Your BMI – what does it really tell us plus some facts that may surprise you?

The Channel 4 show ‘How to lose a stone in 21 days’ focuses heavily on ‘tackling’ obesity using the Body Mass Index (BMI) as a measure.
It claims that an 800 calorie, severely restricted, low carb diet is appropriate for people who fall into the ‘overweight’ or ‘obese’ category.
But what does this really mean and how useful is it?
Let’s start with some background information…
The original theory about weight and height, that was eventually reformulated as the ‘Body Mass Index’, was developed by the Belgian Astronomer and Mathematician, Lambert Adolphe Jacques Queteletwho wanted to see whether the law of probability applies to humans.
He wanted to discover ‘l’homme moyen’, the average man, in order to study human society.  It was developed as a statistical exercise, not a medical instrument.
Lambert himself said that it should not be used to indicate an individual’s level of fatness.
Not only was it created to examine an entire population, but the population it studied was also a White, European one which means that it does not account for the differences in other ethnic groups.
Despite this, American insurance companies started to use the formula.  They recorded weights and heights of clients to judge their risk of illness. This was usually based on a white, wealthy male population and one company even stated that those deemed overweight were less desirable!
It therefore seems that obesity and the BMI index were already being used to shame people’s bodies at the turn of the century!  This is something that I think comes through in the Channel 4 programme itself – it talks about people having ‘lockdown’ belly and being ‘fat’ based on what their BMI states.
Then, in the 1970’s, the famous physiologist and nutritionist, Ancel Keys, went on to show how the index was an accurate description of ‘normal’ human growth and it went on to be called the BMI Index.
It is important to note that again, Ancel looked at thousands of measurements to reach his conclusion which suggests that on a population level, the index may be useful but for an individual?  That remains an open debate for many reasons.
Firstly, it does not take into account the body composition of an individual and assumes a sedentary lifestyle.  A person’s proportion of bone, muscle, and fat is not accounted for.  This makes it almost pointless for an active, fit individual.
You have probably heard of rugby players being classed as obese – this is why!  Their lean muscle mass is not factored in.
It also fails to account for a person’s weight history not to mention their lifestyle.  Very often, a person classed as overweight can actually lead a far healthier lifestyle than someone who is within the ‘normal’ range.
It also fails to take in to account waist and hip measurements.  Our waist/hip ratio is by far a better measurement of our fat distribution and, therefore, our health – physical health at least!
Which leads me nicely on to the next point – BMI does not reflect our psychological state.  Our whole health goes way beyond our weight and physical state, it encompasses our mental, social and emotional wellbeing also.
When considering individual approaches or interventions, 800 calories or not, this MUST be considered.
But the reality is that the index is still being used as a measure of health by so many professionals mainly because it is a simple, cheap, and easy method.
More evidence-based approaches such as body callipers and running machines (which Channel 4 did use in all fairness) are costly, time-consuming and somewhat complicated which makes them less appealing to the health industry.
To go back to the programme itself that sparked this blog, I think it is safe to say that no-one should be attempting such a low-calorie diet alone.  Whilst it does say that it isn’t advisable without support, I think it is safe to say that people will go ahead regardless.
And whilst the show masquerades as a way to improve people’s health, I want to point out that it is the weight loss itself that will improve physical markers (well, in any one given moment at any rate) and not the diet itself!  It has no magical powers – it all comes down to a good old calorie deficit.
BUT, in such an extreme way, it is likely that people will be unable to sustain it and will end up feeling like a failure yet again before regaining the weight, and more!
Same old diet culture…..
Anyone embarking on such a diet needs to be screened, coached and receive therapy throughout.
As for BMI as an assessment indicator, whilst the principle behind it remains mathematically ‘true’…for a population….it’s relevance to a specific individual remains questionable!
This was brought to you by Rebecca Heald, Nutrition Coach.
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