The problem with body mass index

The Body Mass Index is a popular way to measure and assess whether someone is overweight or underweight. Basically, it's just your weight divided by your height. BMI is a simple system, but it does have some flaws. Over at the Obesity Panacea blog, Peter Janiszewski (who has a Ph.D. in exercise physiology) has a nice post explaining why BMI is sometimes useful, and also why it's not a great measurement of individual health.

Right now, I bet you're chomping at the bit with, "Yeah! The BMI doesn't distinguish between fat and muscle!" And you're right. It doesn't. A simple BMI measurement would tell you that The Rock is no different than an out-of-shape couch potato of the same height and weight. Obviously, that's a flaw in the system. But it's not the most important flaw, Janiszewki argues. In fact, he says, this well-known problem with BMI is the easiest one to work around. A simple visual check can tell your doctor whether your high BMI is due to excess muscle or excess fat.

The real problems with BMI, he says, are a lot more complicated. For one thing, the system doesn't distinguish between very real differences in health outcomes for different body types. "Pears" and "Apples" might have the same BMI, but with opposite health results. Even more troubling: You can change your lifestyle, and become objectively healthier, and your BMI might not budge.

This is particularly so if you adopt a physically active lifestyle, along with a balanced diet, but are not necessarily cutting a whole lot of calories. This lack of change in BMI or body weight is all too often interpreted as a failure, resulting in the disappointed individual resuming their inactive lifestyle and unhealthy eating patterns.

However, as we have argued most recently in a paper in the Canadian Journal of Cardiology, several lines of evidence suggest that weight loss or changes in BMI are not absolutely necessary to observe substantial health benefit from a healthy lifestyle. Thus, an apparent resistance to weight-loss should never be a reason for stopping your healthy behaviours.

Read the rest at Obesity Panacea

Image: Wisley Taste of Autumn: Europom, a Creative Commons Attribution (2.0) image from rictor-and-david's photostream


  1. How is The Rock different from a similarly sized couch potato? Both are morbidly obese and have remarkably inefficient bodies that expend most of their energy just carrying around all the added weight of fat and superfluous muscle.

    1.  The big difference is that when the two of them are just sitting around, the morbidly obese person’s fat is burning 2 calories/hour/lb, while The Rock’s muscle is burning 6 calories/hour/lb. Also, most of their body energy goes to the brain and liver, not carrying around body weight. Something like 90% of resting energy goes to osmosis, only 10% is mechanical.

    2. Dwayne Johnson is not even fat, let alone obese, or even more ridiculously, morbidly so.  BMI is so broken as to be useless.  I eat a very balanced diet, excercise usually 5 times a week, and while I do not have a washboard stomach, I do have definition in my abs.  According to the BMI, I am obese.  And no, I am not some ‘roided out looking muscle head.

      BMI is a joke, and is an easy way for damn near everyone to be considered high risk for life insurance policies.  If one is actually interested in the overall health of any one body, the fat to lean muscle mass ratio is a much, MUCH better means of measuring.

      1. Looks no different from the typical (non-meth-head) trailer park resident to me. Just give him a moustache, trucker hat and a mullet, plop him on the sofa in front of his wife’s stories on the tube, and swap his spandex panties for cut-off denim shorts.

        1. You’re trying too hard.  Back in my day, trolling was an art.  Trolling meant something.  Now, people just say ridiculous things.  True, comments like yours tend to gain a few biters.  Maybe that’s the problem.  Maybe it’s that people have forgotten the meaning of trolling.  Maybe people let their feelings guide them to respond to trolls, instead of just ignoring them.  I don’t know.  It’s a sad day for trolling.

          1. Hey –  I remember those days! Back when trolls didn’t try to pass themselves off as legitimate commenters by making spurious accusations of trollhood against those with valid points of view.
            Nothing to see here, reggie… back to your stories and Funyuns.

