BMI: Body Mass Index? Nope. It’s Biologically Meaningless Information.
Would you hire an accountant to give you a medical diagnosis? No, of course not. The idea sounds totally absurd in the abstract, until you realise we’ve all been relying on the medical advice of an early 19th Century Mathematician to determine whether our bodies are overweight or not.
Adolphe Quetelet was a Belgian astronomer, mathematician, statistician and sociologist. Quite the resume to be sure, but if you look closely you’ll notice that none of these professions are medical in nature. So why did this enterprising young Belgian decide to enter the crowded and unregulated free-for-all of the 1800’s Medical Industry?
Well, it’s simple — the US Government asked him to.
Faced with the challenge of an increased population and growing industrialisation, political leaders of the day needed a quick way to determine where resources should be allocated, and by what volume. They decided the best way to determine this was to work out how many overweight people there were. And thus, the BMI was born.
Hold up… What even is the BMI?
Body Mass Index (or BMI) is an equation used to provide an estimate of a person’s relative subcutaneous fat as a percentage of their total weight. The method divides a person’s weight by their height to the power of 2:
BMI = weight (kg) / (height x height)
According to the BMI:
- A score of 18.49 or below indicates a person is underweight.
- A score of 18.5 to 24.99 indicates they are a normal weight.
- A score of 25 to 29.99 indicates they are overweight.
- A score of 30 or more indicates they are obese.
Of course this is all scientifically meaningless — there’s no physiological reason to square a person’s height; and the grouping of numerical datasets into arbitrary weight categories does not make them accurate.
But this understanding, and the fact that Quetelet himself insisted that the equation not be used to calculate individual “fatness”, has largely been ignored by modern medicine for a simple reason: BMI is easy to calculate.
Biologically Meaningless Information.
In fact, BMI is so terribly useless at determining anything health-related that in 2016 a study utilising BMI to determine cardiometabolic health (widely considered to correlate with obesity) incorrectly categorised over 75 MILLION American adults. The study found that almost half of BMI-determined ‘overweight’ individuals were actually healthy, whilst 30% of those within the ‘normal’ weight category were considered unhealthy.
This inability to distinguish between healthy and unhealthy individuals is a common problem for BMI — because it fails to account for bone density and muscle mass (both more dense than fat) — it incorrectly categorises individuals with strong bones and dense muscle as overweight and obese.
BMI’s many failures as a “fatness” metric points to a broader issue with modern medicine in general — the insistence that those categorised as “overweight” have worse health outcomes than “normal” folk — when a large body of peer-reviewed population-based studies routinely demonstrate the opposite.
For example, in a European study published in 2010, which followed 360,000 individuals between the ages of 25 — 70 over 10 years, the risk of death was the lowest for those within the ‘overweight’ BMI category. Another study of 527,000 people produced by the NIH American Association of Retired Persons and published in 2006, reflected the same data, with the lowest death rate also among those in the “overweight” category.
Fact is, human biology is far too complex to rely on reductionist metrics like weight to determine our health. Our bodies do not fit any one given mould. Every body is its own unique creation, the state of which cannot simply be described by an arbitrary number. “Normal” weight as classified by the BMI does not equate to health, and “overweight” classified by the BMI does not equate to unhealthy.
Moral of the story? It’s probably a bad idea to rely on a medical diagnostic generated at a time when cocaine was used to cure headaches, heroin was cough medicine, and an ice-pick hammered into the eye-socket was used to manage mental illnesses.
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