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BMI Fixed?



We all think about whether our weight is normal or not, and to find out in more certain terms many of us turn to the BMI to tell us.

Though how much should we trust the results if the calculation that produces them is wrong? Nick Trefethen has produced a new way of calculating BMI, but has it solved the problems of the equation?



The BMI (Body Mass Index) has been around for a long time, and in that time has come under criticism for the results it produces. A new way of calculating the index has been proposed but there is debate over whether or not it solves the problem of the original BMI.

BMI is calculated by taking your weight in kilos and dividing it by your height squared in metres. The problem with this is it does not take into account weight being fat or muscle. This had led to problems such as Jonny Wilkinson being labelled as overwight due to his BMI. 

The new version of the BMI has been put forward, not by a doctor but a mathematician, Nick Trefethen, who thinks the original is wrong and people place too much faith in it because it looks like such a precise calculation like E=MC².

Trefethen says, "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." 

His proposed version of the BMI looks like this, 1.3 x weight, divided by height to the power 2.5. 

The changes to the formula should mean tall people who were previously graded as overweight could now be normal, and some short people who were normal could now be overweight. 

Tim Cole, a medical professor at UCL thinks that Trefethen's paper with this new equation is just another in a long line of papers where people have tried to improve the BMI. 

He says, "I think he's aware of the deficiencies of BMI but I don't think his index goes anywhere to addressing those shortcomings. BMI is an imperfect index and you can tinker about with the power of height as much as you like but [the equation] shouldn't be expected to do what it can't do which is to measure fatness, because it doesn't include fatness." 

"BMI can't do better than make a guess as to how much fat there is. If your weight is excessive then the implication is that the excess is fat and, of course, people's body composition varies enormously and BMI doesn't know anything about that so it's a very imprecise index when applied to individuals."

Yet we do continue to use it, it is an easy way to get a quick idea of what sort of weight category you fall into, with better alternatives being costly. Also it is useful for comparing the weights of large groups of people of different heights.

Read the original article and calculate your BMI here:

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