Your Body Mass Index is the number that defines whether you are normal, fat, obese or morbidly obese. With the latest news that the Government is looking at bariatric surgery (gastric by-pass or gastric band operations) for those suffering from Type 2 diabetes, the BMI is being quoted as the measuring stick – almost a magic wand – to ascertain who qualifies for such surgery and who doesn’t.

But let’s take a closer look at the BMI. Where did it originate from, and why? You may think it’s a modern development, by medical professionals keen to monitor the health of the general public. Think again. What is now called the BMI was actually invented in 1832 by a Belgian called Lambert Adolphe Jacques Quetelet. A mathematician by profession, he specialised in statistics and was among the first to try to apply statistics to social science. As part of his study into the concept of “the average man”, as defined by variables that follow a standard mathematical distribution curve, he came up with  a simple measure for classifying peoples’ weight relative to an ideal weight for their height. A person’s Body Mass Index – following Quetelet’s formula – is defined as their body weight divided by his / her height squared. Nowadays this has been translated to a measurement of Kilograms per metre squared (or, for the more technical among you, kg/m2).

The formula devised by Adolphe Quetelet was never meant to be used in the way it is now by the medical profession for defining obesity: it was purely a method of defining standard proportions of the average human build. In fact, Quetelet himself specifically warned against using his Index for individual diagnoses because of the many variables.

However, in the early 20th century, as the insurance industry came into its own, underwriters looked for a method of classifying people for health insurance and life assurance policies – and found the BMI, which is still also called the Quetelet Index. Et voilà – a social science formula became increasingly used as a medical industry measure. In 1972 a physiology professor and obesity researcher called Ancel Keys published “Indices of Relative Weight and Obesity”, the result of a study of some 7,400 men in five countries. Having compared various height-weight formulae, Keys decided that Quetelet’s kg/m2 to be the most accurate and so Quetelet’s Index became the Body Mass Index.

Because it was a simple formula, it allowed researchers – especially those in the insurance and obesity industries – to go back through back data and define what they perceived to be past levels of obesity. There are many obvious reasons why the BMI as currently based on Quetelet’s formula should not be the magic measuring stick for defining obesity.

For instance, it doesn’t take into account relative proportions of fat, muscle and bone in a person’s body. Bone is denser (and therefore heavier) than muscle, and about twice as dense as fat. People who go to the gym and work out a lot build strong, dense bones, lean muscle and have low fat – and yet they are frequently classified as overweight or obese! This shows that someone who is health conscious and very fit can score just as high on the BMI scale as someone who is genuinely obese because of wrong eating, lack of exercise or serious health issues.

There is no such thing as “an average man” any more than there is “an average family” with 2.4 children. Gender, age, ethnicity and body frame size can all influence the BMI figure – and yet these are rarely taken into account.

Over the years the classifications have changed too – the figure at which someone is now labelled “obese” seems to get progressively lower. There are many websites offering “find your BMI” charts. You can see for yourself how just that one more gram pushes you from overweight to obese; how just a couple of centimetres makes a big difference.

I am a good example of how it doesn’t work. I fall easily into the Obese bucket on the BMI and yet recent blood tests at my local health centre gave me a completely clean bill of health: liver and kidney both working perfectly; cholesterol and blood sugars fine, and blood pressure absolutely fine, thank you very much. I’m not saying that I’m totally saintly – I should eat smaller portions – but I am a big lady, with big bones and broad shoulders (and broad hips to match, dammit!). I had a sister who was 6 ft 3 ins and in my prime I was 5 ft 9 ins (I’m down to 5 ft 7 1/2 ins now, I’m told!). Totally different to someone of the same height but thin as a beanpole – and different again to someone from a different ethnic group, or 20 years younger . . .

There is no denying that obesity is a growing problem in the UK. But if the best measurement we can come up with is a 200-year-old formula devised by a mathematician for social science studies, then (in my humble opinion) we need to address that too.

Linked content:  Weight Loss