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Body Mass Index (BM) is a widely recognised tool used to assess health and categorise individuals based on their body weight relative to their height. Despite its widespread use, BMI is controversial. This article demystifies BMI, explains what it is and explores its advantages and limitations. If you’re unsure about BMI, you’re in the right place.
Definition of BMI
Body Mass Index (BMI) is a tool used to try and categorise people using their height and weight to establish whether they are within a healthy or normal range.
How to calculate BMI
BMI is calculated using a simple formula:
A person’s weight in kilograms divided by the square of their height in meters (kg/m²)
This result of this calculation then categorises individuals into different weight categories —underweight, normal weight, overweight, and obese.
You can even track your BMI on a chart like this:
photo source: NHS Inform
Is BMI out of date?
Technically, yes. BMI is very out of date.
It was first developed in the 19th century, but then in the 1970’s a physiologist called Ancel Keys built upon the original frame work and made BMI what we know it to be today.
Research or theories that are over 50 years old, like BMI, would be considered out of date by our standards today in 2024. So why are we still using it?
Why we still use BMI today
The reason why we still use BMI today is very simple. It is fast and effective.
BMI is used by health professionals, Doctors, nurses, Personal Trainers and more. It is one of the easiest ways to estimate someone’s health status and potential health issues with little details.
Let me explain a little more.
At the time of writing this article, the average GP appointment in the UK is scheduled for roughly 9 minutes. That is a very short amount of time.
You’ve got to consider the confirming of details, explaining your symptoms and problems, you’re easily a few minutes down. If the Doctor when went on to ask you about your medical history, your families medical history, how much exercise you do etc, that 9 minutes just doesn’t cut it…and that is still not talking you through your options!
To save a lot of time, Doctors will reach for BMI. It is a super fast way to get an idea of how healthy you are in line with the recommendations for your height and weight.
You’re probably thinking: What about age? What about health conditions? What about the exercise I do?
That is where BMI fails.
Why is BMI unreliable?
A BMI calculation doesn’t consider:
- Body composition – How much muscle mass and fat mass you have
- Age, sex and ethnicity – These are all very important and will impact your result. Thankfully, the NHS BMI tool does consider these.
In a previous role, I worked with elite athletes who were at the very top of their game. They would have been classed as obese if we did calculated their BMI. They were heavy due to their muscle mass. It didn’t take much more than a glance to see that they had a lot of muscle mass and low body fat; but this means their BMI would be completely inaccurate. So BMI isn’t always applicable.
It works the other way too.
Someone with a low muscle mass may also appear in the average or normal range despite having a higher body fat.
You need to mindful of this when reading your BMI.
Before you go
While BMI offers a quick screen tool to identify health issues related to body weight, it has its limitations and doesn’t give us the full picture of what is going on.
A BMI reading will work for most, but not all.
Remember that BMI is simply a tool, and it is a tool that you can pick up or drop whenever you want. There is so much more to your health and well-being than just your BMI!
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