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It’s simpel, free and non-invasive. But BMI-measuring also has its fair share of flaws.
It’s simpel, free and non-invasive. But BMI-measuring also has its fair share of flaws.
Ever heard of BMI?
You’ve probably heard of the abbriviation, maybe even said it yourself. And you might know that it stands for Body Mass Index, but do you really know it?
Body Mass Index
In short, BMI measurements is a value derived from a persons mass in kilograms compared to a persons height in meters.
You find your own BMI-value by dividing your weight in kilograms with your height in meters squared.
An example
Say, for instance, you weigh 80 kilograms and is 1.80 meters tall.
You then multiply your height, 1.80, with itself (1.80 x 1.80), getting 3.24 as a result.
You then divide your weight (80) with that number: 80 / 3.24 = 24.7.
What does that number mean?
The result of the calculation is used to determine if you’re outside the normal weight range for your height.
If the number is below 18.5, adults are considered underweight.
If it’s over 25, you’re considered overweight.
How is it used?
According to the Centers for Disease Control and Prevention (CDC), BMI is a well-established and valuable population health measure.
For example, the CDC use BMI data on its Adult Obesity Maps, that show the rate of obesity by state/territory and by race/ethnicity.
Here’s the catch
BMI is also used to measure an individuals health to determine risk of chronic disease – but according to experts, you should not measure your own health solely using BMI.
What about body composition?
The reason BMI is not very usable for measuring an individuals health is that the BMI only takes a persons weight and height into account – it does not tell you anything about body composition.
The Schwarzenegger example
Bodybuilder turned actor turned governor, Arnold Schwarzenegger, is approx. 1,83 meters (6 foot) tall and at the top of his bodybuilding career, he alegedly weighed around 106 kilograms (235 lbs).
This gave him a BMI of 31, meaning he was obese according to this – but if you’ve ever seen a picture of a young Governator, you’d realise how lean he was.
Mathematical issues
The basis of BMI-measuring was developed as early as the first half of the 19th century.
However, a Mathematician from Oxford University argued in 2013, that the BMI-calculation is flawed in a way that makes short people think they’re thinner, than they actually are, and tall people fatter, than they are.
The “new” BMI
The Mathematician, named Nick Trefethen, actually provided a new formula for BMI calculation, which he claimed is more accurate.
According to him, you should multiply your weight in kilograms by 1,3 and then divide the result with your height in meters to the power of 2.5 instead of just 2.
The new result
With this new way of calculating, let’s try with the example from earlier with a weight of 80 kilograms and a height of 1,80 meters.
80 x 1.3 is 104.
1,80 to the power of 2.5 is 4.35.
104 / 4.35 = 23.9 (conciderably lower than the first result of 24.7)
The takeaway
BMI might be an easy and cheap way to find out if your way too much or too little compared to your height.
But the method is flawed and should never be used on its own to assess the health of an individual.
That being said, you can use it to track your weight progress to find out if you should make any changes to your lifestyle.