The bunkum of Body Mass Index

You can’t even go for a stroll these days without it being turned into a health initiative. ‘Green spaces are good for us. People living close to parks are more physically active and less likely to be overweight or obese’, declared the UK environment secretary, Hilary Benn, today. Benn was commenting on the launch of a new scheme – the ‘Natural Health Service’ – that will see chubby people quick-marched around a south London park for 30 minutes on a regular basis to help them lose excess pounds.

This is just one of many schemes that aims to turn everyday life into a focus for government intervention, all in the name of saving us from the ‘obesity timebomb’. According to the UK Department of Health, ‘almost one in four adults in England are currently obese, and if we carry on as we are by 2050, nine in 10 adults will be overweight or obese. The cost of overweight and obese individuals to the National Health Service is estimated to be £4.2billion and is forecasted to more than double by 2050. The cost to the wider economy is £16billion, and this is predicted to rise to £50 billion per year by 2050 if left unchecked.’

In other words, by 2050, the government is suggesting that 90 per cent of the population will be ill and that this ill-health will cost the British economy a small fortune. What is required, therefore, is a determined attack on the flab to make us all into normal, skinny, morally upstanding individuals.

It’s never too soon to start, with the government’s Change4Life advertising campaign aiming to ‘make the subject of weight and physical activity a hot topic’ for everyone. So we’re currently being treated to animated TV adverts in which a child explains how playing computer games and sitting round at home would lead to him and his family getting really fat and unhealthy. The NHS was launched in the 1940s to provide healthcare ‘from cradle to grave’. Now health – and the issue of obesity in particular – is being used by the government to inveigle its way into every aspect of our lives from the moment we’re born till we draw our last breath.

The claim that we face an epidemic of obesity is based on the changing distribution of body mass index (BMI) in society. BMI is a fairly crude way of assessing fatness first proposed by a Belgian mathematician, Adolphe Quetelet, in 1832. It is defined as a person’s weight in kilograms divided by their height in metres, squared (kg/m²). As a recent piece in Slate magazine notes, BMI only gained popularity after the well-known American researcher, Ancel Keys, proposed it as the best way of quickly assessing fatness in an article in 1972.

The other major development was the establishment of cut-off points for weight and health, based on BMI, by the US National Institutes for Health (NIH) in 1985. Now BMI had the force of official backing. As a means of giving health authorities a rough idea of how our bodies are changing, BMI is crude but has the merit of simplicity. However, when applied to individuals, it has the potential to be downright distorting. Firstly, it makes no distinction between fat and muscle mass. Secondly, it is blind to where fat is stored on the body, which some researchers and doctors believe may be crucial to whether it is a health problem or not. Thirdly, it gives a pseudo-scientific precision to the notion that carrying a bit of extra weight is going to kill you.

BMI is actually a poor indicator of future health prospects. There is little difference in mortality rates between people who are of ‘normal’ weight (BMIs of 20-25), ‘overweight’ (BMIs greater than 25 but under 30) and ‘obese’ (greater than 30 but less than 40). Only those who are ‘morbidly obese’ (BMI over 40) and those who are underweight (BMI below 18) have markedly worse health outcomes than those in the middle. Yet on this shaky foundation, a whole industry of weight-watching, health-obsessing, parent-berating government intervention has been built.

BMI’s simplicity has been perfect in this endeavour; anyone can calculate their own BMI if they know their weight and height, and can then immediately start fretting about the result. The result is almost certain to be of concern because the goalposts set for BMI mean that the majority of the population falls into one of the apparently unhealthy overweight or obese categories. Even if we care little for our own spare tyres, the health authorities and the media assure us that we’re condemning our children to a lifetime of ill-health if we allow them to get fat.

Yet, as our BMIs go up, so it seems does our life expectancy, suggesting that obesity is not the killer it is made out to be. Indeed, we’re caught between two ‘timebombs’ at the moment: the obesity timebomb, which threatens to shorten our lives and those of our children; and a pensions timebomb, which will bankrupt the nation because we’re all living too long. Can these two contradictory panics both be right?

We should be very sceptical about the claim that being fat will kill us – and refuse to accept the state’s endless attempts to use the obesity panic to micromange our lives.

Rob Lyons is deputy editor of spiked.