Why the Body Mass Index (BMI) is a Poor Measure of Your Health

If you go to your physician’s office and inquire about your weight status, he or she will measure your height and weight to derive your BMI (weight in kg divided by height in m squared). Then they will compare your BMI to that of established criteria to decide whether you are underweight (<18.5 kg/m2), normal weight (18.5-24.9 kg/m2), overweight (25-29.9 kg/m2), or obese (>30 kg/m2) . Often times, this measure alone determines whether or not you receive lifestyle treatment.

But how useful is this measure anyways? What does it tell you about your health? And finally, how helpful is it to measure when assessing the effect of a lifestyle (diet/exercise) intervention?

Before I get into the various limitations of BMI, I must point out that the measure is quite useful across large populations, as it is well correlated with the degree of adiposity, and of course it is extremely simple to measure in clinical practice.

Nevertheless, here are some of the key issues with BMI, particularly when used on an individual basis.

1. BMI does not differentiate between the Michelin Man and The Terminator

Ok, we might as well just get this abundantly obvious problem out of the way. I have heard countless times how one buff celebrity or another (e.g. Tom Cruise, Arnold Schwarzenegger, The Rock etc.) would be classified as overweight or obese according to their BMI due to their excess amount of muscle. Yes, this is absolutely true. BMI is a measure of relative weight; fat mass and muscle mass are not distinguished. Here’s what is equally true: the large majority of the general population with a BMI in the overweight or obese range does not look like Jerry Maguire or the Terminator. Also, if you seek advice from your physician about your “excess weight”, in case you have body dysmorphia and cannot yourself decide, they will quickly be able to assess whether your excess weight is due to your bulging muscles or your rolls of adipose tissue. So while this is an obvious problem, I would argue not the main issue.

2. BMI does not differentiate between apples and pears

For over 60 years, we have known that independent of how heavy a person is, the distribution of their body weight, or more generally the shape of their body is a key predictor of health risk. It is now well established that individuals who deposit much of their body weight around their midsection, the so called apple-shaped, are at much greater risk of disease and early mortality in contrast to the so called pear-shaped, who carry their weight more peripherally, particularly in the lower body. Thus, two individuals with a BMI of 32 kg/m2 could have drastically different body shapes, and thus varying risk of disease and early mortality.

Fortunately, a very simple measure allows you or your physician to decide whether your elevated BMI is of the apple or pear variety: waist circumference. Current thresholds suggest that a waist circumference above 88 cm in women and 102cm in men denotes abdominal obesity. Interestingly, for the same BMI level, those individuals with an elevated waist circumference have a greater risk of diabetes, cardiovascular disease, mortality, and numerous other health outcomes. Thus, as studies from our laboratory have consistently suggested, waist circumference may be a more important measure of obesity and health risk than BMI. Currently, most researchers would agree that waist circumference should be measured along with BMI to adequately classify obesity-related health risk.

You can measure your own waist circumference by using a tape measure and wrapping it around your abdomen, at the level of the top of your hip bones. Make sure you measure at the end of exhalation, without sucking in your gut – you’re only fooling yourself!

3. BMI does not always budge in response to lifestyle change

Given the number of papers my supervisor, Dr. Ross, and I have published on the topic, I would argue this is the biggest drawback of using BMI: it doesn’t always change even though you may be getting healthier. 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.

First, it is well established that increasing physical activity and associated improvement in cardiorespiratory fitness are associated with profound reductions in coronary heart disease and related mortality independent of weight or BMI. Second, exercise (even a single session) is associated with substantial reduction in several cardiometabolic risk factors (such as blood pressure, glucose tolerance, blood lipids, etc.) despite minimal or no change in body weight. Third, waist circumference and abdominal fat (arguably, the most dangerous fat) can be substantively reduced (10-20%) in response to exercise with minimal or no weight loss. In fact, significant reductions in fat mass often occur concurrent with equal increases in muscle mass in response to physical activity – equal but opposite (and beneficial!) changes which are not detected by alterations in body weight on the bathroom scale, and thus BMI.

