I weight lift... a lot. So BMI is utterly useless for me (as you said)
And while BMI may be useless to you, it's reasonably useful for the vast majority of people in the world.
Overall, I think BMI is probably a pretty good overall metric, just not a good diagnosis for an individual. For instance, a physician might notice that a patient has a BMI above 25, but then see that waist to height ratio or a bf measurement looks very favorable.
From a broad numbers perspective, I think BMI is probably very useful - for instance, if you notice that a higher percentage of a population has a BMI in excess of 27 than in previous years, you can safely estimate that the population is becoming increasingly overweight. I suspect that the situation where people have high bmi's with low body fat is limited to a small number of athletes who are not typical of the population at large.
Simply because a measurement is fast, simple, and gives some arbitrary numerical precision doesn't make it useful. It's also got to, you know, like, correspond to the stuff it's supposed to be measuring. Which it doesn't.
Official discussion of BMI actually admits this.
The panel did not search for evidence to prove that
measures of body composition or metabolic rate are
useful tools.
Either way, BMI clearly has some use. Or do you think this chart is completely random and devoid of correlation? http://upload.wikimedia.org/wikipedia/commons/b/b7/Correlati... Do you think the ordering of countries on the BBC link is purely random and has nothing to do with how many fat people are in each country?
It shows population correlations. In a general population, where a range of bodytypes may be assumed, there will be a correlation between BMI and body fat percentage. There's relatively little argument over that, though as I mentioned above, the actual utility of BMI for epidemiological / population studies is being question.
- The question isn't "is there a correlation?", but "is the correlation between BMI and body fat stronger or weaker than for other estimators of body fat?". For which you'd want to look at the r^2, RSS (residual sum of squares), and perhaps plotted residuals to determine the appropriateness of the model, in comparison with other estimators: waist circumference, height/waist ratio (most people also have a pretty good idea of their waist size), circumference and caliper estimators, etc.
- To paraphrase Donald Rumsfeld, in epidemiological population studies, you conduct research with the data you have, not with the data you want. Height and weight are widely collected and reported, even from sources such as DMV files (though they may also be wildly inaccurate). For large-scale or national studies, this makes for large datasets. The relevance and usefulness of the data may not be as high as, say, DEXA analysis, or even waist to height ratio (http://www.webmd.com/heart-disease/news/20050606/waist-heigh...), but it's there. And for at least some purposes, useful.
Extending the use to individuals, where in most settings (home, gym, doctor's office, clinic, health fair), availability of necessary measurement tools (tape measure, calipers, even the wildly inaccurate handheld electronic bioimpedance monitors) is readily facilitated, there's really no call to rely on BMI.
- The problem with BMI is that it's essentially saying "if you fit our model of body fat relative to height and weight, then here is where you fall on our model of obesity". Again, for a general population this relationship holds reasonably well, but for an individual, where the question specifically is "what is my body fat percentage", BMI is the wrong tool for the job. It fails notably for specific populations of interest -- it will consistently underreport obesity in less-healthy populations, and overreports obesity in more-healthy populations. It can also result in nonsensical intervention recommendations: most unfit people will benefit by efforts to increase lean muscle mass, despite the fact that for those with lower BMIs this will decrease their putative health score by increasing overall body mass. I suspect this is among the reasons strength training is under-recommended for increasing fitness (most literature addresses "diet" and "exercise" generally, but makes little or no distinction between cardio and strength training, or focuses virtually exclusively on cardio).
As I've been repeatedly saying, the other estimators of body fat are much more difficult to calculate. Most people already know their height and weight. Most people don't know their waist circumference and have absolutely no idea how to calculate BF%. That's why sites like this use BMI, so that their visitors can gain some information rather than no information.
Among the problems of BMI is that it is official health policy. It's the definition of obesity according to the US government. It's the basis of insurance coverage / premium assessments and other abuses. Changing policy would mean that doctors' offices would measure your height and waist, to give a far more accurate fitness assessment. Over time, epidemiologists would have this dataset generally available.
Really. It's long past time to retire BMI.
Why are you so wedded to it?
A set of 3-4 tape measurements (height, neck, abdomen for men; height, neck, waist, hips for women) will give a reasonable estimate of body fat. Equipment cost, about $5. http://www.linear-software.com/online.html
Your observation of the weakness of BMI is that it tends to misreport true body fat percentage both ways. I'd hardly call that an advantage of BMI.
Note that since official US government policy defines obesity in terms of BMI, technically, BMI doesn't misreport obesity. What it does is fail to correspond to true body fat percentage in any meaningful way.
Your point is that a better measurement can be made if everyone goes out and buys some new thing and takes some new measurement.
That seems to be in agreement with my point.
> since official US government policy defines obesity in terms of BMI, technically, BMI doesn't misreport obesity
The US government defining something doesn't make it true.
>What it does is fail to correspond to true body fat percentage in any meaningful way.
It is very meaningful. It's much better than a random guess.
As many people in the fitness/bodybuilding field will tell you, eyeball estimates of bodyfat are in fact pretty bloody accurate. If you're interested in equipment costs, I could look up eyeballs on Amazon as I did for tape measures.
And again, I agree that there are more accurate measurements, but they are all significantly more complicated. This time you are suggesting training people to visually estimate body fat percentage. That could work. But it's a heck of a lot more complicated than telling someone to type their height and weight into a website.
But I've not lost 6 pounds while gaining strength. I can see my abs. Turns out I was 12 pounds over my high school weight.
I'd now need ~8 pounds of extra muscle to become "overweight". That's a lot.
I think it's hard to go over BMI based on muscle alone. You may have less of a problem is you're BMI 25 and muscular, but you still probably could stand to lose some fat.
I believe it was $50. There are several on Amazon for that price and lower. If a regular scale is $20, and one that measures body fat is $40, then I don't think it's a big deal. I personally watch the body fat % now rather than weight.