Top 10 Reasons Why The BMI Is Bogus

 

Top 10 Reasons Why The BMI Is Bogus

by KEITH DEVLIN

 

Americans keep putting on the pounds — at least according to a report released this week from the Trust for America's Health. The study found that nearly two-thirds of states now have adult obesity rates above 25 percent.

But you may want to take those findings — and your next meal — with a grain of salt, because they're based on a calculation called the body mass index, or BMI.

As the Weekend Edition math guy, I spoke to Scott Simon and told him the body mass index fails on 10 grounds:

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.

I don't think it is, by any means, a perfect indicator of health.   However, I use it as a goal - I do have good bones and have always exercised and so do not have any fat rolls etc but I am on the high end of normal, whereas I see people who look "larger" than me with a lower BMI, and who wear single digit clothing ....

I have always shot for being the healthiest I can be, in my diet, exercise, vitamins and labs - ie, the whole lifestyle.   

I do think people cherry pick aspects of health that they feel comfortable with - but I know lots of thin, unfit people who never exercise, smoke, drink excessively etc - can't say they are healthier than me,  just because their BMI is lower!!!!

As someone with muscular thighs and large bones in my legs (but normal size muscles and bones everywhere else) and LOTS of breast tissue, the BMI is pretty much BS for me.  What really made me realize how ridiculous it was (other than it being developed over 100 years ago when people in general were much more petite) is that a friend of mine is only 5 feet tall, clearly quite overweight with a lot of fat around her middle and in her bust, but has very small bones and very little muscle, yet SHE has a BMI much lower than mine because her bones and muscles weigh so little!

 

Lora

Thanks so much for sharing.  It makes me crazy that people rely so much on their BMI to determine whether they are healthy or not when most professional athletes (runners, cyclists, gymnnasts, and track athletes excepted) are overweight or obese.  Pro football players (yes, some of them DO have a lot of fat, but many do not) are often MO by the BMI.

Body fat percentage is a MUCH better indicator of being at a healthy weight or not!

 

Lora

(right on the line of normal/overweight BMI, but 24% body fat, which is in the "optimum" range for a 50 year old woman)

 

Thanks so much for sharing this informative and thought provoking article. It makes perfect sense in every way. The only disappointment I got from it is that it totally deflates my joy over being "just" obese after losing 105 pounds. Not super morbidly obese, not morbidly obese, "just" obese. lol. That's okay though. I know without a doubt that I will make it to my goal. I was reading your stats and found that we have so many similarities, both in age and starting weight. I can't tell you how encouraging that is! :)

Thanks for this. It is a mistake for us to ignore body fat and focus strictly on bmi.

That article would be a lot more interesting if it weren't so sensational.  BMI is not perfect but that doesn't make it bogus.  It's a fair measure of most people, most of the time, that can be obtained using instruments widely available.

And #2 is just plain wrong.  It's called regression analysis and it's used everywhere.

Thanks, C. Hulbert, you're right dead on. I think in spirit, the article makes a good point. It's inaccuracies don't do it any favors. BMI is not the entire measurement of a person's health, and certainly not their well-being.

As a former weightlifter (long LONG time ago)(dirt was new at that time ), I found that muscle is not necessarily heavier than fat. I apologize that this is the only stat I can remember, but when he was in peak bodybuilding trim, Ah-nold weighed only about 250. He was both huge and tall. Well, I just included that to further muddy the waters.....

We don't need BMI to embarrass the Americans, sadly. I have been lucky enough to travel outside the states a bit on bidness, and you just don't see the obesity other places. Now the Chinese and Japanese are buying American food products, they're becoming more portly, too. You don't need a statistical measurement to tell you what eyes easily see.

Ok, I'm hungry, so I better go clean something....

Angie in Missouri, US

305(h)+++++++++251(c)+++++++++++++++++++++++++++++++++++++200(g)

I give the BMI chart some merit. If you have another way to measure your health like body fat percentage, great. Sadly too many people here take the "BMI is bogus" stance because they do not want to do the hard work it takes to be thin.

And some of us just know that we have the body composition that it does NOT account for (greater muscle and bone mass,big boobs)... and that, in such cases, it IS bogus.  

I think that there are a couple of people who use it as a cop out of sorts, but I don't think I agree with the "too many" statement. Not to mention that "thin" is subjective.  I would not characterize myself as "thin" because of my chest and thighs, yet I weigh about the same as you (and am only an inch shorter), but would characterize you as thin.

Lora