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Some fallacies aren’t simply a one-and-done stat, but an entire methodology. I cannot for the life of me think of a dumber example of this than the Body Mass Index (BMI).
The BMI was invented between 1830 and 1850 (wasn’t the germ theory of disease still a “theory” back then?) by the Belgian polymath Adolphe Quetelet. It breaks people down into the following categories:
– Underweight
– Normal Weight
– Overweight
– Obesity Class 1
– Obesity Class 2
– Morbidly Obese
OK, so far, so good. Graphically, it looks like this:
And mathematically, it’s all based on the following simple equation:
OK, now we have a problem. As you can see, there is absolutely nothing about muscle composition, body type of either gender (although some more advanced models do take this into account). Regardless, some nerd who’s never picked up a dumbbell in his life is not healthier than a body builder, which this equation would lead you to believe.
Let’s take a look at some examples using BMI-Calculator.net to do our calculations. We’ll start with professional athletes, using ESPN.com’s player profiles.
– Super Bowl winning quarterback Aaron Rodgers: 6’2″ 225 pounds, BMI: 28.9. Prognosis: Overweight
– Freak of nature Lebron James: 6’8″ 250 pounds, BMI: 27.5. Prognosis: Overweight
– The big but dominant pitcher C.C. Sabathia: 6’7″ 290 pounds, 32.67. Prognosis: Obese, Class 1
– The tank of a running back Michael Turner: 5’7″ 247 pounds, BMI: 35.44. Prognosis: Obese, Class 2
– Speedy point guard Deron Williams: 6’3″ 209 pounds, BMI: 26.12. Prognosis: Overweight
– Elite NFL lineman, and a very big man for sure, Carl Nicks: 6’5″ 343 pounds, BMI: 40.7. Prognosis: Morbidly Obese
Lebron James: Fatty
Seriously, Carl Nicks is a very big man, but morbidly obese implies he can barely move. More accurately, there’s hardly anyone this guy can’t move. It’s as if proponents of the BMI didn’t know that muscle weighs more than fat.
Indeed, if this Wiki answer is correct, Denzel Washington weighs 216 pounds. He’s 6 foot even, so his BMI is 29.29. In other words, he’s overweight and just a shade under being obese. And how about the other way. If this Ask answer is correct, than Angelina Jolie—who, at least to me, appears to be one of the more normal-sized, ridiculously beautiful Hollywood actresses—weighs 120 pounds. She’s 5’8″. BMI is then 18.24, or underweight. (At least Calista Flockhart came in underweight too, that’s a small token in the BMI’s defense.)
Kate Harding put together a slide show of pictures of mostly normal people whom the BMI has judged to be “overweight” or “underweight” or “obese.” It’s worth looking at. And if you actually want science not just ridiculous examples, NPR points out that the BMI is basically completely bogus. They give ten reasons, a small sample:
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.
And on and on and on… You get the idea. Yes, there is such a thing as obesity, and yes it is a serious problem, but no, the BMI is not a good way to measure it. How this stupid measurement of nothing has stuck with us for so long is beyond me.
Photo Credit: BBC and TampaBay.com
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Lies, Damned Lies and Statistics Series
Part 1: A Primer
Part 2: Income Stagnaton
Part 3: All Fiat Currencies Fail
Part 4: Iraq War Casualties
Part 5: Female-Male College Gap
Part 6: Male-Female Wage Gap
Part 7: Roger Maris’ Asterisk
Part 8: Women Do All the Work but Men Keep All the Money
Part 9: The BMI
Part 10: A College Degree is Worth One Million Dollars
Swift Economics 
Couldn’t agree more with this post and have personally made the same argument many times. Well said.
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If anything the height should be cubed…assuming humans are relatively uniformly dense….
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Not so many people around me care about BMI because they know it means nothing. Your point is absolutely right.
