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Monday, 1 October 2012

In defence of BMI

Every trainer in the land chokes on their carrot stick when they hear the letters BMI. I used to be one of those trainers too. What exactly is it and why do trainers get their knickers in a twist?

BMI is a calculation which divides a person's weight by the square of their height. A score of 25-29.9 puts you in the 'overweight bracket' and anything over 29.9 classes you as obese. It was originally invented in the 1800's and its purpose was to see what the average man looked like and had very little to do with health. A guy called Ancel Keyes studied this relationship between height and weight (originally invented by Adolphe Quetelet). After looking at different formulas, he determined that this was the best one and it became known as the Body Mass Index. However, Keyes rubber stamp came with a warning. It shouldn't be used for individual diagnosis as it ignores a lot of other factors. Factors which many trainers and are in uproar about. Here are some of the issues trainers have.

Your BMI will go down if you get taller but stay the same weight. It will also go down if you get lighter. You aren't going to get taller as an adult. In fact, you will get shorter as you age. Also, it doesn't take into account sex, ethnicity, genetics, amputees....the list goes on. So it's pretty obvious to see some of the flaws. It was only meant to be used as a guide and further tests would need to be done to ascertain health issues. Now it is used by Doctors, insurance companies, weight loss companies.....the list goes on. So should it be ignored? I used to think so, but now I'm not too sure.

When you collect data together like this and try to see any relationships between the group, people will always get missed off the edges. In the last week I have heard one guy say "BMI is rubbish because I'm apparently obese yet i'm 5ft 8", weigh 13 stone and 5% bodyfat." Well your argument falls flat on your face mate. How many people are that lean? You'd be lucky to do skinfold measurements on 10 random guys in the street and get an average bodyfat reading under 20%. Then you have the slim guys who are TOFI shaped(Thin outside fat inside). They will have an apparently healthy BMI yet may be less than healthy (higher levels of visceral fat around internal organs). Again, in today's society, this isn't indicative of the average person (TOFI shaped people, not high levels of visceral fat).

So what am I getting at? Trainers and other critics of BMI seem to have helped convince those who have dangerously high bodyfat levels that there is nothing to worry about. I agree that it is a pretty arbitary measure and never use it myself. I'm classed as overweight on my BMI. Howver the reality is if your BMI says you are obese, there is a high probability that you need to do something about it. BMI is pretty useful in telling people who are kidding themselves that they need to take action. Not everyone can get their bodyfat checked or get their health checked in a few minutes. However, you can get your BMI done in a matter of minutes. Any trainers who think BMI is nonsense should think about doing the following study. The average person does not exercise 3 times a week and is not muscular (in the western world). So this group should not be included. You'll need an 'average range' for heights of men and women. Anyone very tall or a lot shorter than the average shouldn't be included either. Finally, very thin people shouldn't be included. In the group that is left, any person who they start training who has a BMI score over 29.9, send them off to get some health checks. When they get the results, either put a tick or a cross next to BMI. I guarantee that you will have more ticks than crosses.

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