Does your child have obesity or are they just heavy? A new calculator can tell

Since the early 2000s, public health officials have decried an obesity epidemic among children. But now, researchers suspect that obesity and overweight are being overdiagnosed among children, says Andrew Agbaje, a physician and Professor of Clinical Epidemiology at the University of Eastern Finland in Kuopio.

“Many children have been misclassified as having obesity when they are only heavy,” Andrew Agbaje says.

The culprit? BMI, a diagnostic criterion for obesity that relies on a simple ratio of weight to height squared. It fails to consider key developmental differences between children and adults, Andrew Agbaje explains. “Children have more muscle mass than fat,” Andrew Agbaje says. “But when they stand on the scale, you weigh everything together.”

So how are parents and clinicians supposed to tell the difference between problematic and unproblematic weight? Andrew Agbaje points to another metric that can serve as a more reliable indicator of excess fat – a ratio of waist circumference to height.

Using data from more than 7,000 European children, Andrew Agbaje has developed an online calculator that healthcare providers can refer to before recommending interventions for weight loss.

“We have a tool to replace BMI in the young population so that we do not label them what they are not – and take food away from children when they are just growing,” Andrew Agbaje says.

The failings of the BMI

Given the dominance of the BMI in modern diagnostic criteria for overweight and obesity, it may come as a surprise that BMI was never meant to assess an individual’s health – in fact, its creator specifically cautioned against it.

The BMI – weight in kilograms divided by height in metres squared – was conceived in the 1830s by Belgian statistician Adolphe Quetelet, who aspired to find the proportions of the “normal man”, which he considered an ideal. Although he only used the BMI to compare the average physique of large populations such as countries, health insurance companies in the United States seized on it in the 1950s to gauge the financial risk of potential policies based on the applicant’s weight.

From the insurance company charts, the BMI passed into the physician’s clinic, Andrew Agbaje explains – largely because it was not possible to measure fat directly.

But nearly two centuries after Quetelet, scientists have no excuse to lean on the BMI, Andrew Agbaje says. Fat can be directly measured through techniques such as dual-energy X-ray absorptiometry (DEXA), which uses X-rays to map the composition of tissue and bone on the body. The equipment is too expensive to install in every paediatrician and general practitioner’s office – but, Andrew Agbaje reasoned, DEXA scans can help to choose a better proxy for fat than the BMI.

Growing children and healthy weight

Researchers have proposed several metrics to replace the BMI as an inexpensive and noninvasive way to identify children who might have excess fat – among them waist circumference, waist-to-height ratio and waist-to-hip ratio. Andrew Agbaje aimed to assess how well some of these metrics reflect actual levels of fat among growing children.

A long-term study from the United Kingdom comprised the perfect test case – DEXA scans of more than 7,000 children at multiple points in their development, from age 9 to age 24 years, provided objective measures of body fat while detailed measurements of the bodies enabled researchers to compare the metrics.

“This is an extraordinary data set that cost millions of pounds to collect,” Andrew Agbaje says. “No one else in the world has that.”

Andrew Agbaje says a clear winner emerged among the potential surrogates – comparing a child’s waist circumference with their height best predicted fat mass as measured by DEXA.

Nothing but organs and fat

The waist-to-height ratio “was very effective for both males and females” and remains stable over time, he explains. “It takes a lot to increase your waist relative to your height,” he adds.

But what makes waist circumference-to-height ratio such an effective predictor of fat levels?

“Apart from the outer layer of abdominal muscles, there is no muscle in the waist or abdomen,” Andrew Agbaje says. “Nothing but organs and fat. When your waist starts getting bigger, you know that this is from increased fat storage.”

“The height index approximates one’s entire body span; therefore, the waist-to-height ratio estimates both DEXA-measured total body fat mass and trunk or central fat mass (adiposity) to a maximum of 89% accuracy.”

The dangers of overdiagnosis

“When heaviness is pathological, we should address it,” notes Andrew Agbaje. “But when it is physiological, the same interventions can be dangerous for a child, because children need at least four (4) times more muscle mass than fat mass to lower the risk of cardiovascular diseases.”

Typical interventions for obesity – commonly limiting food but also medication and surgery – for a child who does not have excess fat can impair muscle growth at a key developmental stage.

Unnecessary and potentially harmful interventions may occur frequently when clinicians rely on the BMI to diagnose obesity and overweight among young people, Andrew Agbaje says. About 20% of the children in the United Kingdom cohort were classified as having overweight based on their BMI at 9 years old. But using the waist-to-height ratio, the percentage of children flagged for high fat mass fell to just 7%.

Thus, two thirds of the children who would have been diagnosed as having overweight based on their BMI in the United Kingdom cohort have normal fat mass.

“Parents should not be discouraged by the BMI or how much their children weigh,” Andrew Agbaje says. With the new online fat mass calculator, “they can confirm whether the weight results from increasing fat by examining their child’s waist-to-height ratio.”

Based on the ethnic background of the participants in the United Kingdom study, the online calculator will be most accurate for Caucasian children, Andrew Agbaje notes.