Asymmetrical Information

Most obesity studies use BMI in order to determine whether their subjects are overweight (or underweights). And as has been discussed ad nauseum, BMI is a deeply flawed statistic.

Of course, the same bias exists at the other end of the curve too: Poor health often makes exercise more difficult, which leads to weight gain.

In fact, a less simplistic statement than 'being overweight shortens life expectancy' might be 'lack of exercise shortens life expectancy and often leads to being overweight'.

A moment's comparison to the burgeoning literature on caloric restriction ought to have made anyone a bit skeptical before proclaiming that being skinny shortens one's lifespan in and of itself.

And the other problem with these BMI studies is that if you work out with weights at all, and build up a little bit of muscle, your BMI will increase and very likely push you well away from the lowest BMI category. It's undisputed that that there are a lot of regular exercisers whose BMI looks too "high," simply because they have more muscle, which weighs more than fat. According to the government's BMI calculator, Michael Jordan (stats here) is "overweight." Which is absurd.

To finish the point: If someone like Michael Jordan (or any one of the millions of people who exercise) lives longer than a slightly plump person who never exercises, the BMI studies would apparently reduce this to the claim that people with somewhat higher BMIs live longer. And this finding would then be interpreted as if it's alright to increase your own BMI by sitting around eating junk food (rather than by exercising and adding muscle).

According to the government's BMI calculator, Michael Jordan (stats here) is "overweight." Which is absurd.

Rumor has it that MJ gave up trying to learn how to swim because his extraordinarily low body-fat percentage made it impossible for him to float. He instead would sink whenever he tried to float, which makes it very difficult and nerve-wracking to learn to swim.

Stuart:

That's another reason why the oft-repeated claims of the Fat Acceptance types (like Ampersand over at Amptoons) that these studies show that being overweight isn't a problem should be discounted. Though OTOH I think it's fairly hard to actually fall into the "obese" category of BMI just from being exceptionally muscle.

While BMI is most definitely a flawed statistic, is it still not valuable as a general indicator? Yes, there are certainly marginal cases of muscular people falling into the overweight, or even obese category...that would include me, even if my body-fat percentage were low (which it's not). But aside from these relatively few people, wouldn't it be fair to say that the minority of people whose elevated BMI is not from fat is too insignificant to make a case for not applying it to large groups of people?

We can't ALL be like Mike, after all.

Mark Twain once said of statistics:

"Statistics are like prostitutes, once you've got them down, you can do anything you want with them."

My favorite statistical absurdity is the one about life expectancy in the Roman Empire: age 21.

This is silly on the face of it, because, for one thing, the Roman Legions would have been unsustainable -- length of service was 16 years, from ages 17-18 to 33-34. This was followed by a fairly long, active retirement on a land grant.

Obviously, the "21" is derived by factoring infant and child mortality, as well as mortality from epidemics, etc. Nevertheless, it leads to a distorted image of Romans becoming "old" while in their early 20's.

I repeat my point from the last time we did the BMI-runaround: At the extremes, it doesn't work well. In the middle, it's a pretty good predictor.

And if you're pointing at the extremes to prove BMi wrong, likely, you're not one of those in the (fat) middle.

Stupid typos!

I meant to say: "And if you're pointing at the extremes to prove BMI wrong, likely, you're one of those in the (fat) middle."

Next time, click "Preview".

There have been many studies over the years that consistently showed higher mortality for underweight persons, even when disease and age were considered. When illness (such as a major infection or cancer) strikes a healthy but very skinny person, that person has no energy reserves and is more susceptible to complications and death.

No reliable studies have shown that being overweight significantly increases risk of death. Being obese is associated with increased risk of death with risk rising exponentially with degree of obesity.

And if you're pointing at the extremes to prove BMI wrong, likely, you're one of those in the (fat) middle."

Is this supposed to be aimed at me? If so, it's wrong. I'm 6'4" and around 210, with an "overweight" BMI of 25.6. I used to weigh around 175-180, but that was before I started lifting weights in college. In fact, if I were still lifting weights now, I'd probably weigh 225 or 230. The only reason that I now weigh so little is because I stopped several months ago, so that I could lose some muscle mass (to make it easier to run 6-9 miles at a time).

But I'm (obviously) in nothing like the physical condition of Michael Jordan, and whenever I used to go to the gym, there would be plenty of guys who were more muscular than I was. That's why I have the impression that men who work out regularly at all will probably build enough muscle mass that their BMI will be skewed. Even if they still eat too much unhealthy food, such that their bodyfat is too high, the fact that they're exercising and building muscle mass is probably going to make their BMI rise. Muscle is heavy.

John Thacker is right that it's hard to get into the "obese" category just from muscle mass, although professional-level bodybuilders or football players can certainly do it.

From Dr. T: There have been many studies over the years that consistently showed higher mortality for underweight persons, even when disease and age were considered. When illness (such as a major infection or cancer) strikes a healthy but very skinny person, that person has no energy reserves and is more susceptible to complications and death

Disease and lower energy levels have nothing to do with it. We larger persons simply go around beating the crap out of the smaller ones.

I'm still tring to get to the -4% body fat level I need to be 'normal'. Any suggestions?

Stuart: Under my statement, you do not fall under the "likely" category.

I am guessing that those who pooh-pooh BMI as an indicator do so as a means of making themselves feel better as opposed to adhering to statistical and scientific rigorousness.

Additionally, it is clear from your height (76") that you are on the outside of BMI's usefulness.