Monday, December 3, 2007

The BMI thing...

I've touched on this before, but this Op-Ed. in the Trib. Brought it home to me. BMI is used by statisticians who are making sweeping generalizations, and by the extremely lazy. There are too many factors left unaccounted.
Take me for instance:
wt: 222 lbs (weighed at Ju-jujitsu on Wednesday)
bmi: 31.9
Clinically obese!

Forget that my waist to hip ratio is .85, which is "healthy."
Also forget that I am by most standards in shape, and simply focus on BMI and body fat:
In order to have a "healthy" BMI I would have to weigh less than 174 lbs. That means a weight loss of 50 lbs to be borderline "healthy."
If you assume that BMI and body fat are directly correlated, and that my body fat % would be borderline when my BMI is borderline:
174 lbs-18% body fat= a lean body mass of 142.68.
Assuming only reduction in fat mass to get there, that would put my current body fat % at 36% which would make me indeed obese.
However, rough calculations of my current body fat % put me at most at 18% (16-18%). Which puts my lean body mass at: 182 lbs. 8 lbs more than my "healthy weight" according to BMI.
In short I would have to lose 120% of my body fat in order to be "healthy."
Needless to say that would cause my metabolism to be very low indeed.

The inaccuracies in this system are staggering. The difference between my assumed Body fat %: 35,
and my actual body fat %: 18,
is 18.
That is a deviation of 50% of the assumed value. No one in any other scientific realm would allow for such an expansive deviation. The usefulness of BMI has past. It's time for a better measurement system.


ces said...

It amazes me that BMI is still being used by anyone at all. Certainly, the Hip/Waist ratio is a much better indicator of general health and obesity. Even better still, the US Navy has come up with a body fat estimation calculation that is very simple, yet, apparently, very accurate. The YMCA has a similar formula.

Interestingly enough, the Navy formula and the YMCA formula put me at 16.17% and 15.2% respectively. That's an almost insignificant difference - 6% or so.
One would think that would be close enough for most - and significantly closer to 'real' than BMI.

At 3 or 4 simple body measurements (less than needed to fit a suit!) one gets a much more accurate picture of health.

J. B. Zero said...

exactly, however most of the studies on health and obesity only use bmi. So you take the statistical outliers on both ends of the spectrum and put them at "overweight and obese" it totally skews the data.

J. B. Zero said...

BTW those measurements have me at 20%, calipers have me at 18%.. so pretty close.. and a little too fat for my own good. More on that later.