Is BMI a useful measure of healthy weight? © Getty Images

BMI: We know it’s flawed, so why do we still use it?

A report published by the Women and Equalities Committee says that the use of the Body Mass Index should be scrapped. So why do we still use it?

Barely a day goes by without a news piece or television documentary (a few examples of both I confess I can be blamed for) breathlessly reporting on this current pandemic of obesity. Nearly 60 per cent of us in the UK and US are either overweight or living with obesity.

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Where though, does this ominous 60 per cent figure originate from? Well, it has emerged from population-wide BMI statistics. BMI, or Body Mass Index, is the ratio of bodyweight in kilograms divided by the square of one’s height in metres, and hence is represented as kg/m2.

A ‘normal’ BMI is 20-25kg/m2. Anything below a BMI of 18 would be considered underweight, a BMI of 25-30kg/m2 is considered ‘overweight’, and if you have a BMI north of 30kg/m2, you would then be classed as living with obesity.

The reputation of BMI however, has in recent years been tarnished. At best, it is now considered a poor proxy for fat mass and health; whilst at its worst, it can and often is used as a cudgel to ‘fat-shame’ the larger amongst us in society. But what has BMI done to deserve this, and should it be replaced with anything else?

The problem is that BMI as a measure of individual ‘fatness’ is inherently flawed, because it is derived using purely your weight and your height. Thus it cannot, for instance, differentiate between a rugby player or a powerlifter and a Joe Public of similar height and weight, but carrying substantially more fat.

So why not just measure amount of fat instead?

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The ‘gold-standard’ method of measuring fat-mass is called Dual-energy X-ray absorptiometry or DEXA. This is where two low power X-ray beams, with differing energy levels, are used to scan your body. X-rays work by differentiating tissue density, so it is able to detect the difference between muscle and bone (which has a higher density) and fat (which has a lower density), and therefore be used to calculate fat-percentage.

Another approach that will be more familiar to many of you, will be the use of body-fat scales. These use a technology called ‘bioelectrical impedance’, and function by passing an imperceptible electrical current through your body. Because muscle contains more water, it conducts electricity better than fat does; thus the greater the electrical resistance, the more body fat you have.

DEXA is certainly both very accurate and precise. It is, however, also expensive, requiring specialist equipment and technicians to run, so is just not logistically suitable for use in population-wide studies.

And the body-fat scales, while widely available, ranging in size and cost from your domestic bathroom scales to large coin-operated weighing machines found in chemists, are notoriously inaccurate.

In contrast, BMI is cheap and easy to calculate, and is therefore easily scalable. And critically, despite being imperfect for measuring fatness in particularly athletic individuals, the sad fact is the VAST majority of the population are not rugby or powerlifter types. As a result, for most of us, the higher our BMI, the more fat we tend to carry, thus BMI is a suitable proxy for tracking weight and health of entire populations.

BMI, however, shouldn’t be used by health care professionals, or anyone else for that matter, as a be-all and end-all to inform treatment or advice for individual patients. It has to be used in context with other information, such as fasting insulin and glucose levels, blood pressure, as well as family history of metabolic disease. In fact, the UK Parliament Women and Equalities Committee recently published a report recommending that the use of BMI in determining if an individual’s weight is healthy should be scrapped immediately, as it contributes to health issues such as eating disorders and people’s mental health by disrupting body image and inviting social stigmas.

But why is carrying too much fat bad for you? There is a great deal of misunderstanding of what happens during weight gain or loss, with many thinking that they are gaining or losing fat cells. This is not true. Your fat cells are like balloons; they get bigger when you gain weight, and they get smaller when you lose weight. The actual number of cells doesn’t change by much at all.

Now, the safest place to store fat is in your fat cells. When the cells become full however, the fat ends up in places not designed to store fat in large amounts, such as our muscles or our liver for example, and that is when we tilt into diseases such as type 2 diabetes and other metabolic conditions.

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So, here is the interesting thing. Depending on our genes, our fat cells, or adipocytes, are able to expand to different sizes before becoming full. So East Asians (such as Chinese folk like me) and South Asians don’t have to put on that much weight before increasing their risk of getting type 2 diabetes. Whereas other ethnicities including white people and, famously, Polynesians for instance, can gain a lot more weight before becoming ill, in large part, due to the expandability of their adipocytes.

The degree to which our fat cells can expand is a powerful genetically influenced trait, and informs our differing safe fat-carrying capacities. So, in any given population, there is most definitely health possible at many sizes, with some larger folk of higher BMI being the picture of metabolic health, and other lean and fit-looking low-BMI people with type 2 diabetes.

But here is the critical take-home message: for any given individual, there cannot be health at every size, because if you surpass your own personal safe fat-carrying capacity, you WILL become ill.

The $64 million question is, of course, how can we tell what our safe fat-carrying capacity is? Well, figuring this out is the subject of cutting-edge research, and when that happens, we will enter a new era of how we relate fat-mass and bodyweight to health. Until then, the use of the humble BMI as a proxy for ‘fatness’ and hence health, at least at the population level, will continue to dominate.

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About our expert, Dr Giles Yeo

Dr Giles Yeo a geneticist at the University of Cambridge, whose research focuses on food intake, genetics and obesity. He is a presenter on Trust Me, I’m A Doctor, and his latest book is Gene Eating: The Story Of Human Appetite (£14.99, Orion Spring).