Using BMI for whats up with overweight children

Comments and Key points, about body mass index, of child obesity.

This seems like a good article, and it does discuss child obesity and BMI, but upon closer inspection it was not helpful. It’s key limitation is that it only examined 7-year old children, not other ages.

A study of only 7 year olds. There must be a limitation in that.

It defined "true obesity" as children with body fat percentage (measured by impedence) above the 95^th percentile of their sample. This definition may or may not be OK, but it is arbitrary.

A study where “true obesity” was not studied, just assumed.

This study compared BMI thresholds to see which gave the most "sensitivity" for detecting "true obesity" correctly. They compared 2 types of thresholds, which I call type A and type B.

For type A, they quoted a BMI (for 7-year olds) of 19.3 for boys and 19.2 for girls as obesity thresholds, based on the IOTF definition¹. These values are quite close to, but slightly lower than- the CDC growth chart BMI values. Unfortunately, the reference they give for IOTF definition¹, does not seem to match these values ( 19.3 and 19.2). Instead, that reference¹ says 20.63 and 20.51! So I’m just totally confused because I cannot verify which criteria they actually tested for type A. Anyway, they concluded that type A criteria weren’t very good.

For type B, they referred to a UK dataset^2,3 of body mass index, and used the 95^th percentile BMI threshold value as a BMI obesity threshold. Unfortunately, their article never bothered to actually mention the actual values they used, forcing the reader to have access to the original dataset.

Finally, the study concluded that the best sensitivity and specificity BMI threshold occurred at the 92^nd percentile fo the UK BMI dataset. Unfortunately, their article never actually mentioned the actual BMI values of these 92^nd percentiles. Incredibly frustrating.

So the reader of the article learns methods but no usable facts about child obesity or BMI. I didn’t post this because it was good. In fact, it’s a weak article when it comes to using the percentiles to set thresholds for judging obesity or body fat percentage.

References

Cole TJ, Bellizzi MC, Flegal KM, Dietz WH. Body mass index in children worldwide: cutoff points for overweight and obesity. Br Med J 2000; 320:1-6.
Barlow SE, Dietz WH, Obesity evaluation and treatment: expert committee recommendations. Pediatrics 1998; 102:c29
Cole TJ, Freeman JV, Preece MA. Body mass index reference curves for the UK, 1990; Arch Dis Child 1995; 73: 25-29

I skipped reading the whole thing.

Good choice. Crap article.

That’s actually the point. It’s an example of some of the poor quality science that piles up and others build on it