Badly Misused Indicator

Or, as some health professionals prefer to call it, “Body Mass Index” or BMI.

Let’s get this over with before I start. I’m overweight. I know I’m overweight. I have no idea by how much. Quite possibly a fair bit, but maybe not so much. My BMI is currently a fraction below 35 defining me as “obese”, but even in my mid twenties when I was very physically active and fit and had barely a scrap of spare flesh on me I was still classed as “overweight” (as noted by the doctor who did my scuba diving medical).

Today there’s a piece on the BBC News website about how people are in denial about their degree of excess weight and a load of guff (and that really is what it is) about BMI and how it tells you you’re overweight. Here’s the real news: using BMI as a measure of how overweight (or even underweight) a person might be is complete rubbish.

BMI was never designed to work that way and is in fact a formula fudged up about two hundred years ago to fit the available data such that someone (in the dataset) who was overweight/obese should have a high BMI figure (and someone underweight would have a low one). However, it doesn’t work backwards any more than throwing a whole cake into a mixer splits it into all its component ingredients. Even the chap who came up with the BMI formula said it shouldn’t be used as an indicator of how over/underweight someone was!

To show that it doesn’t work that way all that’s required is a single counter-example. Step up British Olympic champion Adam Peaty. Renowned for having almost no body fat whatsoever, his BMI puts him right smack in the middle of the “overweight” band. Have another for free: All Black rugby player Jonah Lomu (search for some images if you don’t know of him) was, according to his BMI, obese. It also fails to work by classing some people as a “healthy” weight when in fact they probably aren’t.

Yet the entire concept of BMI as an indicator of health has become so pervasive that even medical researchers believe it, which is where we came in. The BBC piece referenced some research done a few years back showing that peoples’ views of whether they were overweight and by how much. The subjects were asked if they thought they were the right weight, overweight or obese and a poor correlation was found between their views and how they would be classified according to their BMI. Well, what a surprise! What would you expect if it isn’t a good indicator?

So the question remains, why are people who call themselves “health professionals” so wedded to the idea of BMI being a good indicator when it clearly isn’t? The text with the calculator on the BBC page even suggests what your BMI ought to be for you to be healthy. You just can’t do that! It’s totally flawed.

It really is time we moved on from this daft measure and found a better way.

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