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You are making many logical fallacies my friend…
1) If you actually did you research, you would learn that all athlete reported weights in sports without official weigh-ins are exaggerated. This is well known. Sports is a like a dick-measuring contest. Everyone lists their bloated “bulking off season” weight, not their lean weight. And everyone seems to have 20 inch biceps (rounded up from their real 17 of course).
2) Steroids (some of those on your list might be on them). A person who has 20 pounds of muscle above what’s naturally possible is just as unhealthy as someone who’s 20 pounds overweight due to fat. Having a sixpack doesn’t mean you’re healthy if you also have 40 pounds of additional steroid weight. The risk that steroid users have are similar to those overweight to being FAT. Ask the pro-bodybuilders dropping dead left and right, despite not having an ounce of fat on their body… The BMI applies well even if you have a steroid enhanced sixpack I’m afraid.
3) It’s called overWEIGHT, not overFAT. It doesn’t matter if the weight is coming from FAT or STEROID BULK… Carrying around too much weight for your frame is too much if your heart can’t handle it, which is why “overweight” steroid users have the same blood-pressure issues and heart issues as fat people. Make sense? It’s over WEIGHT, not overFAT. Steroid bulk is just as bad as fat bulk.
4) The “precision” fallacy. Some of the people on your list are only slightly into the overweight range. No doctor has told anyone they’re dying tommorow because they’re 1 point above the cutoff. It’s a GUIDELINE, not a precise instrument. NO PRECISE INSTRUMENTS EXISTS IN CURRENT MEDICINE. IT IS ALWAYS BASED ON STATISTICAL ODDS (sheesh). It’s just like cutoffs for HDL/LDL or blood pressure or any other metric. The cutoffs are there for CONVENIENCE – this is how medicine works.
5) Some of the people on your list ARE IN FACT FAT. Many of the NFL linemen have a dangerous mix of tons of steroid bulk with tons of fat. Trust me, those dudes do NOT have sixpacks under their uniform. For example the guy you listed HAS A FACE LIKE A SOCCER BALL. His face IS the face of a morbidly obese person.
Having A TON OF MUSCLE below your fat DOES NOT negate the fat. And besides, the main risk is TOTAL WEIGHT, not total FAT. Fat is only an ADDITIONAL risk, but the bulk of health issues come from having too much WEIGHT for your body to handle. (yes between two morbid persons of equal BMI, the one with smaller waist will be healthier)
Very scientific of you… BMI IS ABOUT HEALTH RISKS. IT IS NOT ABOUT IMPLYING ANYTHING ABOUT ANYONES “moving” ABILITIES. IT PREDICTS HOW MANY ISSUES HE WILL HAVE WITH BLOOD PRESSURE AND HIS HEART. Capiche?
If… It is correct. Notice how you’re using examples that are unverifiable? There are plenty of studies using laboratory measured weight. Why critique all of medicine based on a celebrity’s wiki weight?
Kate Harding is a FEMINIST – yes the same ones who promote shady logic and warped arguments to promote ideas like a wage gap. She uses the same sort of logic behind most feminist mis-reporting as was used in the wage-gap – WHEN she does her thing on fat. Wake up
Kate Harding can stroge her fragile ego, but BMI is NOT about how you “look”. It is about HEALTH RISKS. And MOST OF THE PEOPLE IN THE GALLERY DO LOOK FAT anyway. Dare I say all the pictures match what their BMI designation is?
Show the gallery to EUROPEANS in countries like france, eastern europe etc. They will call every person in that gallery FAT too, without knowing their BMI. I dare you, do a blind test. Show the gallery’s “overweight” and “obese” people to those of us in non-fat nations, and you will get responses that match the BMI.
The reason you people see that gallery as “non-fat people” is because american perception of what is NORMAL has been distorted. You’re so used to everyone being so fat, that your mental image of “normal” and “fat” is distorted.
IT DOES NOT MATTER.
