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New body mass index good news for the long and lean

January 22nd 2013

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The way the BMI is calculated has been changed - and it'll leave tall ladies on a high, writes Anna Hunter

Today's ‘Going Up’ takes on quite a literal meaning with the news that those of taller stature can legitimately consider themselves less porky. No crash diets, bootcamps or liposuction is involved; we have maths to thank for this revelation.

We bet that you’ve never really taken a moment to be grateful to maths. All hail maths. For it is maths, and more specifically Oxford University mathematician Dr Nick Trefethen, that has liberated the more bulky, Miranda types among us of beastly BMI points. In short (we'd rather not be): you're skinnier than you were yesterday without having lifted a finger.

This uplifting report was issued by researchers who have revised the formula for BMI calculation, which is currently assessed by dividing a person's weight in kilograms by their height in metres squared. In a letter to The Economist, Professor Trefethen states that the current BMI calculation 'divides the weight by too large a number for short people and too small a number for tall people'.

So, as it stands, Long Tall Sally has broken through the obesity ceiling, when actually she may simply be a little bit overweight, while shorter Sharon has been let off the weight management programme even though her waistline is wider. As I said, this is definitely a happy day for tall people.

Professor Trefethen's modified formula multiplies a person's weight in kilograms by 1.3, and divides the answer by a person's height in metres to the power of 2.5. Counting calories suddenly looks like a piece of cake - although unfortunately you better not eat that, as the Body Mass Index is especially prone to underestimating obesity in women. In fact, BMI is considered to be a somewhat flawed weight assessment tool in general, as it doesn't take into account fat to muscle ratio, distribution of body fat or bone density. Nevertheless, it is a nice rounded number by which to judge where you're at on the global tubby scale, and it's certainly a more comfortable test than the skin-fold pinch.

Dr Trefethen's formula has not yet been adopted by the NHS or other health professionals, so the petites can breathe out; but his BMI calculation is considered to be much more accurate. Which means that 6ft gangly girls just lost a whole point. We don't recommend navigating snowy drives in five-inch heels but if there was ever a health reason for extra height, it's now.

Anna Hunter

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