stigma, and shame,” says Dr. Hill. Being classified as overweight or ‘unhealthy’ as a child is a risk factor for developing an eating disorder, which can lead to more severe health complications down the line.”

If the goal of BMI is to assess risks for chronic health conditions, it can sometimes backfire. Researchers have shown that the shame of being overweight is more harmful to heart health than excess weight in itself.

Related: Why You Should Be Asking Your Pediatrician Not to Talk to Your Kids About Their Weight

What Are Alternatives to BMI?

With all of these limitations, you may wonder why pediatricians still use BMI as a marker for health. For one, early obesity as measured by BMI strongly predicts later obesity. More than half of obese children go on to be obese in adolescence, and around 80% of obese adolescents will still be obese in adulthood. It is also an inexpensive screening tool, one with little to no barriers to access.

There is good news, though. Doctors can assess health in other ways that don't involve BMI. Some doctors use more nuanced strategies, according to Dr. Hill. They analyze and consider other markers, like heart rate, blood pressure, and growth trajectories. They speak to children about their physical activities, lifestyle, and relationship with their body (and with food), and they consider their mental well-being.

What's the Bottom Line on BMI

Overall, height and weight are not great indicators of one’s general health. “Without a full picture of a patient’s health and measures like blood pressure, heart function, genetic history, cholesterol levels, sleep hygiene, activity status, and mental well-being, it’s impossible to judge whether someone is considered ‘healthy’ from a medical standpoint," says Dr. Hill. “BMI is an overly simplistic and flawed metric that cannot possibly assess the health of a whole person, let alone a child."

In the future, more healthcare providers should use alternative methods to assess a child's health, and they should always consider a person's unique body type and medical history. But in the interim, you are your child's best advocate. Do your research. Speak with their pediatrician, and help them live a healthy lifestyle.

Related: 6 Lessons in Body Positivity to Teach Your Child by Age 5

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