Tuesday, October 23, 2007

Ignore RDA it's all BMI baby!

Yep it's time for BMI, hey isn't that an airline - no silly me it's Body Mass Index; so in the same way I did for RDA let's look at BMI.

BMI is a highly difficult calculation, actually it's not - take your weight in kilograms and divide it by the square of your height in metres (height times height) and out pops your BMI; simple huh. So let's start with the first point and that it's a measurement whose units are kg/m2 what does that mean? Well take a plank of wood say and give it a measurement of 1kg/m if I had 10 metres of it then it'd weight 10 kilograms; easy. Take a sheet of wood weighing 1kg/m2 then if I had 10 m2 then it'd weight 10 kilograms. A swimming pool full of water at 1kg/m3 then 10 m3 would weight 10 kilograms. Length, width, depth - m, m2, m3. BMI is m2 so you're a sheet.

Well no you're a volume, except it's possible to ignore one dimension in this context if you know it shares the same ratio with what you're measuring compared to everyone else; in this case we're ignoring two dimensions and just measuring one (height) so we're assuming everyone's width and depth ratios are identical? Okay maybe they are, well let's pretend anyway. So we've got our magic number that's popped out of our calculation and which is meaningless on its own - I've got a BMI of 25 is that good/bad/indifferent?

To find out what that figure means we need to compare it to other people's BMI, if overweight people have a BMI between 25-30 then I'm approaching overweight, if underweight people have a BMI of 25-30 then I'm underweight, again easy huh. So we take a bunch of people divide them into categories and assign their magic number. So how do we assign people, we can't use BMI to determine who is/isn't overweight/underweight as that's what we're trying to find out, hey we'll use insurance tables that'll work.

If we say that underweight and overweight people are risky then we can measure their BMI and see where they fall in terms of risk assessment - yay! We can trust the tables of course. Oh they'll need prodding and poking to weed out smokers etc. and they'll need updating when it's realised that the insurance companies themselves don't use them, but by and large we can trust them.

Congratulations we now have a simple system for determining how underweight/normal/overweight you are. Well not quite, trouble is muscle is denser then fat so let's try Rugby's Johnny Wilkinson 1.77m and 88kg. 88/(1.77*1.77)~=28 oops sorry Johnny you're overweight, your mum's going to get a letter from your school.

So to recap BMI treats you as an area, it appears to use pre-determined categories to place you, and doesn't account for muscle (or as it happens bone structure, sex, and ethnicity). In fact to use it to determine a person's category seems to be rather useless in this regard and there's a reason for that - it was never meant to be used like this.

BMI was designed for 'sedentary' people of average body composition for simple classification and that's it. It's not an all-purpose general population diagnostic tool. So why is it being used as one?