We just got a letter from Islington NHS saying that Jed is overweight! How can that be – Jed at nearly 5 years old is lean and fit. There’s barely any fat on him at all.
Apparently this wonderful news is the result of the National Child Measurement Programme whose mission seems to be to use simplistic tools to convince people to be more healthy. A quick search online led me to this survey from the Baylor College of Medicine in Houston that shows that the relying on BMI to determine whether someone has unhealthy levels of body fat appears to be wrong around 1/6 – 1/4 of the time.
I’d heard that muscle is more dense than fat – it turns out muscle is about 18% more dense than fat and so someone who has bigger muscles could well be in the “overweight” BMI range. There are other, more accurate ways to determine the levels of body fat: waist-to-hip ratio, skin-fold thickness measurements, and weighing in water (hydrodensiometry) are just a few. This last one is my favourite – I think a better heuristic than BMI is buoyancy. If it’s hard to stay afloat you’re probably OK.
The worst thing about the whole programme is the large-scale misuse and miscommunication of science. The BMI is clearly a flawed measure but we’re being told the gospel-truth according to science that Jed is overweight. It’s clear that he’s not and it seems likely that up to a quarter of these reported results are wrong. This kind of bad science just undermines the credibility of all science reporting and people will just ignore it.