Friday, 28 May 2010

On the Flaws of the Body Mass Index

I’ve been promising an essay on the BMI (Body Mass Index) for a while now. And I tried, I honestly tried. I wanted to write a fully comprehensive essay, with a balanceds argument for and against the BMI and its use. But I just couldn’t. There are too few pros for the system, and far too many cons. It left me wondering why it is used at all, but one of the two things I found in its favour explains exactly why it is used.

It’s easy to use.

It involves no expensive equipment, no complicated formulae, no extensive testing. It gives an immediate piece of quantitative data on how fat a person is, and therein lies its other advantage. It gives a number that can be quoted. Instead of a doctor simply saying you’re (and by you I mean a hypothetical fat person, not you in particular) fat, they can say your BMI is X, which means you are this fat. It also means they can tell you exactly how much weight that a person needs to lose to not be that fat any more.

That’s it. That’s the whole reasoning behind it being used.

Now compare that to ever increasing evidence that the BMI is an utter load of crap.

For starters it doesn’t take into account bone density or muscle mass, meaning that many incredibly fit athletes show up as obese on the scale. The Wikipedia page gives a list of exceptions to the BMI that makes me wonder why it is used at all.

Secondly, it was never meant to diagnose individual people. It was meant to study populations of people, so how the hell is that translatable to use on individual people? How the hell did this become the primary tool for diagnosing overweight and obese individuals?

Add in to this the fact that the cut off points are incredibly arbitrary. I’ve never seen any good reason why you’re suddenly overweight when you’re BMI hits 25. I’ll be happy to read anything anyone can show me that proves otherwise, but I will continue with my argument.

I know that the cut off points between underweight/normal/overweight/obese are arbitrary because in 1998 the USA changed theirs to fall in line with the World Health Organisation. Over night millions of people were suddenly reclassified as overweight without gaining a single pound, and without their health changing one jot. All because a committee of “exerts” saw fit to move the goalposts.

The BMI system is also incredibly biased against people at the higher end of the spectrum. Take a look at the chart on the Wikipedia page. There is a huge chunk of the chart taken up by overweight and obese, but only a tiny sliver taken up by the underweight category. That’s shows a clear bias and just seems plain wrong to me.

Speaking of plain wrong, the BMI is exactly that. The World Health Organisation defines overweight and obesity as such:

Overweight and obesity are defined as abnormal or excessive fat accumulation that presents a risk to health.

That means that the BMI is rubbish. Studies have shown again and again that people in the overweight category have no significant health problems compared to “normal” people. I fact, they seem to cope better with certain diseases that the supposedly healthier normal category.

In addition to this, the BMI categories just don’t fit into reality, not mine at least. Check out this flickr stream, I hope you will find as I did, that the BMI overweight people looked normal to me and the BMI normal people looked too thin. That’s just my personal view, but those photos should definitely make you rethink how the BMI applies to real, flesh and blood people.

There are a whole host of other reasons why the BMI is crap, and it should be scrapped for use in diagnosing “overweight” and “obese” people. If you currently remain unconvinced of just how crap the BMI really is, may I point you to the links below. They cover similar ground to this post, and hopefully a few extra things.

Read the links and the comments over at this post at the Rotund.

This post at the Guardian's Comment is Free also takes down the BMI.

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