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Is the BMI figure unfair to tall people?

Is the BMI figure unfair to tall people? Take 2 people of exactly the same proportions but one twice as tall as the other. They will also be twice as wide and twice as deep so 8 times the weight. As BMI is based on height squared it implies that taller person is fat, even though they both have exactly the same proportions. Why?
Ian Simpson from Argyll & Bute (Age 45-54)

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12 Responses

  1. BMI is based on weight / height squared (kg/m2) and infact the taller you are for a fixed weight the lower it should be.

    Cut off points for overweight in adults for overweight is a BMI of 25 to 30, and for obese 30+. This is suprising not that much. Less than a couple of stone over ideal weight for shorter people can push them from normal to obese.

    The people that BMI measurement really mis-judges are atheletes and body builders who have extra muscle mass and low body fat mass. BMI does not differentiate between these and takes no acount of body composition.

    The critical aspect of adiposity is how much body fat we have and where it is located. Abdominal obesity, carrying fat on our waist line, is most associated with risk of cardio-metabolic disease (diabetes, heart problems ect). If you are apple shaped rather than pear shaped you should probably seek advise and get a check up.

    The amount of body fat we have can be measured by bio-electrical impedance and whole body canning can reveal where it is located. But combining BMI with a measurment of waist circumference is fine. Men should definately be below 40 inches and ideally below 37.

    Remember most modern trousers sit on the hips not on the waist, so if you jeans are a 34 inch waist your true waist size is probably nearer 38.

    Jason Halford – Liverpool Obesity Research Network (LORN) http://www.liv.ac.uk/obesity

  2. With all due respect, Jason Halford’s first sentence, while correct, may 1) indicate he missed what I perceive to be the point of the query (that does not actually specify unfairness assumed to be some high-skewing inaccuracy in the classification of one’s relative weight); 2) show that he simply didn’t follow the logic of his introduction with more contemplation based on observation of experience, 3) demonstrate he simply may not have had an opportunity to observe or analyze a sufficiently large sample of tall people of varying degrees of fitness or 4) simply indicate he was thinking about shorter tall people than me (i.e., those around 6’0, not the 6’5-or-above height I perceive as “tall”). He does correctly cite athletes and body-builders as the prime examples of how BMI does not account for body composition, but I think his apparent dismissal of height’s effect on BMI should be re-considered.

    I’m just a social science PhD candidate, but, as a 6’8″ man (2.03 meters), I’m confident that a BMI based on weight/height squared penalizes most tall people by over-stating our potential “over-weight-ness” — particularly for those tall men who possess big bones. Most of the masses on earth grow in cubed dimensions, but the human body doesn’t. So basing BMI on cubes is wrong, but squares “favor” those in the middle of the bell curve (try the calculation at another extreme of less than 5’0 for various weights and it doesn’t make complete sense either). My own informal surveys, empirical observations and logical train of thought suggest there should be some compensation for extreme height when one uses the BMI to make health judgments as insurance companies in the U.S. often do. Based on judgment, one might apply an exponent of 2.2 or 2.3. vs 2.0, perhaps, but some proper research should be done, if only because its application in determining life insurance premiums has significant financial implications. Moreover, an “unfairly bad” BMI reading scares the less-informed giants among us.

    I am currently 214 pounds, about the ideal weight for me in terms of many medical measures. I’m in the normal range, but barely (23.5) and I’m very fit right now. If I gained a dozen pounds (easy to do if I stop working out daily) — or just gain five pounds and weigh myself at night — I’d be overweight per BMI. I (and, more importantly, my wife) feel I look increasingly, and unhealthily, gaunt when I drop down towards 200. After extensive diet and exercise two years ago, friends who saw me less often confessed they feared I had cancer when I weighed 205 or so. Even I saw myself as scarily cadaverous (many bones showing through my skin) when I bottomed out at 197 and loosened my regimen a bit. The nearly-dead-from-cancer look I had at that 197-pound weight represented a 21.6 BMI that placed me dead center in the range of “normal”. And I actually have a thinner frame and lighter bones than many fellow tall men. I know several reasonably fit, but big-boned, wider people who are over 6’5″ – nut supposedly overweight based on BMI.

    As for the Mr. Halford’s 40″ waist measurement that has, I’m sure, been reliably correlated with heart problems in men 4’-to-6’4″ or so, I also question its applicability to those of us over three S.D.’s from the population norm. I happen to have a 38″ waist size (pants) when I’m very fit, but I’ve flirted with 40″ without being fat in anyone’s eyes. When joking about the validity of BMI and 40″ for tall guys, I discovered that all but one of my fit but tall, big-boned and wider friends have waist sizes 40″ or more. I’d ask Mr. Halford what he would conclude if a man with 42″ waist appeared to have minimal fat. I could show him at least two men like that who regularly play in our gym’s nightly pick-up basketball games.

    All tall people’s dimensions are usually big, so why not the waist? I personally have a 38″ inseam and 38″ arms, not to mention a hat size of 8 (all never found in stores). In fact, I think a 38″ inch waist may be nature’s attempt at symmetry, though I’ve never checked with shorter folks on the correlation between their own arm and leg measurements and their waist. (Of course, my dimensions indicate an overall symmetry not found in short-legged, “long-torsoed”, long-armed, big-handed and -footed superstar Olympic swimmer Michael Phelps, for instance. At 6’4″, he’s less subject to BMI skew, but his asymmetry and muscularity would probably place this man in peak condition near the cut-off — though I know Mr. Halford would probably speculate the same.)

