Looking for a Good Measure of Obesity in Kids

After four decades of agitation about childhood obesity it seems like we might safely assume that scientists can measure and track it with confidence. But it turns out that this is not exactly true. Why? Because of flaws in something called the BMI z-score. Body fat quite normally changes in children with age. What’s healthy for a child who’s five is not the same as it is for the same child when they’re 15. So for some time now, researchers have been using the BMI z-score to measure obesity in kids.

However, it seems that the BMI z-score has some serious problems. With a new paper in Obesity, Justin Ryder, Aaron Kelly, and David Freedman explain:

“The use of the CDC z-scores has the potential to affect and lead to incorrect conclusions drawn by the authors of studies, particularly if many participants have BMI values above the 97th percentile, which reduces the rigor of these studies.”

More Severe Obesity Means More Problems with Z-Scores

BMI has its problems, but it is a simple measure and it “correlates well enough with direct measures of adiposity” to be useful. But the same cannot be said for BMI z-scores. This is because z-scores come from CDC growth charts. And for children above the 97th percentile, the values on the growth chart are extrapolations – best guesses, so to speak.

For some time now, scientists have been pointing out that BMI z-score is a poor indicator of adiposity for kids and youth. Those best guesses are coming back to bite, it seems. Because severe obesity in children has risen so much, the problems with z-scores are becoming intolerable.

Problem Solving Requires Measurement

Perhaps the most famous maxim of problem solving is that you can’t fix something if you can’t measure it. So this little problem with a measure of obesity in kids is actually quite a big problem. When Ryder, Kelly, and Freedman conclude that “metrics matter,” they’re putting it mildly.

Click here for the new paper by Ryder, Kelly, and Freedman. For more on the issues with BMI z-scores, click here, here, and here.

Children in the Sea, painting by Joaquín Sorolla / WikiArt

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January 20, 2022