24 Jan, 13 | by BMJ
In 1832, whilst trying to define ‘normal’, Adolphe Quetelet, Belgian polymath, defined an index – the Quetelet Index – which later became known as the Body Mass Index (BMI) and is used as an indicator of obesity which has become recognised as an important marker of early mortality. 
The formula is well known to us though rather cumbersome to calculate. It is weight in kilograms divided by the square of height in metres. We are all familiar with the swathes of colour that map out underweight, normal, overweight and obese. We may very well say to people in our clinics that ‘according to your BMI you seem to be obese!’.
However, there is a problem with the index in that it doesn’t quite catch the nature of obesity and human growth. Those who tend to be below average height score slightly lower on this index than perhaps they should. The opposite is true of people who are taller than average. Clearly we can’t change our height but when certain recommendations  have explicit levels of BMI we need to be sure it is representing what we feel it should. The problem is mainly to do with the power that the height is raised to and Quetelet seems to have recognised that too (though he wasn’t so concerned with obesity). Babies seem to require a power of 3 to correctly recognise their growth in all dimensions and children would probably benefit from a figure of 2 since they grow more vertically. Adults fall in between and 2.5 seems to fit better.
So, Professor Nick Trefethen of Oxford University, made all these points and has suggested a refined formula following a letter to The Economist where he said:
“millions of short people think they are thinner than they are, and millions of tall people think they are fatter”
His formula is:
BMI = 1.3*weight(kg)/height(m)2.5
In the diabetes literature there is concern over the suitability of BMI in those with short-stature  and its usefulness when compared with other measures .
So, will you be using the new (slightly more cumbersome) formula in your clinic?
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2. NICE Clinical Guideline 43. Obesity. http://www.nice.org.uk/cg43
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4. Lee CM, Huxley RR, Wildman RP, Woodward M. Indices of abdominal obesity are better discriminators of cardiovascular risk factors than BMI: a meta-analysis. J Clin Epidemiol. 2008 Jul;61(7):646-53. doi: 10.1016/j.jclinepi.2007.08.012.