So in the end, while BMI surely has its strengths in ease of use and pretty good reliability in large populations, on an individual basis, the greater focus should be on healthy behaviors: physical activity and a healthy diet. And if you must measure something, check your waist circumference.

Enjoyed this post? Please sign up for future updates on similar topics to be delivered to your email inbox or RSS reader.

Peter

Reference:

Ross R, & Janiszewski PM (2008). Is weight loss the optimal target for obesity-related cardiovascular disease risk reduction? The Canadian journal of cardiology, 24 Suppl D PMID: 18787733

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38 Responses to Why the Body Mass Index (BMI) is a Poor Measure of Your Health

  1. steve says:

    All of the notes I’ve seen on waist circumference give two numbers – one for women and one for men. How does this vary as a function of height? What is the danger level for a 6’7″ woman or a 5’2″ man?

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    • Hi Steve,

      Yes the numbers for WC cutoffs differ between men and women – many people don’t know how these were values were developed. Sadly, these values were extrapolated from the BMI’s of a large sample of men and women. At a BMI of 30, the average waist circumference in men was 102cm and in women 88cm. In terms of height, a few studies have actually found that yes, height does have an impact on health risk. Specifically, for a given waist circumference shorter individuals appear to be at bigger risk of disease. Come to think of it – this could be an interesting post on Obesity Panacea. Thanks for the tip!

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      • Natalie Sera says:

        Seems to me that waist circumference is ALSO a blunt tool. All I have to do is look at myself. I’m female, 5’3″ (160 cm) and my waist circumference is 39 in. (99 cm), and yes, I have T1 diabetes. BUT I have had 2 angiograms, due to having had coronary artery spasms, the last one being when I was 63, and they were both normal. I do not have any other obesity related diseases either — I do not show insulin resistance, although my total daily dose is on the greater side of average (about 35u a day), I have no arthritis, and I can easily do yoga poses on the mat. So I’m suggesting that perhaps a large waist does not necessarily indicate large amounts of visceral fat — maybe it’s subcutaneous and not a cause for worry. I know that the ways of distinguishing subcutaneous from visceral fat are expensive and probably not going to be used for evaluation of an individual’s risk in most cases, but again, the physician should be more observant and thoughtful when doing this evaluation.

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  2. How many Terminators are out there? Look around you. I’d say a max of one in a hundred men.

    All the Terminators already know the BMI doesn’t apply to them.

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    • Hi Doctor -

      certainly, the Terminators know that.

      Many others believe they are Terminators, too! (frequently in a silly display of over-estimation), which you may encounter in your practice!

      And since it doesn’t apply in a black-and-white way, it’s intended or populations, not individuals, the public jumps on it, the media misrepresents it, and hier we are, in this pickle where patient-generated risk factors are of significant concern in the primary care arena!

      The BMI has a bad reputation, mainly from patients misunderstanding its value in evaluating populations, oftentimes from not wanting to face up to the fact that they’re overweight and must do something themselves to reverse that, and how it’s frequently reported in patient discharge summaries, etc.

      It can be tracked – very easily, but how about considering not using it with the patient. Use it as a research metric, not anything else.

      respectfully,
      njw

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  3. Stephen says:

    For a first level diagnosis, BMI does alright. It’s quick and cheap.

    Ten years ago, i had a stress test. I got on the treadmill, the doctor set the speed. A few minutes later, he came back, noticed that not much had happened to my breathing or heart rate, and cranked up the speed. Minutes later, he did that again. Then he came in and told me that i was in pretty good shape.

    Today, my Dunlop’s disease is about the same as it always was. (My belly dun lopped over my belt.) I can still run three or four miles (5-6 km) comfortably. But though i look pretty much the same, i doubt that i’m in the shape i was ten years ago. I can’t keep up with the dog anymore.

    And that’s the great predictor of future outcomes.

    Treating a heart attack is treating the last few minutes of a decade plus long disease. We’d be better off starting a decade earlier.

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  4. Thanks, this was a very helpful article. I took away a couple of useful points,

    First if we can add a few simple criteria to the BMI to make it more useful, that’s great news. Given our semingly nearly infinite capacity for rationalization, I’m sure a lot of us need a more objective way of forcing us to realize we are not the Terminator when we come up with a large BMI, and to distinguish our shape in ways that we might not be able to do for ourselves objectively.

