A new way of calculating Body Mass Index (BMI) has been proposed - but does it really solve any of the BMI's well-known problems?
How often have we heard that Brad Pitt at the time of Fight Club, and England rugby player Jonny Wilkinson in his prime, were "overweight" - according to their BMI?
Any system that tells people whether they are "normal", "underweight", "overweight" or "obese" is bound to be controversial, but one obvious weakness of the BMI is that it doesn't distinguish between fat and muscle.
First devised by Adolphe Quetelet more than 150 years ago, BMI is calculated by taking your weight (in kilograms) and dividing it by your height squared (in metres).
In simple terms, it is a way to compare the weights of groups of people of different heights.
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Letter to the Economist 05/01/13
Sir, The body-mass index that you (and the National Health Service) count on to assess obesity is a bizarre measure... As a consequence of this ill-founded definition, millions of short people think they are thinner than they are, and millions of tall people think they are fatter.
Prof Trefethen's letter in full (third in list)
His explanation of his formula
But mathematician Nick Trefethen, Professor of Numerical Analysis at Oxford University, thinks that the old formula is wrong, as he explained in a letter to the Economist newspaper published earlier this month.
He thinks that people have put too much trust in it in part because it looks so precise - like, say, Einstein's famous equation E=MC².
"That's an equation of physics and it's really right. The BMI formula looks similar. It seems to have the same character but it doesn't reflect a precise truth about our world, it's an approximation to a very complicated reality," he told the BBC.
With that in mind he has proposed a new formula: 1.3 x weight, divided by height to the power 2.5.
