Body Mass Index
Thursday, February 23, 2017 at 10:50:24 PM GMT+1
WTH Is BMI?
The Body Mass Index (BMI) is used to characterize our bodies. The calculation is the body mass divided by the square of the height (kg/m²). It was orginally called the Quetelet Index in 1832, after Adolphe Quetelet, a mathematician from Belgium[1]. Quetelet was trying to characterize the “normal” man and found that the weight/height relationship didn’t fit in a standard Gaussian distribution. After some studies, he concluded that other than after birth, puberty, and diet contests, the weight increases as a square of the height and came up with this index.
It is not a direct measure of fat. A young, muscular Arnold Schwarzenegger with a high BMI is very different from an old, sagging Arnold with the same BMI, for example.
I include it in our table simply as a relative measure of our physique. I only highlight really big numbers in scarlet purely for the mathematical reason that it is a really big number (>35), and not as the equivalent of walking through town with a nun yelling, “Shame! Shame!” behind you.
It is also not used by the all-knowing, all-powerful Algorithm to calculate our scores.
What Is Normal?
In an effort for the insurance industry to charge us more money, it came up with the idea that people above the “ideal” weight should be charged more money because they tended to die sooner. Something about statistics and lies.
Math and stuff were used and in 1972[2], someone decided that Quetelet’s index, re-dubbed body mass index (BMI), seemed to fit well.
The current accepted values as determined by the World Health Organization (WHO) are:
18.5 < BMI ≤ 25.0 Normal
25.0 < BMI ≤ 30.0 Overweight
30.0 < BMI Obese
These have changed in the past, such as in 1998 when the upper range of Normal was changed from 27.5 to 25.0 by the National Institute of Health. This decision made 30 million normal Americans “fat” overnight.
Is It Flawed?
The BMI is a model, and like many models, it may not reflect reality (see: runway models, but probably not at work).
It is not meant for individual measurement, since it does not take variables like age or gender into the equation. It is best used for a general population of people.
Correlation is not causation, but many studies show a correlation between BMI and mortality[3].
Should I Care?
Sure, why not.
BMI is simply an indicator that’s a little fancier than pure weight. Including a second physical value (height) give BMI intrinsically more information. However, use it as a relative measure of change for yourself, and not as an absolute guide.
Unless they are delusional, most people have sense of whether or not they are “healthy” anyway. Just remember, don’t sue me, as I only play an actuary on the internet. In real life, I pretend to be a health advocate.
References
[1] Adolphe Quetelet (1796–1874)—the average man and indices of obesity↩
[2] Beyond BMI↩
[3] An interesting critique of BMI from a paper that now requires a subscription to read.