  2. I used to work as a diesel mechanic, and still carry  around the… burly build that someone working with large equipment will have. Yes, I’m fat. But the doctors are always amazed at how a guy who’s 5′ 6″ and 280 lbs could have a completely normal blood pressure and be otherwise healthy. According to the standards, to be just “overweight” I would have to lose 100 pounds.

    Never mind all the muscles, never mind the barrel chest. I don’t fit this 19th century “scientific” ratio, therefore I’m sick. According to the BMI, I should be in immediate danger of death, and have problems getting around, and so on.

  3. BMI isn’t supposed to be the equivalent of an IQ test for your body.  It’s one broad measure among many things that can assess health.  Anyone actively concerned with their health and fitness has more specific and accurate measures available.  It doesn’t really matter that it’s out of whack for an athlete because an athlete would never use BMI as a measurement.
    What it can do is give someone who’s not involved in health and fitness a comparison of their own height/mass ratio against norms.  

  4. I was curious, after running a timed mile, about whether I was an the “good” ballpark or “bad.”  I was somewhat surprised to see that a doctor had suggested (and then backed off) the idea that a mile-run time is good measure of total fitness for 50-somethings like me.  (Heh, on my second attempt I ranked in “good” shape by his measure.)  He  backed off though, not because anyone found fault with the idea.  No, they worried that too many 50-somethings would go out there and kill themselves trying.

    Anyway, long story short – find a performance measure that works for you, and find steady improvement.  Maybe that’s how you feel after a weekend hike, or your time on a bike loop.  Whatever.

  5. We don’t have a good enough understanding of the human body to quantify ‘overall health’ numbers are pretty meaningless when talking about bodies. People are all obsessed with their weight, and they put themselves on this emotional roller coaster that’s completely avoidable if they only realized that weight fluctuations are normal, especially if you’re exercising – your body is doing a million regulatory things, it’s not as simple as “exercise = fat burning = weight loss” we have this tendency to oversimplify things like health (usually in order to sell something), to the detriment of all.

  6. i was a big fat fatty like 5 years ago, with a BMI of 28. i started running, and 2 years later, i was a 24, which is healthy. then i fell in with the hortonites, and now i’m back up at 27. 

    so what i’m saying is, the result of all my plyometrics, ab rippers, and yoga x (“it’s not a cult or a religion, it’s just oms”) is that i’m now 15 pounds overweight again according to bmi. 

    so what i’m saying i’m saying is, bmi seems sort of stupid to me — it seems to motivate you to diet, rather than to become active. like, muscle is a penalty.

  7. Yes, BMI is flawed.  I think it is equally flawed to suggest that fitness alone will guarantee success.  It has to be the total package: fitness and a healthy diet and a healthy lifestyle.

    Now, beyond that, the words “healthy diet” are continually up for heated debate.  I think it is generally agreed that a low-saturated fat, low-sugar, low salt, low-alcohol, low-processed foods diet is healthiest, and not too many total calories each day.  The EXACT makeup of that diet is subject to interpretation.

    To suggest that you can eat whatever you want as long as you exercise: FLAWED.
    To suggest that you can eat a low-calorie diet and not have to exercise: FLAWED.
    To suggest that you can take this metabolic diet pill and not have to eat well or exercise: FLAWED.
    To suggest that you can eat well and exercise like a fiend and remain stressed out all the time and not ever deal with personal mental wellness: FLAWED FLAWED FLAWED.

    It’s the total package. 

    When are these “scientists” going to get it through their thick skulls?

    1. I think the “scientists” are backing up your plan for moderation.  It’s the public health and diet industries who have the “simplified” or “extreme” plans.

