Searching past threads, there seems to be general agreement (including my own) that BMI is not a
measure of body fat composition.
So what good is BMI? And why do some reputable books and articles still suggest using BMI as a
measure of "fitness"?
As we know, BMI is weight divided by square height in kg/m^2. The interpretation of BMI is the same
for both male or female; and it does not take "build", age, or even some subjective notion of
fitness into account. For example, see nhlbisupport.combmicalc.htmOpen ↗ from the US NIH.
"Normal" (whatever that means) is BMI 18.5 to 24.9. Period.
How could that be the same for male and female? Isn't it common scientific knowledge that all things
being equal (height, weight, age, fitness level, etc), females generally have higher percent body
fat than males, on average?
Of course, there are exceptions. That is why statistical samples yield some kind of bell-shaped
curve. But the NIH's own percent body fat tables show that for age 20-24, for example, "optimal"
%fat is 18.9-22.1% for women, whereas it is 10.8-14.9% for men.
(The "margin of error" is reportedly +/-3 percentage points. Dontcha just love it when statistics
are reported to a greater degree of accuracy than the standard error?)
Of course, the correlation between BMI and labels like "underweight", "normal", "overweight", and
"obese" is based on statistical studies and linear regression (of some other method of statistical
correlation). I can only assume that the standard error is so great that it overrides any difference
due to gender.
Does anyone know the "margin of error" for the BMI? Can someone point me to a report of the
statistical study? Just how old is BMI, anyway?
My purpose in making those comments is not to write a provocative manifesto, but to highlight some
context to why I question the usefulness of BMI as an individual.
My conclusion is that BMI is virtually useless -- except perhaps for its original purpose, which I
believe was to simplify motality tables for insurance purposes.
(Right? But IMHO, even that is suspect because BMI fails to take "fitness" into account.)
I welcome knowledgable opinions on the subject.