BMI: An inaccurate guide to good health

Body Mass Index, more commonly known as BMI, is a method largely employed by healthcare providers to determine which category a person’s weight falls into– underweight, normal weight, overweight or obese. 

Through this classification of body size, a person’s health may be assessed in tandem with other tests carried out by medical professionals. The controversy around BMI, however, is posed around its inaccuracy in representing an individual’s physical fitness due to factors such as registering all body’s mass as fat, and differences within a population.  So how did this scale become a standard measure of wellness and more importantly, why is it still in use? 

Developed first in 1832 by Belgian statistician and mathematician Lambert Quetelet, the measurement is a calculation of a person’s weight divided by their height, giving a proportion expressed in kilograms per square metre. The index we use today was termed by physiologist Ancel Keys in 1972, who after many comparative studies ,largely around men of different stature, described it as “if not fully satisfactory, at least as good as any other relative weight index as an indicator of relative obesity”. Both Quetelet and Keys have expressed the measurement worked best in population studies and would not be useful when applied to the individual. However, due to the simplicity and uncostliness of BMI as a health tool, we see this is not the case today. 

Now we know the scale is a measure of relative body fatness, it begs the question: whom or what is it in relation to? One of the most obvious oversights seen in BMI calculations is, in measuring a person’s weight all their mass is assumed to be fat content. As a result those with more muscle mass, like bodybuilders and athletes, are likely to be registered as overweight on the scale in comparison to the average person, who may very well have more fat per body weight. This trend is seen between the sexes, with women ranking lower on the BMI despite generally having higher fat mass.

What about the distribution of fat in the body? Although it has been well-established excessive fatty tissue increases the risk of chronic diseases, studies show where fat is accumulated is just as, if not more important. An overreliance on BMI to accurately assess this risk also leads to the disregard of other factors such as genetics, lifestyle, age and sex. This bleeds into weight bias, where those marked as “overweight” or “obese” fail to have the full scope of their health properly assessed by medics. 

Relative body fatness fluctuates between ethnic and racial populations as well. Indices and their cut-off points were standardised using measurements from European groups, according to the National Institute for Health and Care Excellence (NICE). In a 2012 report, they stated these cut-offs may not be appropriate for other ethnic groups. An example of this is seen in those of Asian descent, where a smaller increased weight correlates to an increased risk of type II diabetes in comparison to white and Polynesian populations. Studies have shown Black populations have higher lean muscle mass in comparison to their white and Hispanic counterparts, and as such may be inappropriately represented as overweight. Additionally, Black women have been found to be metabolically healthy at higher cut-off points for BMI than non-Black people. 

As BMI has been shown to miscategorise “healthiness” in women, children, elders and non-White populations, many alternatives have been made to either adjust BMI measurements to increase accuracy per individual or create whole other methods of measuring body fatness. Examples include measuring waist-to-hip ratios (though this presents similar drawbacks) or precise dual-energy X-ray absorptiometry. 

Regardless of the technique, measuring individual fatness should not be the single or most important determinant of someone’s fitness, inside or out of the examining room. 

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Mariam Jallow

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January 2022
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