BMI/waist-to-height/waist-to-hips

[Guest guest]

At 12:07 PM 9/29/2006, you wrote:

>Maco,

>

>Your argument proposing deceptively low BMIs for shorter people is

>flawed. BMI is proportional to 1/height SQUARED. So greater height

>reduces BMI by the square of the height. Therefore a 7 foot tall

>person will have a deceptively low BMI.

Because mass/weight will increase as a cube of any one dimension, the fact

that BMI only corrects for a square of an increase yields excessive numbers

for taller people. Weight in kg/(height in meters, squared) is the formula,

I think.

If you compare a person of a certain physique who is 60 inches tall with

someone who is 10% (6 inches) taller, you would expect that taller person's

weight to be (1.1) X (1.1) X (1.1) that of the shorter person. If you only

correct for the square of the height, the expectation, given

proportionality, is that the taller person's BMI is going to be an

overestimate relative to the shorter person by the ratio between the

heights of the two similarly proportioned people, so that in this case, the

BMI for the 5' 6 " person will be 1.1 that of the shorter person because the

height-corrective factor is a square rather than some flavor of cube.

Comparing your averagish 5' 4 " woman with a man who's 6' 3 " (roughly 10%

taller) will give people of equal proportions (unlikely, that, but to make

the point . . .) BMIs so that a 24 on the part of the woman will correspond

with a 26.4 on the part of the man.

Of course, you were saying that a woman should " shoot for " a lower BMI,

which is true (because that lower BMI will correspond to a higher BMI for

taller people, anyway); I'm just pointing out (confusingly, sorry) that for

a given body " situation, " the shorter person is already going to have a

lower number, which should psychologically generate the same conclusion you

were initially supporting without any additional effort on the part of the

shorter (statistically more likely to be female) person.

Waist to height at least compares a one-dimensional measurement (h) with

another (2-pi-r) as does waist to hips. Waist to hips is probably better,

but I think its easier to consistently measure waist and height than them

pesky hips.

;-)

Maco