    Second, I agree it would helpful to have health providers tell us things that make us healthier regardless of weight control, and I don’t see them as mutually exclusive. Knowing what we should be focusing on is important in any long term effort. If the only target we know about or focus on predominantly is weight, the results seem to be significantly less than optimal in most cases. Taking weight control as a secondary goal on the other hand, may be helpful to keep the obesity situation in our mind, yet focus first on the things we can do something about more effectively. Exercise and nutrition goals can be determined with scales and measuring tapes being relegated to a secondary role since those measures can change very slowly and sometimes don’t change at all even while we are successful at becoming healthier.

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  6. Dan says:

    Thanks for the informative article. I am starting to work on a obesity prevention project with children and wondered if ‘waist circumference’ could also be a good measure for children as well.

    Also if it’s possible could you provide a few citations for these types of article? They’d be helpful those who’d like to do some further reading.

    Thanks again!

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    • Dan says:

      Sorry – didn’t notice you did provide a reference at the bottom of the post. Thanks!

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  7. Bear says:

    There’s a more subtle problem with the numbers – it gives short people a false sense of confidence and tall people a lot of pointless grief from anyone who uses flat cutoffs.

    The problem is that weight goes by a cube (following the square-cube law) while BMI only uses the height squared. That means that somebody who’s 10% taller than another person will also have a 10% higher BMI, all other things being equal, since their weight goes up by 30% but their height squared only goes up by 20%.

    I don’t know the statistics for height but 5% for an average guy who’s 5’8″ tall (I think) is only 6″, or about 6’2″. A lot of men are at least that tall, esp. younger men. That means that, all other things being equal, the same build will give you about a 24 BMI at 5’3, a 25 BMI at 5’8″, and a 26 BMI at 6’1″. The shorter guy wrongly thinks he’s well under the cutoff and the taller guy wrongly thinks he’s well above the cutoff.

    This doesn’t matter in population statistics but it’s a real pain when insurance companies or employers use blind cutoffs for things like preferential insurance rates, etc.

    This also affects motivation. I know I’m obese but when my doctor tells me what I should weigh according to my BMI I just laugh. But a slightly higher BMI in line with the adjustment I mentioned above and I can say that, yeah, that sounds like a good goal. That 6 kg or so is the difference between a scrawny college freshman and a healthy and active man in his 20s.

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  8. Phil D says:

    I am 6’2 (1.87m) and weigh 116 lb (98 kg)
    I have a 36 inch (91.5 cm) waist and a 44 inch (112 cm) chest
    I am 53 years old and run about 10 miles (16km) per week and push a few weights to stay in shape I have moderate 16 inch (40cm) biceps and have a nascent six pack, which you can see through the layer of fat when I tense my muscles.
    My cholesterol is 5,0 mml and my blood pressure is 120/80.
    I think I am doing fine,
    But… apparently, I have a BMI of 28.2 and am overweight, nearer obese than normal.
    According to the figures, I need to lose 22 lb (10kg) yes 22 POUNDS, before I can be classed as normal.
    I am no Arnold Schwarzenegger, but I am definitely NOT obese, nor even overweight.
    10 years ago I actually achieved my top end acceptable BMI when I was very ill, and that is exactly what I looked like… VERY ILL.
    Recently my new doctor told me my BMI is too high and I should lose weight. When I protested he actually LOOKED at me and said, well you do seem to have a lot of muscle, maybe you can carry a bit more weight.
    Either BMI is relevant or it isn’t! My ex-wife is in the bottom end of the BMI limit, but she is CLEARLY emaciated and lacking in energy.
    Doctors! Please, I EMPLORE you stop believing in arbitrary numbers and limits and treat the person in front of you.
    BMI may work for the statistically normal, but two minutes examination will tell you much more than 2 minutes calculation.
    STOP treating us by NUMBERS!!!!

