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BMI is crap! & biased against taller people

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BMI is crap! & biased against taller people

Postby Sanov22 » Sun Sep 05, 2010 11:49 pm

I saw this argument in the paper a while ago, after reading it I don't really know why BMI continues to be a scale which people (even doctors) use! People who have the same size-to-weight proportions should have the same BMI regardless of their height, right? But they don't - if a short person with a healthy BMI was "grown", so that everything remained in exactly the same proportions, the taller version would have a high BMI.

Here's a simplified example to prove it:

First of all consider a cube with a height of 10cm = 0.1m, and let's say it weighs 1kg. It's BMI is 1/(0.1 x 0.1) = 10

Now imagine you make another cube by placing four of those smaller cubes together - so the new cube has a height of 20cm = 0.2m and a weight of 4 x 1kg = 4kg. It has exactly the same size-to-weight proportions as the smaller cube, it's just twice as big. So it SHOULD have the same BMI, but alas:

4/(0.2 x 0.2) = 100 !!!

The problem with the BMI formula is that it seems to presume people are two dimensional. If you try this same trick ^ with a square you get the same BMIs. So basically if you take a short person and "grow" them into a taller person with the same proportions, the formula allows for all of the extra weight but only two dimensions worth of extra size, instead of three.


Hope you found this as interesting as me! :)
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Re: BMI is crap! & biased against taller people

Postby tigerlilly » Sat Sep 25, 2010 2:30 pm

I fully agree. The BMI is fundamentally flawed and should be removed completely. There are far better criteria to measure, and pigeonhole, people into regarding weight and health.
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Re: BMI is crap! & biased against taller people

Postby dragonfly » Sun Sep 26, 2010 10:40 pm

oh absolutely, it is a general average thing to go by. not every one is going to fit into the ideal perfect range. a bodybuilder who weighs 120kilos, is FAT... yet has only 5% body fat, the rest muscle.
and someone like my sister who is the same height as me, can get away with weighing 65kilos, yet I would look freaky at 65kilos. (i was 95kilos many years ago, and was told I looked boney then? so take another 30kilos off that, yeah i'd look freaky!) lol
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Re: BMI is crap! & biased against taller people

Postby court » Mon Sep 27, 2010 3:40 pm

I'm 70kgs give or take atm. The top of my healthy weight range according to BMI puts me at 64kgs. I was 66.5kgs, struggling desperately to maintain it to my detriment, super fit, muscualr and with my body fat % at 24% (as a woman that is pretty low I believe?)... BMI is horrible. A horrible horrible way of determining the "ideal".
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Re: BMI is crap! & biased against taller people

Postby Shay » Mon Sep 27, 2010 4:18 pm

The initial mathematical logic used (ie 10 vs 100) is flawed. But yeah, as someone who works in public health i'd have to agree BMI isnt the only way to determine a person's health status. I think people have stuck to it for so long because its a universal measurement, but these days we also take tricep measurements, body fat percentage, waist circumference etc.
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Re: BMI is crap! & biased against taller people

Postby Sanov22 » Fri Oct 01, 2010 12:04 pm

The initial mathematical logic used (ie 10 vs 100) is flawed.


How so?
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Re: BMI is crap! & biased against taller people

Postby Sanov22 » Fri Oct 01, 2010 12:55 pm

Oh wait... haha, yes you are right. In my attempt to keep the numbers 'nice' I somehow came up with 1/(0.1 x 0.1) = 10 instead of 100 .... oops! And even worse, I wrote that you need 4 of the original cube to make up a new cube that's double the size... whaaaaaat was I thinking that day? You need 8! :oops:

OK, so, with corrections,

First cube has height 10cm, weight 1 kg, BMI 100

Second cube has height 20cm, weight 8kg, BMI 200
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Re: BMI is crap! & biased against taller people

Postby 2ndTimeBecky » Sun Oct 03, 2010 8:40 pm

I agree that it is flawed, but I feel also for short people.... I am short, around 157cm tall, and the "healthy" range for my height is 45kg!!!!!!!!!!!!!!!!!!!!!!! to 60kg MAX. While 60kg is not unreasonable, I was annorexic in high school and was around the mid to low 50's and looked quite ill, bones all over the shop... I go on clothes size now, and measurements.... :-D
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