Even though a proportionally larger person cannot be considered more obese,
muscle strength increases more slowly than muscle mass as body size
increases (roughly the 2/3 power rather than linearly), and such larger
person would be less fit. This might be one reason for dinosaur predators
maladaptation as they could not move fast, contrary to popular belief. If
you also consider that red blood cells are of the same size no matter what
the body size, there clearly are limits to fitness. So, BMI probably
overestimates obesity releted to fat content, but might be a good
approximation of fitness. That, however, needs to be demonstrated or,
perhaps, has already been studied.
Jaroslaw Wechowski MB BCh PhD (econ)
Warsaw School of Economics
-----Original Message-----
From: Evidence based health (EBH)
[mailto:[log in to unmask]]On Behalf Of Ted Harding
Sent: Tuesday, February 08, 2005 1:30 AM
To: [log in to unmask]
Subject: Re: BMI: the theory? [and some evidence]
On 07-Feb-05 Puliyel wrote:
>
> Dear Ted
> You are correct to conclude that BMI (Weight divided by
> square of height ) is not a rational index of obesity.
> However your suggestion of using the cube of the height
> would make it an even poorer index.
> The reason is simple: Suppose a person grows 1 foot taller.
> He does not automatically become 1 foot wider and 1 foot
> deeper like a cube. If he does, he is obviously getting obese.
Dear Jacob,
Thank you for the comment. However, I was not proposing that
people were cubic in shape, nor grew by equal distances in
all directions!
My "theoretical" reasoning adopted the premiss that proportion
was preserved: width = Cw*height, depth = Cd*height, so that
volume (crudely)= height*width*depth = Cw*Cd*(height cubed).
Specifically, if someone is 60 inches tall, 20 inches wide
(shoulder to shoulder) and 10 inches deep (sternum to scapula)
then if that person grew one foot (20%) taller and maintained
the proportions, they would be 72 inches tall (12 inches taller),
24 inches wide (4 inches wider) and 12 inches deep (2 inches deeper).
Since their shape would not have changed, they would be neither
more nor less obese than before.
The real point of my theoretical discussion was that if all
proportions (proportional volumes, shapes, dispositions) of
different tissue types were preserved then there would be no
obvious reason to conclude that the healthiness of the second
body should be other than identical to the healthiness of the first.
But the BMI would have increased by 20 per cent. Hence BMI
can fluctuate by a considerable percentage without the slightest
implication that there is a body change which might be health
related (and remember that e.g. distinctions between "healthy",
"overweight" and "obese" correspond to approx. 20% increment of
BMI: <=25, 25-30, >30).
Best wishes,
Ted.
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Date: 08-Feb-05 Time: 00:27:10
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