      1. This is usually how the conversation goes:
        Scientist: It’s complicated, to evaluate a persons health around weight and fitness and diet.  When you look at caloric intake, you also have to see the effects of the types of fats, the types of carbs and the amount people are consuming.  Fruit and vegetable consumption is generally associated with better health, supplementing with vitamins generally isn’t, and salt has been controversial but likely bad overall, even if you’re not hypertensive.  For exercise, there’s the importance of cardiovascular exercise, but strength building exercises have been more beneficial towards quality of life then we expected and some benefit is also seen with proprioceptive based regimes.  Of course it’s more complicated than just that, and we’re finding out new things every week.
        Journalist:(long pause)
        Journalist:So if I had to say it in one sentence, you’re saying the important thing is that salt is bad, right?

      2. What “public health industry”?  That beast doesn’t exist.

        And I’m not even talking about disreputable scientists.  The reputable scientists are subject to their own myopia.  Papers about holistic approaches to diet, lifestyle and weight loss don’t make it into NEJM, JAMA and Cardiology.

        I know many of these scientists personally, and they are fine people.  And their research is valuable.  But they are so focused down on controlled studies, that they cannot and do not produce generalizable findings that fit in reality with what you need to do to be a healthy person.

        And don’t get me started on media reporting the latest greatest research.  The whole thing sucks, when what we all need is a swift kick in the ass and to grow some horse sense about our lives.

          1. Sure, the information is all “together” in a table… but it’s just next to itself, not really doing anything.

        1. I knew I was breezing by two groups with “public health and diet industries” but thought it fair.  There is after all a career path associated with “public health,” and part of the job might be pushing things like BMI.

  8. Carrying extra fat is one risk factor for high blood pressure, diabetes and heart attacks.  This is pretty well known.  Carrying a lot of extra weight for a long time greatly increases the risk.  That’s unarguable.  The fact that BMI is not an exact and conclusive measurement is no reason to disregard it and ignore your weight if you score high.
    High BMI is like a smoke alarm going off.  The fact that they can give a false alarm is no reason not to go see if there’s a fire.

  9. BTW, I should mention that I come from this at the other end.  I’m way skinnier than society wants a male to be.  I’m 6′ and 145.  I was actually shocked the first time I ran the test and found that they put me in normal BMI range.  I mean a street kid in Mexico once made fun of me (“feed me!”).

    But I guess late in life I’ve come to accept the skinniness.  And after converting some fat to muscle at the same BMI, people assume I’m a runner, which amuses me.

    1. I’m solidly in the middle of Healthy BMI and have been for 35 years.  This is due to genetics, recreation and physically active jobs.  I’m considered skinny by most people and very healthy by my doctor.  It’s clear that in the US expectations have changed in the last 20 years.  

      1. Yes, they changed the definitions for obese and overweight in 1998. Keep that year in mind every time you read an article talking about how in the last X years, the percentage of overweight and obese Americans has gone up dramatically. The number of years will always go back to a bit before 1998.

        That’s like if they change the definition of murder to include suicides and then say that the murder rate has gone up 1000%.

          1. So you don’t think 5 to 10 % in 10 years is huge?  We are talking about 15,000,000 to 30,000,000 people. Entire states, if you grouped them together.

    2. I had no idea what my BMI was. I’d never been interested in knowing. So, I went online and tried a few of those calculators, which all put me in the range of 20-22. I’ve always been labeled as “skinny”, though I’ve recently tried my best to cover up the muscles that had started to become visible in my late teens and early 20s. In addition to the BMI, all deeper health indicators suggest I’m very healthy. Blood pressure, cholesterol, heart rate, blood sugar, etc. are all abnormally low. My resting heart rate is around 50, and after a sprint around the block will not exceed 100.
      But my point is that I don’t feel any of these quick or more invasive indicators, even used together, are especially accurate for broad assumptions. Quite simply – I cannot be this healthy. I eat beef and pork in large portions several times per week. I drink a gallon of whole milk per week and have 12-24 oz of soda per day. I consume sugar like it’s oxygen. I go days without eating vegetables. That’s not an exaggeration – it’s not uncommon for me to go 3-4 days without consuming even a single leaf of lettuce. I despise veggies. I have a sedentary job, and a largely sedentary lifestyle outside of work. The only real exercise I get is in the bedroom, and I doubt a few hours per week are enough to keep me healthy.
      As far as I can see, my only bright spot is that I take in much less salt than the average American (and haven’t eaten a frozen meal since college).
      And yet the doctors always commend me on the active, healthy lifestyle that I must be maintaining. I’m convinced it will all catch up with me by the time I’m 40 or 50, if I make it that far.

