BMI: body-mass index or body-mass inaccuracy?


Content warning: this article contains mentions of body weight and language that some readers may find upsetting.

Body-Mass Index (BMI) has been one of the main measures of body weight and health status since the late 20th century. Although convenient, reliance on an oversimplified metric based on height and weight alone, especially in medical environments where alternative measurements are generally available, may have had harmful effects on both the physical and mental health of the population.

The origins of the idea of BMI (first known as the ‘Quetelet Index’) was developed between 1830 and 1850 by Adolphe Quetelet, a Belgian mathematician and self-proclaimed “social physicist”, in his study of “the average man”. It is defined as an individual’s mass (in kilograms) divided by their height (in metres) squared, with under 18.5kg/m² being categorised as ‘underweight’, between 25 and 29.9kg/m² ‘overweight’ and over 30kg/m² ‘obese’.

The contemporary term BMI was coined in the 1970s, when the Quetelet Index was described as “at least as good as” the many other relative weight indices developed in the decades after World War II when the relationship between body mass and cardiovascular health first became apparent. It is worth noting that the statistics for Quetelet’s ‘average man’ would be a core idea in Francis Galton’s development of Eugenics a decade later.

BMI is extremely convenient as a general measurement because it requires no specialist equipment and is quick to calculate. However, this metric alone cannot define what ‘overweight’ or ‘obese’ means when it omits so many factors that contribute to body composition.

30% of those who were a ‘healthy weight’ according to their BMI had metabolic or heart health problems

In a 2016 study in the US, 29% of people who were classified as ‘obese’ on the BMI scale were metabolically healthy (i.e. their blood pressure, cholesterol and blood sugar were at ‘normal’ levels).

Conversely, 30% of those who were a ‘healthy weight’ according to their BMI had metabolic or heart health problems. Other studies have implied that adults in the ‘overweight’ category have a similar risk of death as their ‘normal weight’ counterparts. So as far as cardiovascular health metrics go, this quantity seems unreliable when identifying heart-health-related problems.

According to experts, one of the main flaws of the BMI metric is that it fails to consider the amount of muscle versus the amount of fat that a person has. Muscle is denser than fat, so generally athletes and bodybuilders have a high BMI, implying that they are an unhealthy weight.

It also disregards bone density, metabolism and other aspects of body composition. The accuracy of the metric therefore varies wildly between ages, sexes and ethnic groups, which is no surprise to many. For example, the WHO recommends a lower cutoff for obesity in Southeast Asian populations.

When Quetelet created the measure in his study of ‘the average [white] man’, he paved the way for not only eugenics, but for a metric that may have been physically dangerous to marginalised groups.

It seems inappropriate to continue to use this categorisation of obesity when it is not even applicable to most individuals. Global obesity rates may have tripled over the past 50 years, but perhaps part of the problem is the outdated definition of obesity, oversimplifying what it really is.

The search for a more appropriate alternative has been ongoing for quite some time, with various theories arising. In March earlier this year, researchers published an article in the European Heart Journal claiming that the ratio of waist size to height is more accurate when it comes to determining risk of heart disease.

Much more research is required before we find a perfect measure

The researchers considered age, gender orientation and blood pressure before they reached this conclusion and stated that waist size is a good measure of visceral fat (belly fat), surrounding vital organs such as the liver. The United Kingdom’s National Institute for Health and Care Excellence advises to keep your waist circumference to less than half of your height to maintain a lower risk of health problems.

Another proposed alternative is using the waist to hip ratio as an indicator of abdominal obesity. According to MedicineNet, dividing the circumference of your waist by the circumference of the hip at the widest point helps you to identify your risk of heart/chronic diseases, with above 0.8 for women and above 0.95 for men classifying as ‘abdominal obesity’, suggesting a higher risk.

Although progress has been made to find a more suitable measure of identifying obesity, much more research is required before we find a perfect measure. Until then, scientists recommend not to focus too much on the specific value of the BMI and to adjust daily routines to increase movement, no matter how small. A recent study by UCL showed just replacing a few minutes of sitting with a few minutes of moderate activity can improve cholesterol, waist size and BMI, amongst other physical benefits.

BMI has been an integral part of the definition of obesity and health these past two centuries, but health is so much more than a short-sighted, simple calculation. We should not allow a number to categorise health and affect self-esteem as much as it has in the past. Scientists agree that BMI has had too much influence in the healthcare system and a more appropriate replacement needs to be found. It is vital that our classification of obesity adapts with the time, and that we move away from a measure which some are calling a ‘scientific embarrassment’.

Image: Ekaterina Grosheva via Unsplash

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