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BMI: The Problem with Simplifying Health

Symbolising the darker side of BMI and it's view of our health. Image shows a black sky with grey smoke rising and a gold halo above.

Seemingly, BMI is one of the simplest and most accessible ways to quickly calculate an individual’s health. It is commonly used in fertility, to calculate if an individual could benefit from losing weight, which in turn can help increase fertility. However, many call into question the accuracy of this measurement, here’s why.

What is BMI and how is it calculated?

BMI – Body Mass Index – is a measurement which calculates if a person’s current weight is healthy, but this measurement is deeply problematic. It was created as a way to estimate people’s body fat levels, defining them into one of four categories: underweight, normal weight, overweight or obese. This is done by taking the individual’s weight (lbs), dividing this by their height (inches), and multiplying this by seven hundred and three.

Origins and links to eugenics

To fully understand the issue which BMI presents, we need to go back to its origin. In the early 1900s, Adolphe Quetelet – a mathematician, notably not a physician or medical expert of any kind – came up with the concept of BMI. Quetelet intended BMI to be used as a way to measure populations, not individuals, and designed it purely for use in statistics, rather than in healthcare. This period saw the emergence of what is now commonly referred to as ‘racist science’, and much of Quetelet’s other work involved eugenics and criminality, in which he attempted to prove people of colour were a kind of subspecies of human, and were born criminals. His work would be used for many years to justify racism within medicine and science. Furthermore, Quetelet himself made it clear BMI was intended to be used to take statistics of populations and was not intended to measure an individual’s body fat, build, or health.

Does it work?

Coming back to modern day, there are many other issues associated with BMI which bring its reliability into question. For starters, BMI does not take into account nearly enough factors to give an accurate idea of a person’s health. It does not factor in things such as a person being very tall, or that a healthy female will require more body fat than a healthy male. In the case of female, breast size can also be a factor as being a C-cup could be enough to put you into the obese category.

From a scientific point of view, BMI makes very little sense. There is no physiological reason to square a person’s height (a key part of the BMI calculation), and it ignores an individual’s waist size, which is a vital indicator of body fat levels. BMI also doesn’t make any allowances for the difference between individual bodies, such as relative proportions of bone, muscle and fat in the body. Due to this, many athletes will find themselves classified as overweight, or even obese.

Despite the CDC’s claim that “BMI is a reliable indicator of body fatness for people”, many medical professionals have pointed to a fundamental error in their logic. If a person is fat or obese, they will have a high BMI, but they assume that this also works the other way around. As explained previously, a high BMI doesn’t necessarily mean a person is overweight, since it does not take enough factors into consideration when measuring.

From a psychological point of view, BMI can have a very damaging effect on people, both as individuals and as a society. BMI has been linked with eating disorders and body dysmorphia, and it’s hardly surprising. For those of us able to stand in front of a mirror, without feelings of dread and shame – which is often only achieved after years of personal reflection and working hard to rebuild the self-esteem that much of the beauty industry seems determined to alleviate us of – seeing that your BMI puts you in the overweight category may be just enough to bring all that hard work crashing down. For those who already dread looking at themselves in the mirror or standing on the bathroom scales, BMI can be earth-shattering. The problem comes from the fact that you cannot define a person’s health using only two numbers. Our health is a complex mix of nutrition, lifestyle, exercise and genetics. Surely it’s better to be overweight with good nutrition and daily exercise, than to be a healthy weight with a poor diet. The complexities of the human body can’t be reduced down to two numbers divided and multiplied to make another number, and that final number certainly shouldn’t be given as much power as it has been in the past.

So, why do doctors use it?

Unfortunately, BMI is currently the most accurate way we have of determining someone’s health – without performing thorough and expensive tests. It is accurate for a large number of the population so it makes sense for doctors to use it. If you’re having fertility problems it is likely that, at some point, your doctor will measure your BMI, and may recommend you lose or gain weight. This is because there is evidence that having a BMI under 18.5, can cause irregular menstrual cycles or cause ovulation to stop altogether. Similarly, having a BMI over 30 can cause irregular menstrual cycles and irregular ovulation. However, there is also evidence to suggest that overweight women with ‘normal’ menstrual and ovulation cycles are less likely to conceive, compared to healthy weight women who also have ‘normal’ menstrual and ovulation cycles.

BMI is somehow both hugely complex and wildly over-simplified. It takes an incredibly intricate thing – the human body – and attempts to whittle it down to a number. Its origins have been convoluted and distorted over the years, masking serious issues which bring into question its reliability as a measurement of health. As much as we would like to send BMI to the bottom of a river in a locked box, it remains the dominant measure of human health and is used by most medical professionals. It is, however, vital that we remember just how deeply flawed it is.