      1. All I can say is that if you wanted to try eating more vegetables, there are ways to dress ’em up and make ’em taste good without overloading them with crap.  But it would take effort and curiosity.

        1. Thanks. I try.
          The few I can stand I prefer fresh and raw. I’ve resigned myself to calling ginger, garlic, sweet potato, sriracha and horseradish veggies (but what’s in a name?) because, if not for them, then I’d probably eat the equivalent of 1 serving of vegetables per week.

  10. My mother has run at least one marathon every year for the past ten years, is in fantastic shape, and is in the “obese” category of BMI because she’s 5’2. Short people who don’t have what I’d call a skinny body type – she’s got very wide shoulders and hips – are royally shafted by BMI. If my mum weighed what BMI says she’s supposed to weigh, she’d be in very bad condition.

    The reason BMI is good though is because it’s better than using weight alone. That’s not much of a review – not quite as bad as the next bad option – but it’s the reason it’s used at all.

    1.  Back in the 90’s I starved myself to get down to the “ideal weight”. My doctor became alarmed when I hit 120lbs at 5’6″ because my ribs were clearly visible. My body type didn’t react well, not to mention that I was in the throes of disordered eating to reach the weight the doctors told me was “healthy”.

      Now I watch what I eat, I try my best not to eat anything that comes out of a box and is filled with chemicals, and I’m now what doctors term “obese”. Thing is that I’m far healthier by metabolic standards than a friend of mine who is at a “healthy weight” but who is sedentary. I’d venture to say I’m far healthier than she is by actual health measurements, though I’m the one who is seen by society as the “unhealthy” one. That’s seriously screwed up thinking.

    2.  Yeah, one of the problems with BMI is that is doesn’t take into account your physical frame. I have what under older systems of measuring would be described as a “large frame” because regardless of my weight I have broad shoulders and wide hips (can’t make bones smaller). When I weighed around 120 in highschool my doctor ordered me to gain weight because while my BMI was well within normal for my height it was clear my body was not built for that little mass on it and it was negatively affecting my health.

  11. The best indicators for health are metabolic indicators like blood pressure, cholesterol and blood sugar. I’m “obese” (misses size 14-16), yet I work out with a personal trainer twice a week, I eat well and I feel great. My metabolic indicators are right where they should be for “healthy” people, yet I’m told I’m sick because I don’t fit into the narrow definition of “healthy”. 

    Thin != healthy, fat != unhealthy. I wish people would get this in their heads. Research shows that 95% of people will gain back the weight lost and more when they stop actively dieting. Weight cycling is far worse for blood pressure and other body processes than being overweight is. BMI is a useless measure. I haven’t lost a pound yet I’ve lost a size so far in working out. Muscle is replacing fat, and I’m leaner and stronger. That’s my measure of fitness and health. I only wish doctors and insurance companies thought the same way.

  12. BMI is a really good tool. You can use it at home, simple as that.
    It gives a very rough result, sure. It’s like measuring a persons height with an ungraded metre long stick.

    But if that stick is what you have at home, and all you need is to see if you need to take more precise measures – it’s wonderful. Simple. Accessible.

    Don’t hark on it for not showing millimetres. Pick up your tape measure when you need it.

    1. The issue is that people are generally too concerned with weight generally, and weight as a proxy for body shape specifically. 

