Sunday, 24 January 2010

So I'm not fat after all

I was reading a copy of Cycling Weekly from 17th September 2009 at a friend's house last week and I came across an article on BMI (Body Mass Index) and its applicability or otherwise to cyclists and other athletes. BMI You can read more about BMI here: (sorry - having problems with hyperlinks)

The CW article's main contention was that, as BMI was developed in the 1830s and further in the 1970s as a tool to be applied to allow generalisations about statistics of large populatons where having a higher body weight (relative to your height) increased your risk of disease. When used on the general population (generally relatively sedentary and with average body compositions), BMI might have a role but, as a marker for regularly active people, CW concludes that it is irrelevant and atheletes should, instead seek to measure their body composition through estimation of body fat composiiton (and a number of methods for this are recommended). Hurrah for that!

Now, I've long had a bit of a problem with BMI. As a biologist, I was, quite correctly, trained by my biometrics lecturer and eventual supervisor to regard with suspicion any simple numerical index that was providing a single number summary of complex biological relationships. In the language we used, the reduction of complexity to a single-value index resulted in the loss of most of the useful information. And so it is with BMI, I submit. Were I to calculate my own BMI (for example), it would have remained almost unchanged for the last 5 years (you don't need to know what it is, to spare my blushes), despite having trained, sometimes trained hard, for triathlons over that whole period (or the bits when I wasn't injured or ill). I have changed shape (in a good way) but not weight in the last five years as a result of swimming, cycling and running a lot. But whereas I might have lost fat, I've also put on muscle (on legs from cycling and shoulders from swimming) and, as we all know, muscle is heavier than fat, volume for volume. So, I haven't changed height, I haven't changed weight, I have become fitter and healthier by all sorts of measures, but my BMI hasn't really changed and it says I am overweight for my height.

So, do atheletes need to worry about this? Maybe not, if they are content to obtain their own body fat measurements as above. But consider how BMI can be, and is being, used more widely in society. In one example, some foreign governments are already using BMI as an indication of the relative weight (and by extension, health, and again by extension from that, fitness of prospective British adoptive parents looking at overseas adoption. Given the proclivity of the insurance companies for simple measures of risk (ever wondered why your postcode seemed to attract higher premiums for house insurance, even when you live in a nice place? Somewhere else sharing the first part of your postcode may have more of a crime problem but you share their cost of incresed risk - mmm, insurance coompanies wouldn't generalise, would they?), would they see BMI in a society with rising obesity levels as a quick (quack!) measure of risk for setting life insurance premiums. One to watch! As for me, I need to have my body fat composition measured as a more relaiable guide to health and risk of disease!

PS I did hear a radio programme about diet and weight gain once which claimed (and I have no reason to disbelieve it) that simply eating 100 calories per day in excess of your metabolic needs was enough to bring about a weight gain of about a stone (= 14 lbs = 6.35 kg) per decade. It doesn't sound like a lot of calories per day. It isn't: a typical chocolate digestive biscuit contains about 80-90 calories. So a single extra biscuit a day over your necessary calorie intake might be enough to bring about the kind of weight gain with age that I saw in many of my sedentary elderly relatives. Scary!

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