BMI questions

Factual Questions
1

  1. What is the advantage of body-mass index over the old height-weight tables? Is it just convenience (at least for those who measure their weight in kilos and height in meters) or does it give different results?

  2. When they do those studies that report that some large percentage of Americans are overweight or obese, do they use BMI or body fat percentage? If BMI, do they adjust for the fact that it doesn’t correlate perfectly with body fat percentage?

2

Various studies have been done to look at diffent indices for how much someone should weigh. They’ve generally settled on BMI as giving you a single index for height/weight that correlates best to the resulting mortality and health problems (which are, after all, what we should be most interested in).

This doesn’t make BMI a magic number, nor even the best number for all purposes. But experience shows it to be valid in setting weight targets.

This may vary between different studies, but most recent ones use BMI. Sometimes you will see a disclaimer that BMI may not be accurate (particularly for very athletic people who have a high percentage of muscle to fat, which raises their BMI.)

3

That’s exactly what I’m wondering about. If there’s a significant percentage of people who are athletic, it could give misleading results. That is, a report based on BMIs may say that 50% of Americans are overweight, when actually only 35% are overweight and 15% are in real good shape (numbers here are pulled out of the air, of course).

A disclaimer is better than nothing, but is ignored by the media. They just want the sensational big number to draw attention (and make everyone feel fat). So has anyone done any studies to find out how the BMI figures should be adjusted?

4

I think the bodyfat % is a more accurate indicator. Say a man weighs 300 lbs but has 10% bodyfat. He would probably look like a champion bodybuilder, but on the BMI scale would look no different than someone that was just way overweight.

As far as aggregate stats, I doubt this is throwing things off. Just by going out in public with my eyes open, I think there are way more truly overweight Americans than very athletic ones. I’ve noticed lately I see more really heavy people, say 100 lbs overweight or more, than I used to.