Why the BMI is Bogus

I saw this link on Tiptoe's blog and let me just say: I love it, I love it, I love it!

This should be required reading for any health journalist who uses the letters "BMI" in a story--which is basically all of them.

In its entirety (because it's just that good) is the "Top 10 Reasons Why the BMI is Bogus":

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.

The CDC says on its Web site that "the BMI is a reliable indicator of body fatness for people." This is a fundamental error of logic. For example, if I tell you my birthday present is a bicycle, you can conclude that my present has wheels. That's correct logic. But it does not work the other way round. If I tell you my birthday present has wheels, you cannot conclude I got a bicycle. I could have received a car. Because of how Quetelet came up with it, if a person is fat or obese, he or she will have a high BMI. But as with my birthday present, it doesn't work the other way round. A high BMI does not mean an individual is even overweight, let alone obese. It could mean the person is fit and healthy, with very little fat.

5. It's bad statistics.

Because the majority of people today (and in Quetelet's time) lead fairly sedentary lives and are not particularly active, the formula tacitly assumes low muscle mass and high relative fat content. It applies moderately well when applied to such people because it was formulated by focusing on them. But it gives exactly the wrong answer for a large and significant section of the population, namely the lean, fit and healthy. Quetelet is also the person who came up with the idea of "the average man." That's a useful concept, but if you try to apply it to any one person, you come up with the absurdity of a person with 2.4 children. 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.

Note: if you click on the story link, you can listen to the NPR segment as well.

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Anonymous said...

As a recovering anorexic, I relate to the sense of body distortion - am I normal, is really what I want to know. At my current weight, I am overweight - 20 lbs so. Yet, I am active and am proportionate, have working use of all my muscles. It shocks me that my boyfriend doesn't consider me chubby when my BMI clearly states that I am. I was hoping to lose enough to be in the "normal" BMI range - reading ur post, however, was shocking. I thought the BMI scale had much more scientific authority that it does. Oy, I'm just gonna settle for whatever weight three meals a day and 2 snacks gets me. I am tired of hating myself. Hope you get better, Carrie.

Tiptoe said...

I knew so many of us would enjoy this segment from NPR. I was ecstatic. Let's hope it reaches the masses and professionals, so less weight is given to the BMI. After all, aren't we all more than a BMI category?

now.is.now said...

I love what anonymous said. Eat 3 meals, 2 snacks, stay active and then... you weigh what you weigh. Love that concept. Thanks!

And, regarding your previous post - I have totally thought the same thing regarding the pants. A few years ago, I had a very naturally thin roommate who forced me to try on her pants to prove to me that I had lost too much weight. I was literally SHOCKED when they were big. It was a wake up call. Now, I would never fit in her pants. But I guess I'm not supposed to be able to either....

Mostly, I just totally hate body image.

Anonymous said...

Good.... but I've seen several reports (and I take ALL epidemiological studies with a grain of salt, even these) showing that obesity is not the big problem it is painted to be, and that obese people are not necessarily in poor health. And IMHO, the waist measurement indiciator is not much better than BMI. I recently went (for the last time) to a doctor who felt it appropriate to measure my waist as part of a gyno exam. I was just barely under the "bad" waist size- and yet, I am healthy, fit, and of normal weight. Another meaningless number.

Anonymous said...

P.S. there was a site, alas I can't remember it, that listed some people who come out overweight or obese on the BMI scale.
They included Andy Roddick, George W. Bush, and Yao Ming. Need I say more?

Carrie Arnold said...

Anon-- you're right. Waist-to-hip ratios aren't foolproof either. I'm pretty narrow through the hips and I tend to carry my weight more in the thighs, which means that my "ratio" always leaves me scratching my head. (I knew the numbers because I was sewing some pajama pants- I'm not measuring tape happy!)

I think the AED obesity prevention task force put it best: weight is not a behavior, and you can only change your behaviors.

Adrianna Joanna said...

I feel this person made legitimate points, and at some points she was hilarious. 200-year-old hack, lol.
I just wish this person hadn't assumed that most of us don't eat right or exercise enough and that being fat is automatically bad.Like others have pointed out, weight is primarily genetic and is not at all an indicator of health, yes, even if that weight is made mostly of the evil fat.

How the hell does she know what all our lifestyles are like, or what we should all be doing anyway? We are all different. I'm more active than many people thinner than me, but I am not as thin as they are. It's kinda hard to run a mile if you walk with a limp. It's hard to eat all those healthy nuts and fruits if you are allergic and live on gluten-free. That sort of thing.

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I'm a science writer, a jewelry design artist, a bookworm, a complete geek, and mom to a wonderful kitty. I am also recovering from a decade-plus battle with anorexia nervosa. I believe that complete recovery is possible, and that the first step along that path is full nutrition.

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