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  12. Adriano Petrich says:

    There’s another factor. BMI does not work well with over tall people as it goes quadratic to height instead of cubic if you are tall as I am 6’10″ (2.08 m) in order to have a ok BMI i would have to be anorexic.

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  13. CJohn says:

    Trivia: BMI itself goes back 160 years, when it was defined by Belgian Adolphe Quetelet, who provided mathematical evaluations of medical risks to insurance companies. It has remained largely unchanged since then. 

    http://m.ndt.oxfordjournals.org/content/23/1/47.full

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  14. Peter says:

    Wikipedia actually has a fairly good article on the limitations of BMI. As stated, it is good for evaluating a population, not so much for individuals. It deals properly only with averages and “There *are* no average people”.
    With respect to the comments about tall vs small; mass varies as the cube of height – not the square BUT tall people are not just scaled up short people. There is also a difference in body shape. This means that neither height^2 nor height^3 work correctly.

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  15. kjb says:

    I would also like to include the difference between musculature…there are endomorphs, ectomorphs and mesomorphs. Mesomorphs naturally have more muscle in their build. I think mesomorphs probably tip into the ‘obese’ BMI a lot more easily, even though they are in fine shape.

    I even had this problem in the military. I was in incredibly good shape, but because I am a tall (5’10″) mesomorph woman, my weight would consistently be 2 pounds over the weight limits…and then I would have to go to the embarrassment of being tape measured. Only to find out that my measurements were just fine and I was not ‘overweight.’ Meanwhile, I would see plenty of short (5’0″ to 5’4″) tubs of lard who would somehow pass the weight standards! It was ridiculous.

    I would like to see weight standards for different musculature types. No matter how much I diet, I cannot get rid of my heavily muscled thighs and glutes. Just won’t happen.

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  17. Laura says:

    There is a simple way around this whole BMI debacle, discovered by biological anthropologists around 30 years ago. Use *sitting* height, not standing height, and you do away with many of those body build variation isssues, since for any given torso height, a leg weighs about the same, whether long and skinny or short and stout. Yes, some of us who are tall mesomorphs really ARE “scaled up short people” (ie have a long-torso with comparatively short or medium- length legs), not built like supermodels (ie, relatively short-torsoed people with really long legs). I think the article was published in the American Journal of Physical Anthropology in the early-to-mid 1980s.

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  22. Martin B. Brilliant says:

    It’s easy enough to find out where BMI came from, and what’s wrong with using it to estimate an individual person’s health, but: where did the cutoff points for healthy and overweight, etc., come from? What are they based on?

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    • Martin B. Brilliant says:

      Specifically, two questions:

      1. Who set the magic cutoff numbers at 18, 25 and 30, and why? Did they really think people live longer with BMI below 25 than above it?

      2. Now that we know people live longer with BMI above 25 than below it, are we correcting a mistake, or has something changed?

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      • Martin B. Brilliant says:

        I think I’ve found the source of the classification: “Physical status: the use and interpretation of anthropometry.” Report of a WHO Expert Committee. WHO Technical Report Series 854. Geneva: World Health Organization, 1995. Accessible at http://whqlibdoc.who.int/trs/WHO_TRS_854.pdf

        Chapter 7, on “Overweight Adults,” says: “For adults, the Expert Committee proposed classification of BMI with the cut-off points 25, 30, and 40 for the three degrees of overweight…. This classification is based principally on the association between BMI and mortality….

        “It has been widely concluded that the relationship between BMI and mortality us U-shaped or J-shaped…. Minimum mortality for white men who were 50 years of age at entry and were followed for 30 years was at BMI between 24 and 25 … ” and the plot actually shows higher mortality at 19 (low normal) than at 30 (borderline obese).

        Why then would they call the BMI range of 18.5 to 25 “normal”? Because their focus was on overweight. Individuals in that range would not be targeted for weight reduction because weight reduction would increase their mortality. In that sense they would be classified as “normal.” Only when BMI exceeds 25 would weight reduction be desirable.

        In the light of that history, it’s no surprise that newer studies have “rediscovered” the relationship between body weight and mortality, and found that “normal” weight people, as a group, have greater mortality than “overweight” people.

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