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BMI: The problem with simplifying health

25 Apr 2023

Seemingly, BMI is one of the simplest and most accessible ways to quickly calculate an individual’s health. Commonly used 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. It was created as a way to estimate people’s body fat levels, defining them into one of four categories: underweight, normal, overweight or obese. This is done by taking the individual’s weight (lbs), dividing this by their height (inches), and multiplying this by 703.

Origins and links to eugenics

To fully understand the issue that BMI presents, we need to go back to its origin. In the early 1900s, Adolphe Quetelet – a mathematician, notably not amedical 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. 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.

Critically, during this period the world saw the emergence of what is now commonly referred to as ‘racist science’. 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 "born criminals". His work would be used for many years to justify racism within medicine and science.

Does it work?

Returning to the 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.

From a scientific point of view, BMI makes very little sense. There is no physiological reason to square a person’s height and it ignores an individual’s waist size – 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 overweight, they will have a high BMI, but they assume that this also works the other way around.

Being overweight will give you a high BMI but having a high BMI doesn't always mean you're overweight.

From a psychological point of view, BMI can have a very damaging effect on people. Unsuprisingly, BMI has been linked with eating disorders and body dysmorphia. For those of us able to stand in front of a mirror, without feeling dread and shame (lucky you) seeing that your BMI puts you in the overweight category may be just enough to bring all that self-love crashing down. While for those of us 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 and chain smoking? The complexities of the human body can’t be reduced to two numbers divided and multiplied to make another number, which 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 thorough and expensive tests. It is accurate for enough of the population so it makes sense for doctors to use it. If you’re having fertility problems at some point, your doctor will likely measure your BMI and may recommend you lose or gain weight. This is because there is evidence that having a BMI under 18.5 or over 30 can cause irregular menstrual cycles or stop ovulation altogether. However, there is evidence to suggest that overweight women with a ‘normal’ menstrual cycle and ovulation 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 single number. Its origins have been convoluted and distorted over the years, masking serious issues which bring into question its reliability. 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.

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