Weighty problem for scientists as effectiveness of BMI is called into question

A study has revealed that obsessing over Body Mass Index is pointless since what really matters is the size of your waistline.

Waistline of 102 centimetres in men and 88cm in women indicate obesity. iStock
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You may have read the press coverage, seen the social media or noticed the posters. Whatever way, you will know the Ministry of Health is working hard to get the nation back in shape.

Everyone knows the problem – with almost two-thirds of the population overweight or obese, the health of the UAE is going pear-shaped.

Still, maybe you’re one of the lucky, healthy minority.

You may know your Body Mass Index (BMI) – that is, weight in kilograms divided by the square of height in metres – is below the figure of 25 officially regarded as overweight, let alone the 30 figure deemed obese.

If so, it is time to wipe that smug smile off your face.

New research has confirmed concern that the BMI is not a reliable way of telling if you are unhealthily fat.

An international study of more than 41,000 people in Australasia and Europe has shown it is entirely possible to have a healthy BMI and still face a 60 per cent higher risk of early death from cardiovascular disease.

The results, in the current issue of Obesity, show that the global obsession with BMI (even the World Health Organisation uses it) has obscured the greater reliability of a much simpler measure: waist circumference.

The rule could hardly be more basic.

Get a tape measure and put it around the narrowest part of your torso. If you’re a man and the figure is at least 102 centimetres, then you’re obese. For women, the figure is 88cm.

That’s it: no need to fret about measuring your weight, or how to square your height. Apparently what really matters is just the size of the spare tyre around your waist. It is that which holds the key to ill-health from overeating.

This may be good news for some people, as those waist measurement standards are pretty hefty.

Indeed, the researchers found that 6 per cent of people with obese BMIs may not, in fact, be at greater risk – because their waist measurements are just fine.

But there is bad news for the similar proportion of people whose BMIs are fine but whose waist measurements are anything but.

They typically face a 30 per cent increased risk, and double that in the under-65s.

While it may be unreliable on its own, a BMI exceeding 30, combined with an obese waist circumference, was linked with up to a doubling of the risk of death from cardiovascular disease.

If you find all this depressing, spare a thought for obesity researchers. For them, this is just the latest evidence that their field is a scientific basket-case.

As governments look to them for reliable insights into combating the global obesity epidemic, they are having to face the fact that they have even got some of the basics wrong.

The whole debacle is a salutary lesson in the dangers of so-called “physics envy”, believing that what has worked for the most successful of sciences can perform miracles elsewhere.

There is no doubting the success of physics in its own terms. When it comes to predicting the properties of subatomic particles or clashing black holes, it has been brilliantly successful. The recent discoveries of the Higgs boson and gravitational waves bear witness to that.

But that success comes largely from the fact that, despite their apparent complexity, physics problems can often be simplified and summed up using equations. And these lead to grand assertions with a decent chance of proving correct.

It is a strategy that has not worked well in the “softer” sciences, with their far harder challenges – like nutrition.

Take the case of the BMI. This took shape back in the 1830s in the work of a Belgian statistician named Adolphe Quetelet, who was keen to sum up human characteristics in mathematical form.

His studies of the weight and height of humans led him to claim that the former tended to be proportional to the square of the latter.

Why this “law” should hold is not clear but that didn’t prevent his ideas being picked up by the insurance industry looking for a simple test for those who are “disproportionately” hefty, and thus likely to die early.

By the 1970s, Quetelet’s law had been given a new name – the BMI – and had become central to research into obesity. It has stayed there ever since, impressive-looking, unexplained and – it is now clear – unreliable.

The study in Obesity makes a strong case for ditching the BMI from the toolkit of dietary science. But the wonder is it didn't happen decades ago.

Fans of sports such as boxing and rugby have long had access to data that reveals there’s a big problem with BMI. All it takes is a calculator and the height and weight of some heroes of these sports.

Many of these elite athletes turn out to have BMIs consistent with them being overweight slobs.

Charles Martin, the current IBF world heavyweight boxing champion, has a BMI of more than 29, which is borderline obese.

It’s nonsense, of course. The BMI formula simply cannot cope with the fact that muscle is more dense than fat, thus cramming more mass into a given height, and fooling the formula. Yet do not hold your breath waiting for BMI to be disavowed by researchers. They have proved remarkably resistant to ditching dodgy practices.

Despite repeated warnings, they still try to find links between diet and ill-health via studies that ask people to record their eating using diaries.

Unsurprisingly to anyone but, it seems, nutritionists, the results have proved unreliable.

This month, in a scathing analysis of almost 50 years of such studies, Dr Edward Archer, of the University of Alabama, said that people understated what they ate to the point where the data was “incompatible with survival”.

Even so, the findings of these studies are now the basis of US government dietary advice.

Even that most basic law of dieting – that a calorie is a calorie, implying it is quantity not quality that matters – has proved to be nonsense.

Lifted straight from the physicist’s playbook, this bit of thermodynamics holds true for simple systems like engines.

But shovel 2,000 calories of sugar daily into human “engines”, and they will need drastic repairs far sooner than those getting those calories via fish, fruit and vegetables. Human biochemistry is a tad more complex than a steam engine.

It is clear that governments have been fed a diet of pseudoscience by experts for decades. The proof can be seen breathlessly waddling around us every day. We deserve better.

Robert Matthews is visiting professor of science at Aston University, Birmingham. His new book, Chancing It: The Laws of Chance and What They Mean for You, is out now.