      Take, for example, someone who is both overweight and sedentary. An exercise regimen is unlikely to have a major impact on his weight, but it will have a massive effect on his health. A focus on calorie restriction will have a much better impact on his weight, but even people of normal weight will suffer adverse health effects from being sedentary.

      1. We could suggest that The Rock knows that he is, in fact, The Rock, and isn’t wondering if his sedentary lifestyle has got out of hand … and so BMI isn’t for him.

  13. I’ve been morbidly obese my whole life.   While that is unhealthy, there are also time when I’m what I call “Fit” my weight will be down a little but I will also have the stamina and health to run circles around most people.  

    This has always surprised people.  

  14. In the early 90s they already had skin caliper estimates of body fat percentage, and a few years later they devised other ways of measuring the same thing. That’s a much more useful measure than BMI.
    Then… some epidemiologists devised BMI as a measure because the hoary dataset they were working with had only height and weight to work with. Well, statistically and academically, it worked fine for them. Still does, you can show rough correlations for BMI with all sorts of stuff.
    And finally, because medical science is prone to fads, everyone decided that this new measure was the greatest thing since sliced bread and started using it, even though they had superior measures already available.
    Just shows that people are mostly kind of retarded.

  15. I’ve never understood why it is based on the square of someone’s height, instead of the cube.
    In my understanding the volume of a solid object increases with the cube of any increase in height.

    So, if I am twice as tool as someone in exactly the same proportions as me, I should weigh 8 times as much. Not 4 times as the BMI would suggest.

    Or am I missing something?

  16. Right now, I bet you’re chomping at the bit with, “Yeah! The BMI doesn’t distinguish between fat and muscle!”

    No, in fact, we were champing at the bit.

    But others have been let loose at this point, so I have nothing more to add.

  17. Both psychological and physical health are important. If you exercise simply as a penance to work of your bad lifestyle sins you ain’t doin’ it right and will be prone to manipulation by all sorts of misinformation. You should exercise simply for the enjpyment of it. Like anything else really. The problem is how to get from penance to enjoyment. The psychology of the ‘fitness industry’ is seriously whack, based as it is on good ol’ Christian guilt.

  18. I had to sit in the Bod-Pod and have my numbers crunched for my work: one point below obese!  Woo-hoo!

    And, yup, those six drinks a day have to go somewhere.  But my bp is below average.  I can plank, pull ups, push ups etcetera at an above average ‘Mens Fitness’ level.  I run up hundreds of stairs regularly…

    If I wasn’t such a lush I’d really be quite the physical specimen.

    Oh well: at least there is room for improvement.  But that BMI junk can take a flying leap:

    As my Cartman briefs proclaim: I’M NOT FAT I’M BIG BONED!

  19. BMI also doesn’t take into account breasts. I have large breasts which I imagine add at least 5-7 lbs to my weight..where is that accounted for?  My BMI goes from “overweight” to “normal” if those 5 lbs are eliminated.

  20. BMI and in fact simply using wieght as a measurement of fitness is idiotic. This is why if people want fitness tips from me, the first one is always “throw out your scale” it doesn’t help.

  21. Is it wrong that I misread the first line as “The Body Mass Index is a popular way to measure asses”?

  22. There was an article on BBC yesterday about how exercising three minutes per week in a certain way was absolutely, positively, scientifically proven to be just as good as exercising two hours per day.  I can’t find it today, so maybe someone who’s not crazy saw it and pulled it.

    1. There are some recent studies showing that brief intense exercise is healthier than commonly believed.  Go out and run a few minutes of sprints, rather than miles, etc.

  23. Yup, at 6’2″, and 137lbs, I don’t even appear on some BMI charts. I eat everything, and lots of it. I don’t exercise. No soda. I gain about 2 lbs during thanksgiving, but it comes off a couple days later. Strange!

    1. I’m 6′-2″ and used to weigh from 135 to 143 for about 20 years. People kept asking me how long I had to live. But it was a perfectly healthy weight for me. I ate constantly, but I never gained weight.

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