How Much Should You Trust BMI?

Richard Kelly 18th March 2019

Most people are familiar with BMI from check ups with their doctor.  It is a measure of a person’s weight against their height.

Now, in theory, this would seem like a sound measurement to take.  After all, there should be a healthy range of weight to be at when measured against a person’s height.  That sounds completely logical.  Except for the fact that it doesn’t take into account the composition of a person’s body.  And this is a key consideration.  BMI doesn’t take into account the weight of your bones, your blood, your nerves, your muscle, your body fat, your brain, your connective tissues or the various other fluids in your body.  It just gives you a measure of overall weight versus your height. 

And this is a huge problem.  Anyone who has any muscle mass whatsoever will be over their BMI because muscle weighs more than fat.  It actually weighs about three times more than fat if you were to measure the same amount of surface area.  So if you were to take a kilo of fat and a kilo of muscle, the muscle would be a third the size.  And muscle is only one area where it is a concern. 

I can tell you that I have come across a number of yogis and runners who have frighteningly low bone and muscle mass, which are good indicators of osteoporosis and degenerative diseases in later life, who sit in the ‘normal’ range.  The concern here is that when these people visit the doctor and are told their weight is normal, they will take no action.  That leads to complications later in life which could have been avoided much earlier. 

Equally, when someone is told they are ‘underweight’ and that they should put on more weight, what is it they need to put on more of?  There is a healthy amount of body fat an individual should have, and simply packing on weight indiscriminately isn’t a healthy approach to solving this problem.

So, we have a problem with BMI for some individuals in the ‘overweight,’ ‘normal,’ and ‘underweight’ categories.  So perhaps we should look at what BMI was originally created for.  BMI was designed by Adolphe Quetelet in his search to discover the average man in the 1830s.  He used the weight of a person measured against their height to determine what the average size should be. 

In the 1980s this was picked up by insurance companies in the United States as a means of predicting lifespans, theorising that someone at the upper end of the range would be more likely to die earlier and therefore was more of an insurance risk.  This measure than made its way into healthcare.  And the NHS has used it since the late 1990s.  However, the question that should better be asked is, why are Healthcare Practitioners using a statistical model which doesn’t take into account body fat, to measure body fat?

What is even more baffling is that there are modern tools which would be far better at measuring and providing genuinely helpful data to people.  My gym, for instance, has a machine that is able to tell me my weight, body fat, muscle mass, bone mass and fluid levels.  I can have all of that information to hand in under a minute and I can see from that information how I am progressing and what areas of concern I should be paying attention to. 

Nor do I accept the fact that the general population wants an oversimplified number to guide them on their body fat.  We live in an age of data, and we have seen that the more data that is made available, the more interest people take in it.  Rather than trying to simplify things, the healthcare sector would be better served by educating the population it serves.

Equally, being told on every visit to the doctor that you are overweight or underweight can cause depression and anxiety, especially when people are trying to make a change to fit within the ‘normal’ range of this made up scale. 

Lets take me as an example.  According to the machine I use at the gym, my BMI is 28.5.  In order to be ‘normal’ weight on the scale id need to have a BMI of 25.  But according to my machine I am 43.9kg of muscle (that’s almost seven stone to those luddites who still use Imperial measures).  I’m also within the healthy range of body fat, at 20% body fat, which is 19kg.  In order to reach a ‘healthy’ BMI I’d need to lose 11kg of weight.  Realistically, if I were to lose weight from where I am right now a good half of it would come from water within my body (which is 56kg), and maybe the rest from fat.  If we assume that this was the case, and 6kg came from water, and 5kg came from fat, which would be very unlikely as it was, to take me to a ‘healthy’ BMI, I’d have 14kg of body fat, which would be 11.5% body fat.  That pretty close to athletically lean, and remember, I’d be barely scraping into ‘healthy’ BMI.  In fact most athletes you see will be that body fat during competition.  If I would to apply the same calculations I’ve just done to work out body fat percentage if I was in the middle of the BMI range and at the lower end I’d be 7.5% at a BMI of 22.5 and 4% at a BMI of 20.  Four percent is the body fat competitive bodybuilders go onto stage at.  It isn’t sustainable long term, nor is 7.5%. 

The best I could hope for, then is a BMI of around 24.  And that would require me to have a perfect diet, training programme and lifestyle.  It is possible, but the level of dedication needed to get there is incredibly high. 

But that isn’t true for everyone.  The more ‘average’ you are, the more realistic a measure BMI is.  If you aren’t that trained and if you aren’t that tall or short then BMI can have some relevance.  However, the question remains, what is the point of a measure that can only be accurately applied to some people, especially when there are other tools out there that can accurate measure body fat, which is, obviously, a measure of fatness?

And until Healthcare Practitioners start to use modern and correct measurements on the population they will never get to grips with issues like obesity or degenerative problems faced by older populations.  

If you are interested in creating a sustainable long term plan to lose fat, increase muscle and create a sustainable long term healthy lifestyle, please click on the link below to visit my online training page



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