July 18, 2009

10 Reasons BMI Measurements are Complete Horse Shit

The list comes from NPR but the title comes from Ken Wheaton* on Twitter.

* Just some guy linked from the "Twitter" section on RealClearPolitics.com.

A synopsis:

1. The person who dreamed up the BMI said explicitly that it could not and should not be used to indicate the level of fatness in an individual.

The BMI was introduced in the early 19th century by a Belgian named Lambert Adolphe Jacques Quetelet. He was a mathematician, not a physician. He produced the formula to give a quick and easy way to measure the degree of obesity of the general population to assist the government in allocating resources. In other words, it is a 200-year-old hack.

2. It is scientifically nonsensical.

There is no physiological reason to square a person's height (Quetelet had to square the height to get a formula that matched the overall data. If you can't fix the data, rig the formula!). Moreover, it ignores waist size, which is a clear indicator of obesity level.

3. It is physiologically wrong.

It makes no allowance for the relative proportions of bone, muscle and fat in the body. But bone is denser than muscle and twice as dense as fat, so a person with strong bones, good muscle tone and low fat will have a high BMI. Thus, athletes and fit, health-conscious movie stars who work out a lot tend to find themselves classified as overweight or even obese.

4. It gets the logic wrong.

[Obese people have a high BMI but a high BMI doesn't mean you're obese.]

5. It's bad statistics.

Averages measure entire populations and often don't apply to individuals.

6. It is lying by scientific authority.

Because the BMI is a single number between 1 and 100 (like a percentage) that comes from a mathematical formula, it carries an air of scientific authority. But it is mathematical snake oil.

7. It suggests there are distinct categories of underweight, ideal, overweight and obese, with sharp boundaries that hinge on a decimal place.

That's total nonsense.

8. It makes the more cynical members of society suspect that the medical insurance industry lobbies for the continued use of the BMI to keep their profits high.

Insurance companies sometimes charge higher premiums for people with a high BMI. Among such people are all those fit individuals with good bone and muscle and little fat, who will live long, healthy lives during which they will have to pay those greater premiums.

9. Continued reliance on the BMI means doctors don't feel the need to use one of the more scientifically sound methods that are available to measure obesity levels.

Those alternatives cost a little bit more, but they give far more reliable results.

10. It embarrasses the U.S.

It is embarrassing for one of the most scientifically, technologically and medicinally advanced nations in the world to base advice on how to prevent one of the leading causes of poor health and premature death (obesity) on a 200-year-old numerical hack developed by a mathematician who was not even an expert in what little was known about the human body back then.

To #10 I would add, "...and help make the case for a "health care crisis" in this country."

Science Posted by JohnGalt at July 18, 2009 3:11 PM

I saw this going around this week and wanted to take a victory lap. Thanks jg, I am way too modest to bring this up...

but I wrote in Feb 2007 that the weenie little Belgian that "discovered" this theory should have studied a little Calculus.

"Related Rates" dictate that a cylindrical man would add mass as the square of his height, and a spherical man (and we all know a few of those) would add mass as the cube. A realistic BMI would have to have an exponent between two and three. I offer this, humbly of course, as #11.

Posted by: jk at July 18, 2009 3:51 PM

Actually, a cylindrical man would add volume as the square of his radius, and proportionately with his height. (But relating volume to mass is further complicated by #3 above.)

Posted by: johngalt at July 19, 2009 10:45 AM

Somebody please pass the pie.

Posted by: Boulder Refugee at July 20, 2009 1:09 PM | What do you think? [3]