The BMI (body mass index) is a widely used measure of obesity in children and teens.

It has been criticized for its lack of accuracy on an individual basis.

The prevalence of obesity among children and adolescents has reached alarming levels, affecting nearly one in four children between 2020 and 2024.

This surge in obesity is associated with a range of serious health conditions, including cardiovascular, metabolic, neurological and musculoskeletal diseases, as well as premature death in adulthood.

This is why it’s grossly irresponsible to declare that the body of an obese child is “perfect just as it is.”

Timely and accurate detection of overweight and obesity in children and teens is essential for initiating interventions that can mitigate these risks.

What’s better than BMI for showing obesity in kids?

A  study has identified waist circumference-to-height ratio as an inexpensive and more accurate measure of obesity in children and adolescents compared to the BMI.

The research, a collaboration between the University of Bristol, the University of Exeter in the UK, and the University of Eastern Finland, was published in Pediatric Research.

The findings suggest that this measure can more effectively detect excess fat mass and distinguish it from muscle mass, addressing a critical need in the ongoing battle against childhood obesity.

For years, health professionals have relied on weight-to-height ratio charts and BMI adjusted for age and sex to diagnose obesity in children.

However, these methods have limitations as they do not distinguish between fat mass and muscle mass.

This can result in inaccurate diagnoses, as two children with the same BMI may have different body compositions.

For example, one child may have a higher proportion of muscle, while the other may have a higher proportion of fat.

Nevertheless, something needs to be pointed out here. There’s only so much muscle mass that children and even adolescents can have, being that children’s bodies lack the ability to pack on muscle mass in the same way that adults’ — particularly men’s — do. 

So for instance, a nine-year-old girl with a BMI of 30 is clearly obese. 

An adult woman bodybuilder with a BMI of 30, on the other hand, will have a much smaller amount of body fat and much more muscle, percentage-wise, when compared to the young girl.

While dual-energy X-ray absorptiometry (DEXA) scans provide precise measurements of fat and muscle content, they are expensive and not widely available in primary healthcare settings.

Recognizing this challenge, the American Academy of Pediatrics (AAP) has called for urgent research into affordable and accurate alternative measures of obesity.

Emerging studies in adults have indicated that waist circumference-to-height ratio can predict premature death better than can BMI, suggesting its potential as a valuable tool in obesity diagnosis.

However, there has been a lack of evaluation on how well this measure aligns with DEXA-measured fat mass and muscle mass in children and adolescents.

The threshold for detecting excess fat using waist circumference-to-height ratio in this population also has remained unclear, prompting this comprehensive study.

The Study

The study utilized data from the University of Bristol’s Children of the ’90s study, also known as the Avon Longitudinal Study of Parents and Children.

It included 7,237 children, with a nearly equal gender distribution, who were followed from age nine to 24.

Their BMI and waist circumference-to-height ratio were measured at ages nine, 11, 15, 17 and 24 years.

The study aimed to determine the level of agreement between these measurements and DEXA-measured fat and muscle mass.

The Results

Results showed that the waist circumference-to-height ratio had a high agreement (81-89%) with DEXA-measured total body fat mass and trunk fat mass, but a low agreement (24-39%) with muscle mass.

In comparison, BMI had a moderate agreement with both fat mass (65-72%) and muscle mass (52-58%).

This indicates that BMI cannot reliably differentiate between excess fat and muscle mass, making it less effective for diagnosing obesity.

Now you might be wondering, Well, what’s the point of all this since it’s visibly obvious when a child is fat or even just chubby, and there’s no such thing as a child who looks like a miniature bodybuilder?

The study is important because it also identified optimal waist circumference-to-height ratio cut points for predicting the 95th percentile of total fat mass.

For males, the cut point was 0.53, while for females, it was 0.54.

These thresholds detected excess fat in eight out of 10 males and seven out of 10 females who truly had excess fat according to DEXA measurements.

Additionally, the cut points accurately identified 93% of males and 95% of females who did not have excess fat.

Andrew Agbaje, a physician and pediatric clinical epidemiologist at the University of Eastern Finland, explained that the investigation showed new information that could be helpful in updating future guidelines for childhood obesity.

The average waist circumference-to-height ratio (WHtR) in childhood, adolescence and young adulthood is 0.45, and it remains consistent across different ages and individuals, unlike BMI, continues Dr. Agbaje.

Using WHtR might be a more reliable and cost-effective method than BMI for detecting excess fat in children and adolescents in clinical settings.

By examining their child’s WHtR, parents can better understand and monitor their child’s health regarding body fat.

Lorra Garrick is a former personal trainer certified by the American Council on Exercise. At Bally Total Fitness, where she was also a group fitness instructor, she trained clients of all ages for fat loss, muscle building, fitness and improved health.