It was LATER found out that it was a VERY CONVENIENT measure that was a GOOD, CONVENIENT PREDICTOR of certain health risks. Nobody says that if you’re 28.5 you’ll die tommorow, but if you’re 28 you’ll live to a 100 years.
The entire criticism of BMI is based on a STRAWMAN.
So what? Centimeters, mili-liters and grams are EVEN OLDER. No really. Then medicine found that certain grams of tumor growth correlate to certain cancer risks.
Medicine found that this many mili-liters of this substance in the body poses this much risk. OMG, but the unit was invented longz agoz omgz!!?!?!
Again, IT IS MODERN STUDIES THAT FIND AND CONFIRM THESE CORRELATIONS. IT doesn’t matter when the unit was originally invented, no more than it matters when centimeters, ounces and CCs were invented.
It is for CONVENIENCE. Of course if you ALSO factor in waist you get EVEN BETTER predicting powers.
But then again, if you add 50 medical tests and bone density measurements, and dexa scans and 100 other things you get EVEN MORE precise.
This is what’s known as the NIRVANA FALLACY. It is criticizing something for not being PERFECT. But it was NEVER meant to be PERFECT, only CONVENIENT. The more extra measurements you add, the more predictive power you get. Eventually we’ll scan people’s genes and be able to tell them with even more precision.
The point is this:
a) IT IS PRETTY GOOD FOR ITS SIMPLICITY. If one is well into the overweight category, statistically they have more health risks than someone in the normal range. PERIOD. If one is “obese”, she (kate) needs to face up to the fact she has serious health issues.
b) IT IS NEVER USED PRECISELY. THAT’S WHY IT HAS RANGES. No doctor tells you that you are “doomed” if you are a few points either way. It’s only a FIRST MEASURE. Doctors do additional tests (duh!).
The old “I’m not fat, i’m big boned” excuse. Most people overcome it by the time they’re out of high-school…
1) Listen dude. I AM A NATURAL BODYBUILDER who has the most muscle that one can THEORETICALLY put on his frame without steroids. Guess what? I’m right on the very top of “normal”. In fact in natural bodybuilding BMI is often used as a good way to give newbies a good goal of what’s possible naturally. The most muscle you will be able to put (while being ripped) is also the top of the BMI.
Go google “natural bodybuilding contests” and “drug tested bodybuilding contests”. These are the most naturally muscular people on the planet. Their heights and official contests weights will be on there. WHAT DO YOU KNOW… Almost nobody is overweight… and NO ONE is deep into overweight territory. In fact when they get someone who’s ripped but overweight, they pull out all the doping tests they can… Weirdness galore… right?
You might say “but what if you have both muscle and fat!?!?” —> Well then you are OVERWEIGHT. If you have 20 pounds OVER normal, it doesn’t matter that they came from muscle. It’s about HOW MUCH TOTAL WEIGHT you can CARRY AROUND.
2) Bones only vary by a matter of small amounts. Bone-density variance can only contribute like decimals to the BMI measurement, not entire points. “Omg, my above fat bones make me 0.1 more on the BMI than if I had thinner bones, omg, the bmi is wrongzzzzz”
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My brother is a natural bodybuilder he is 200 pounds 5’6” which according to the bmi is 32.3. Which is considered obese. Incidentally in Asian populations the bmi was low balling the body fat percentage by about 5% for an average person. So, the bmi is an outdated calculation based on a specific group of people that don’t accurately represent the american population on a whole. However if you live in Belgian or the area around there then it’s pretty damn accurate.
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BMI is much maligned, but you know what? It will actually tend to *UNDERSTATE* obesity levels in samples compared to a bodyfat% metric. ie, there are more “skinny fat” people slipping through as “healthy” based on their absence of lean muscle than there are muscular people being classed as “overweight”.
http://www.nature.com/ejcn/journal/v62/n10/fig_tab/1602846f1.html
So yes, take BMI with a pinch of salt and use common sense – you above anyone else know if you are more Homer Simpson than Jonah Lomu.
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