A third of British children are either overweight or obese when they leave primary school, according to the NHS – and the figure is rising year on year. But how do they know this? Our children’s schools haven’t been visited by a civil servant with a tape measure and a set of scales, so how do they calculate it, asks Michael Donlevy?
The NHS uses the Body Mass Index (BMI). It’s a well-known formula for calculating your ‘healthy’ weight in relation to your height, which the government applies to annual health surveys of about 6,500 households. But what is BMI really? And how accurate is it?
The first shock is that the BMI is ancient. It was devised in 1869 by Adolphe Quetelet, a Belgian astronomer who decided that, in adults of ‘normal build’, weight was proportional to the square of their height. That’s how he devised the formula we still use today: your weight in kilograms divided by your height in metres squared. If your score is greater than 25, you are overweight. If your score is 30 or more, you are obese.
One word leaps out of that definition: ‘adults’. Yet the NHS, the government and the World Health Organisation all use BMI to measure childhood obesity. ‘In children, BMI needs to be entered into the appropriate gender BMI charts as their BMI needs to be calculated as a percentile,’ says GP Dr Ian Campbell. ‘The charts were recently revised so they’re up to date.’
Yet the method is flawed: BMI doesn’t account for body type, bone density, stage of growth or genetic factors. It is, quite simply, over-simplistic. It also fails to differentiate between muscle and fat. Brad Pitt, who is 1.83m tall, weighed 92kg while packing on muscle for his role in Troy, which gave him a BMI of 27.8. Did he look overweight to you?
Randy Schellenberg, a Canadian scientist who has studied the vagaries of BMI, says, ‘The formula ignores one fundamental law of physics: that volume, and therefore mass and weight, increases by the cube of the scale factor rather than the square.’ In layman’s terms, he’s making the point that, according to the BMI, we are two-dimensional.
There is also a problem with Quetelet’s use of the words ‘normal build’: BMI doesn’t work for very short people. In the US, Schellenberg says, this has been blamed for a huge rise in the number of pre-pubescent girls who have been diagnosed with anorexia.
He points out that BMI is also inaccurate among tall people, citing an exaggerated but interesting example. In Gulliver’s Travels by Jonathan Swift, the giant Brobdingnagians are 10 times the size of Gulliver, but with the same proportions. This gives them a BMI of 244.4.
Neville Rigby, former director of policy for the International Association for the Study of Obesity, agrees that the tide has turned against BMI. ‘One of the biggest problems is abdominal obesity, which BMI doesn’t take into account,’ he says. ‘Stomach fat, leading to a big waistline, is usually the killer.’ Research in the States, involving more than 17,000 obese Americans, found that the worst combination of cardiovascular risk factors – irrespective of weight – was in people with large waists and narrow thighs.
Australian researchers claim the waist:hip ratio is a more accurate indicator of obesity than BMI. During an 11-year study of 9,000 adults, researchers found that men whose waist:hip ratio was 1.0 or more were more likely to suffer from the cardiovascular problems associated with obesity. Professor Tim Welborn, who led the research, says, ‘Somewhat surprisingly, waist:hip came out far better than anything else, including BMI, cholesterol and blood pressure.’ He even called for the WHO to reconsider its reliance on BMI.
This throws up another problem. ‘It’s possible to be obese yet have a perfectly normal BMI,’ says Rigby. It might sound as if all these experts are contradicting each other, but they’re making a consistent point that BMI cannot take into account body composition. A short child with a big tummy and stick legs might have a lower BMI than his tall, well-built friend, but a greater risk of heart disease. Girth is more important than weight.
The waist:hip ratio also has the benefit of being straightforward – all you have to do is divide your waist in centimetres by your hips in centimetres – yet there is another flaw. ‘Waist circumference doesn’t work for children,’ says Campbell.
Rather than support BMI, maybe this shows we are becoming too obsessed with tape measures and scales. ‘I’ve written often about the need to avoid medicalising weight in children,’ says Campbell. ‘Far better for parents to focus on healthy living, diet and exercise, and the feel-good factor that comes from good food and activity.
‘Evidence shows that many parents don’t, in fact, recognise overweight in their own kids, so I encourage parents to compare their child with others. Do they look heavier? Does he fall behind in running games? Is he getting teased at school? Does she have difficulty getting appropriately aged clothes to fit? And the rest of the family need to look at themselves, because what’s good for the kid is also good for the parents and siblings – a healthy diet and exercise.’