BMI: IT’S FLAWED, SO WHY DO WE USE IT?
A report published by the Women and Equalities Committee says that the use of the Body Mass Index should be scrapped. Is there a better way to calculate if you’re a healthy weight?
Barely a day goes by without a news piece or television documentary breathlessly reporting on the current pandemic of obesity. Nearly 60 per cent of us in the UK and US are either overweight or living with obesity. Where 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 be classed as living with obesity. The reputation of BMI however, has in recent years been tarnished. At best, it’s considered a poor proxy for fat mass and health; at its worst, it can and often is used as a cudgel to ‘fat-shame’ the larger among 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 ‘fatness’ is flawed, because it is derived using purely your weight and your height. Thus it cannot, for instance, differentiate between a rugby player and a Joe Public of similar height and weight, but carrying substantially more fat. So why not just measure the amount of fat instead?
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 DEXA is able to detect the difference between muscle and bone (which has a higher density) and fat (which has a lower density), and therefore can calculate fat percentage.
Another approach is the use of body-fat scales. These use technology called ‘bioelectrical impedance’, and function by passing an imperceptible electrical current through your body. As 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 accurate. It is also expensive, requiring specialist equipment and technicians to run,
so is not 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 simple to calculate, and therefore easily scalable. And critically, despite being imperfect for measuring fatness in 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 healthcare professionals, or anyone else, as a be-all and end-all to inform treatment or advice for individual patients. It has to be used in context with other data, such as blood pressure, fasting insulin and glucose levels, as well as family history of metabolic disease. In fact, the UK Parliament’s 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, as it contributes to issues such as eating disorders and 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 isn’t true. Your fat cells are like balloons; they get bigger when you gain weight, and 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, the fat ends up in places not designed to store fat in large amounts, such as the muscles or liver, and that is when we tilt into diseases such as type 2 diabetes and other metabolic conditions.
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. 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, it is possible to be healthy at many sizes, with some people 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 BMI as a proxy for ‘fatness’ and hence health, at least at the population level, will continue to dominate.
“BMI cannot, for instance, differentiate between a rugby player and a Joe Public of similar height and weight, but carrying substantially more fat”