Waistland by Deirdre Barrett

Author:Deirdre Barrett
Language: eng
Format: epub
Publisher: W. W. Norton & Company
Published: 2012-07-12T16:00:00+00:00

How Good Is the BMI? A Lesson in Lilliputian Physics

All these studies I’ve been citing on health and weight use the BMI as their measure, so you might assume it’s the most scientific method of assessing a person’s best weight. Oddly, this is not the case.

Gulliver meets the Lilliputians. (Courtesy of the Estate of Arthur Rackham. Bridgeman Art Library)

Not only would the BMI be better if the norms were adjusted downward and distinguished by gender, but replacing it with a very different formula would be best of all. Proportions can’t tell us enough about body composition and, as I’ll say more about in a minute, measures of muscle-to-fat ratio are preferable. Less obviously to many people, BMI doesn’t really measure even body proportions accurately. I’ve had more than one physician argue with me until they review their junior high geometry slowly, so it’s worth examining this point a bit—especially if you’re unusually tall or short.

The basic flaw in the BMI formula is that the denominator of height is only squared, which means that a short person will have a lower BMI than a tall person with the same body shape. The BMI’s “ideal” is based on a model where one should keep the exact same circumference while getting taller and taller. It’s a fundamental law of geometry that volume, and therefore mass and weight, increase by the product of three dimensions, not by two.

The best illustration of this I’ve seen is one Randy Schellenberg uses in writing about anorexia nervosa.34 Schellenberg applies BMI calculations to Jonathan Swift’s fantasy novel Gulliver’s Travels, in which Gulliver encountered two strange races of people: the tiny Lilliputians and the giant Brobdingnagians. The Brobdingnagians were proportioned like Gulliver but were ten times his height, while the Lilliputians were one-tenth his height. In reality the Brobdingnagians couldn’t exist, because of the engineering principle that as something increases in size, it must be made of stronger materials to support the same proportions. Lilliputians would run into the problem that some proportions would dictate membranes less than one cell-width thick. People already exist in pretty much the whole size range our design allows. However, the imaginary Lilliputians and Brobdingnagians make an excellent illustration of what’s wrong with the squaring of height in the BMI formula.

If Gulliver was six feet tall and weighed 180 pounds, his BMI would be near the top of the WHO-US “normal” range:

[ 180 / (72 x 72) ] x 703 = 24.4

A typical Brobdingnagian, however, if proportioned identically, would weigh 180,000 pounds—because if he’s ten times taller, then he’s also ten times wider and ten times deeper front-to-back, or 10 3 10 3 10 = 1,000 times the weight. He would have a BMI of:

[ 180,000 / (720 x 720) ] x 703 = 244

A typical Lilliputian would weigh only .18 pound and would have a BMI of:

[ 0.18 / (7.2 x 7.2) ] x 703 = 2.44

These extreme examples may seem ridiculous and irrelevant, but let’s scale Gulliver up by 19 percent to 7’2’’, the tall end of the range for NBA basketball players.

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