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SCOTT SIMON, host:

This is WEEKEND EDITION from NPR News. I'm Scott Simon.

Americans keep putting on pounds, at least according to a report released this week from the Trust for America's Health. That study found that nearly two-thirds of states now have adult obesity rates above 25 percent. But you might want to take those findings and your next meal with a grain of salt - not good for water retention, by the way - because they're based on a calculation called the Body Mass Index, BMI.

Our math guy, Keith Devlin of Stanford University, thinks he's found some faults with the BMI formula. He joins us in our studios this week.

Keith, delightful to see you.

Professor KEITH DEVLIN (Stanford University): Hi, Scott. Nice to be here in person for once.

SIMON: Body Mass Index, that's what, height times weight or something?

Prof. DEVLIN: To calculate the Body Mass Index you take your weight in pounds and divide it by your height in inches squared, and you multiply the answer by 703. That gives you a number between one and 100. That's your Body Mass Index.

SIMON: 703 because...

Prof. DEVLIN: Because the formula...

SIMON: ...that's in the tax code or something.

(Soundbite of laughter)

Prof. DEVLIN: That's because the formula was originally invented in Belgium. It was based upon on measurements in terms of kilos and meters, and you have to convert it to pounds and inches. And the appropriate conversion factor turns out to be 703.

SIMON: This formula is, I imagine, the source of the old joke: I'm not overweight. I'm just three inches too short, right?

(Soundbite of laughter)

Prof. DEVLIN: It actually reminds me of another joke of the man who has his head in the oven and his feet in the refrigerator. On average he feels just fine. But of course he's going to be dead very quickly.

(Soundbite of laughter)

Prof. DEVLIN: I would put the BMI in that category of silly statistics that should've been abandoned many years ago.

SIMON: Well, what's wrong with it? I mean we should stipulate the Centers for Disease Control and Prevention, along with many doctors, use that statistic.

Prof. DEVLIN: Yeah. And it's gained this kind of aura of authority, which is really bizarre to any of those of us in mathematics. When we come to look at it, it's quite bizarre. It goes back to a Belgium mathematician called Quetelet, who early in the 19th century developed it. He's a very good mathematician. He was the mathematician who really helps get off the ground modern statistical social science, making decisions about populations based on figures and hard data.

And he found that if you take people's weight divided by the square of their height, you get a mathematical formula that for the entire population does agree with the data that you've got. Now, he was a smart guy. He was very careful to say all this does is tell you what's happening at the level of society. But he explicitly said you can not apply it to individuals because it could be very, very misleading. And indeed, it is very misleading.

SIMON: Well, you found some individuals, right?

Prof. DEVLIN: In fact, if you look at famous athletes and sports people, very often you can find on their fan sites information about their heights and weights. And looking around, I found that the following individuals all officially classified as overweight: Kobe Bryant, George Clooney, Matt Damon, Johnny Depp, Brad Pitt, Will Smith, and Denzel Washington.

SIMON: Tubs of lard, all.

Prof. DEVLIN: And let me add, Keith Devlin.

SIMON: And you cycle like about 900 miles a week or so, right?

Prof. DEVLIN: I cycle close to 200 miles a week. My waist is 32 inches. Most people say I look pretty skinny and I should eat more hamburgers. But I just tipped the BMI at over 25. I'm at 25.1 most of the time, and that's officially just in the overweight category.

SIMON: Why is this formula so wrong?

Prof. DEVLIN: It ignores the waist. Now, if you're ignoring someone's waist measurements, you're ignoring the most obvious measurements of obesity. Your grandmother and my grandmother would say your belt is too big, you need to lose weight. And your grandmother, I think, would be doing a better job than the CDC is doing.

The real scientific reason why it gives you the wrong answer is that the body is made up of different materials. In particular, we have bone, we have muscle, and we have fat. Bone is very dense. It's actually twice as dense as fat. Muscle is less dense than bone but it's still more dense than fat. The BMI actually classifies as overweight the very most fit members of society who exercise and turn their fat into muscle.

SIMON: So, Mr. Fancy-Schmancy Math Guy...

(Soundbite of laughter)

SIMON: ...you got a better idea?

Prof. DEVLIN: First of all, on the CDC's own Web site they list alternative methods, which admittedly cost a little bit more money. Because they do involve looking at the body of the individual concerned, taking a caliper to your midriff and measuring how much fat there is there, putting you in a pool of water and measuring your density, and so forth. A simpler method actually would just be to measure people's waist size.

SIMON: During the break, Keith, you and I are going to go in and measure...

(Soundbite of laughter)

SIMON: ...dunk ourselves into water and measure our bone density, or whatever you said.

Prof. DEVLIN: I've been cutting down on my food all week in case you asked me that.

SIMON: Keith, thanks very much.

Prof. DEVLIN: Okay, my pleasure, Scott.

SIMON: Keith Devlin, our math guy.

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