Body Mass Index: a big fat lie

spiked and Wellcome Collection have launched a new website to debate the best and worst ideas, phenomena, developments and practices in the history of medicine, in the run-up to two big live debates in London on 26 June and 17 July. Here, Patrick Basham and John Luik nominate Body Mass Index as the worst idea.

There are four central problems with Body Mass Index (BMI), a method of comparing someone’s weight and height first proposed by the Belgian researcher Adolphe Quetelet in the nineteenth century and which has found increasing currency as a measure of the ‘obesity epidemic’ in recent years.

First, it fails to provide an accurate measure of overweight and obesity. Second, it is arbitrary in its classifications of normal, overweight and obese. Third, its classifications of overweight and obese generally do not correspond to increased risk for premature death or serious illness.

And fourth, its unscientific character has allowed it to be transformed into the major tool for misrepresenting the risks of overweight and obesity and justifying a ‘war on fat’.

The BMI provides an inaccurate measure of overweight and obesity because it cannot distinguish between fat, muscle, organ and water. BMI is simply the ratio of body weight (in kilogrammes) divided by the height in metres squared (kg/m2). As such, it tells us nothing about what the weight is made up of. Bodybuilders, for instance, may have high BMIs despite having little fat. The BMI, being simply a measure of weight and height, cannot account for body frame, for instance the differences between men and shorter women, nor is it an accurate measure of children’s weight status. Slight Sri Lankan children in Australia have more body fat than white Australian children with the same BMI.

But the BMI is also arbitrary in that it simply reflects someone’s view as to where to draw the line between normal, overweight and obese. There is no scientific reason why someone with a BMI over 25 should be labelled overweight or someone with a BMI over 30 as obese. Professor Tim Cole of the UK Institute of Child Health, who devised the overweight and obese classifications for children, admits this. As he said: ‘The idea that these numbers are cast in stone is absolute nonsense.’

Moreover, there is little connection between the BMI classifications of overweight and obese and increased risk for disease and death, either in children or adults. A recent Aberdeen study found that children’s BMIs were not associated with increased risk for stroke or heart disease in later life.

The studies by Flegal (at the US Center for Disease Control) and Gronniger have found that the lowest death rates are for those in the overweight category of BMI 25-25.9. Indeed, this group was most likely to live the longest. Gronniger’s study reported that moderately obese men had the same mortality rate as ‘normal’ weight men. Other studies have found that those who are overweight have the lowest risk for both total mortality and cardiovascular mortality compared with ‘normal’ weight individuals.

Finally, the BMI’s very arbitrariness and unscientific character allows it to be manipulated by both special interests, such as the weight-loss and pharmaceutical industries and politicians. The BMI has become the red flag of the obesity epidemic – immensely useful for frightening everyone into believing that they are too fat and in imminent peril of death, not to say increasing their already obsessive interest in dieting and weight loss.

With the change in where the BMI overweight line was drawn, millions of people went to bed normal and woke up suddenly ‘overweight’, despite no evidence that their weight posed any danger to their health. If two-thirds of the population comes to believe that their BMIs say that they are either overweight or obese, then there are immense profits to be made and political empires to be built in providing them with the means to slim down and become ‘normal’.

Patrick Basham and John Luik are co-authors, with Gio Gori, of Diet Nation: Exposing the Obesity Crusade, a Social Affairs Unit book. (Buy this book from Amazon(UK).)

For more information on the live ‘Best and Worst’ debates, and to join the debate, click on the icon below.