Top 10 Reasons Why The BMI Is Bogus Weekend Edition math guy Keith Devlin graded the body mass index, a popular measure of determining healthy body weight, and failed it on 10 grounds.

Top 10 Reasons Why The BMI Is Bogus

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Keith, delightful to see you.

P: Hi, Scott. Nice to be here in person for once.

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

P: 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...

P: Because the formula...

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


P: 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?


P: 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.


P: 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.

P: 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?

P: 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.

P: And let me add, Keith Devlin.

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

P: 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?

P: 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...


SIMON: got a better idea?

P: 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...


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

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

SIMON: Keith, thanks very much.

P: Okay, my pleasure, Scott.

SIMON: Keith Devlin, our math guy.

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