Thoughts on the Body Mass Index.

[Chris Hobson]

I would like to begin by stating that this is partly an opinion piece and I would be happy for people to come on and disagree with me or to correct any misconceptions that I may be harbouring. I'm sure most people here will know what the BMI is, a number obtained by applying a mathematical formula to a combination of your weight and height. The first thing to say about it is that it is only a very crude, rough and ready means of deciding whether you are overweight, underweight or somewhere in between. This in between zone is referred to on the BMI scale as ideal! The reasons that the BMI needs to be taken with a large pinch of salt are mainly that it takes no account of the fact that humans come in lots of different shapes naturally, and that it takes no account of the difference between muscle and fat. It can also be particularly misleading when applied to children who naturally change shape as they are growing.

Non of this seems to prevent various government health watchdogs and fake charities from misusing the BMI scale to claim that something like three quarters of the UK population are obese. Before I was diagnosed in May 2013, I had expanded slightly from about 75 kilos to something like 88 kilos. As such I was basically slightly overweight but, according to the BMI scale, border line obese. I lost about 10 kilos when my pancreas packed in working but put it back on again once I was put on diabetes medication. Since then I have taken up doing triathlons and have lost weight as a result (See the sports section for my piece of wibble on the subject of triathlons).

The thing that has brought all this to mind is the arrival of a BMI calculating weighing machine in the men's locker room at my health club. For 50p it will provide you with a little ticket containing, in my case, the following information:

04/12/16
WEIGHT: 11st10lb - 74.5kg.
HEIGHT: 5' 8.5" - 1.74m.
B.M.I. : 24.6
Your ideal weight for a B.M.I. Between 18.5 and 25 is:
8st 11lb to 11st 13lb
56.0kg to 75.6kg

This would seem to confirm my view that the scale is hopelessly skewed toward trying to convince us all that we are too fat. Although I am now very lean and fit, I am only just scraping in to the ideal category by the skin of my teeth. If I were to put on just 2 kilos I would be classed as overweight Which, to me, seems absurd. Any thoughts please?

 

[grovesy]

I thought that in most circles it has been accepted that is not perfect but there is nothing to replace it. I also thought it was not designed or meant to be used for children.

 

[Northerner]

I agree with you - it's skewed because it assumes that most people are carrying extra fat rather than denser, lean muscle. It also fails to take account of body type i.e. ectomorphic/mesomorphic/endomorphic. Not sure where you classify yourself regarding body type @Chris Hobson, but I am most certainly a classic ectomorph - small frame. I am 5'9" and currently weigh 9st 10, making my BMI 20.1. Unlike you, when I was at a higher BMI of just under 25 I was definitely carrying excess weight, chiefly around my middle 😱 Since quitting booze at the beginning of the year that has miraculously disappeared and I am now a lean and healthy - but not skinny or emaciated - person 🙂

So we appear to be at pretty much opposite ends of 'ideal', yet we are the same height! 🙄 It needs to be just one piece of information, taken alongside other observations 🙂

 

[Chris Hobson]

" I also thought it was not designed or meant to be used for children."

This is probably correct but it has not prevented obsessed health professionals from weighing and measuring children at school and then sending letters to parents telling them that their, perfectly normal, kids are overweight.

 

[Ditto]

It was only for insurance purposes originally I believe. 🙂

 

[robert@fm]

One of the fundamental errors in BMI is that it is calculated according to the square of height, rather than the cube as it should have been; thus, one of the hidden assumptions in the published BMI ranges is that everyone is around 1.75m tall, which is one of the reasons why BMI is particularly unsuitable for use with children.

 

[PinkGrapefruit]

8st 11lb! How can that be the bottom end of the normal range. Crackers!

There are calculators which look at body fat percentages on the basis of height, weight, but also wrist, forearm, thigh, neck, waist, and tummy dimensions. These seem to be far more accurate than just straight BMI, but slower and more complicated to start.

 

[Deleted member 16779]

One of the fundamental errors in BMI is that it is calculated according to the square of height, rather than the cube as it should have been; thus, one of the hidden assumptions in the published BMI ranges is that everyone is around 1.75m tall, which is one of the reasons why BMI is particularly unsuitable for use with children.

I've seen several articles which suggest that height to the power of 2.5 is a better number to divide by than 2 as it has a correspondingly higher effect for taller individuals such as myself. I'm at the top end of normal BMI under the 'height squared' calculation, but I'm by any measure, very thin, much more towards the middle with the 2.5 calculation.
https://people.maths.ox.ac.uk/trefethen/bmi_calc.html

 

[Radders]

I've never understood why the BMI range is the same for men and women, when men have a higher proportion of muscle which weighs more than fat.