The unethical use of BMI in contemporary general practice

We seem to be regularly force-fed the idea that the body-mass index (BMI) is an appropriate method by which to measure obesity in our patients. Indeed, Read Code 22K5 implies that obesity is only attained when the BMI is calculated as 30 or greater. However this is not only overly simplistic, but such a glib understanding can in fact be harmful to the health of a significant proportion of our patients. The BMI can be traced back, rather surprisingly, to a famous Belgian. Adolphe Quetelet's interest in the emerging discipline of statistics in the mid 1830s saw him collect data on men's heights and weights at various ages. From this study, which he hoped would allow him to determine the ‘average’ man, he formulated what became known as the Quetelet formula, but which is now known as the BMI, and is calculated by dividing an individual's weight in kilograms by their height in meters squared. It can all seem so clear and precise that it may be something of a surprise to discover that its proper use in contemporary clinical practice generally requires that other factors also be taken into account. However, these factors are invariably ignored. In short, it was designed with Belgians in mind, and does not work so straightforwardly on such populations as Bangladeshis, Botswanans, or boxers. If its use is to properly help determine whether someone is obese and hence in need of some intervention to help restore health, or to reduce their risk of ill health, then those who use it should understand it rather better than most of them seem to do. The BMI is a surrogate measure of fatness, and as fatness is an indicator of health (‘if you have a BMI of over 25, you have an increased risk of serious health …