Body Mass Index (BMI) has been in the news recently. I’ve picked out two stories from the past fortnight on this little formula – because that’s all it is – a person’s weight in kilograms, divided by their height in meters squared.
The evidence base for BMI
Just as other public health advice (5-a-day, 14 and 21 alcohol units, 150 minutes exercise a week, 30% of calories from fat, 10% of calories from saturated fat...) is not evidence based, so BMI categories are not evidence based. They are also numbers ‘plucked from thin air’.
Here’s an extract from the fabulous Doctoring Data by the brilliant Dr Malcolm Kendrick, where he traces the origin of the notion that a BMI of 30 = obese:
“The figure itself was agreed at a meeting in Geneva, which generated a long and very boring publication. It stretched to more than six hundred pages. I know, I read it. Somewhere about page four hundred and something I finally came across the passage that I had been hunting for. It was pretty much as I had expected:
‘The method used to define BMI cut-off points has been largely arbitrary’.
Largely arbitrary. An interesting combination of words. I suppose it sounds a bit better than ‘plucked from thin air.’ Or… ‘we just made it up, because we felt that we had to say something.’ The world is gripped with terror that so many of us are now obese. I call it the ‘largely arbitrary’ epidemic. (p245)
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