    Rather than dismiss this Big Question so cursorily, I believe it is yet another opportunity to point out the potential weakness of any “one-number” measurement that seek to embrace the great diversity of humankind. If one knows nothing else about the male human being measured, judgments based on the one-number BMI and the 40″-plus waist need be issued with a few caveats, especially when applied to groups whose population incidence puts them at the extremes (e.g., my two-meter-plus-height cohort)

    P.S. – As opposed to BMI or waist size, I like my 74-year-old doctor’s unscientific “rib test” for whether one is overweight. If you can see the definition of your bottom ribs on each side of your torso, you’re fine.

  3. I agree with Dan and think the height factor should be much closer to cubic then squared. I really don’t see why it should be exactly cubic because that’s the way weight is related to height.
    I am 196 cm with a weight of 95 kg. Accordingly, my BMI is 25, a bit at the high end. However, when I measure bodyfat with a calliper I’m at a pretty low 11% bodyfat and I’m not muscular at all.
    BMI is just not very scientific and severly under and over estimate obesity values for short and tall people.

  4. I think Dan it is better to critise what has been said rather than was we never said

  5. Like Dan Merrick, I, too am 6′ 8″ tall. I have a 40.5″ waist and am of an athletic build. I currently weigh 290 lbs with a weight loss goal of 15lbs (to take me to 275). At 275 I was in great shape, had a 38″ waist. I have a large frame with broad (54″) shoulders. Every BMI I look at shows me near mortal obesity. I find this laughable and question the validity of such an index without a BIG asterisk (i.e. “This applies to people who are between 4’8″ and 6’0″ in height”)

    I share the same inseam and sleeve length (38″) but not hat size (7.5 fits just fine.)

    I think that this BMI calculation should include a disclaimer. I can’t imagine how most of the basketball players would fare. My gosh, Shack would be on a death bed (at 7’1″ and 310 LBS he would be somewhere around 36!)

    I wonder how all this will play out with the public option of Obama Care?

  6. I’ve published a couple of papers regarding obesity without giving the BMI calculation so much as a second thought. I know for individuals there can be issues regarding body composition, but for population studies I and others assume it to be OK.

    I work out a good deal and am in good shape, but I’ve gained 5 pounds and now I am just over the overweight line, yet I am not IMHO. My body fat is probably in the high teens/low twenties. So now I find myself interested by this notion that BMI may be unfair to tall people such as myself – I am also 6’8″.

  7. I am 6’4″ and not broad shouldered or long armed. You couldn’t tell I’m tall from a photo of me – I’m what you’d call normally proportioned.. I recently lost 10 lbs to get down to 200 which just gets me under a BMI of 25. Everyone I’ve seen since the minor weight loss has asked if I’m sick because I look a bit gaunt. That proves to me that BMI is pretty useless for the over six footers.

    Please, BMI advocates, don’t get defensive about it. The BMI was never meant to be anything other than a handy rule of thumb, so don’t sweat it. Just be aware that BMI fits best for 5’4″ – 5’11” men (ie 90% of men)

  8. I believe that the answer to this question may be found at the website I have provided below, which has a BMI calculator for tall people.

    Although athletes and tall people may be somewhat unfairly judged by the standard BMI calculation, even this adjusted scale will find most people too heavy.

    Athletes who are very muscular and tall people with a very high BMI (over 25) are still vulnerable to higher health risks. The more mass, the harder your body has to work. Think dogs….small ones live longer, big ones die sooner.

    So, if you are on the skinnier side, you willl probably live longer.

    I am 6’4″ and weigh 175 pounds. I run alot.

    http://inside.mines.edu/~gmurray/BMIApplet/BMIApplet.html

  9. I haven’t found anything that supports skinny people living longer than muscular, fit people. I have found a great deal to do with genetics, metabolism and just dumb luck. I’m [still] 6’8″ but now 295 (went up instead of down but my body fat % dropped significantly). I’m still a 40″ waist and now 56″ across the chest. My arms are currently 19″. I have more strength and energy than I had in my 30’s (i’m 50 now). Maybe I wont’ live to be 80 but I’ll be OK with that as long as I feel this good until the last day. Still morbidly obese according the the BMI.

  10. Ken, I’m really unimpressed with the science behind your applet. Looks like you plucked the numbers out of thin air and made it fit an arbritary zero point. I’m surprised your college allows you to put crap like that on their website.

  11. Thanks for the great discussion and interesting points raised. I am 1.94cm and have recently gone from 109 kg to 104 kg. This has been a great achievement and everyone comments on how good I look. Problem is, I’m still ‘BMI’ overweight and will be until I reach 94kg. Impossible! Even stranger than someone of my height is supposedly healthy at 75kg! The last time I was 75 kg I was about 13 years old!

  12. BMI for tall people is lacking some serious science. I am 6’6+ and I weigh 230 pounds. I have semi-chicken legs. I am an avid rower and cyclist and my body fat percentage through a water displacement test is just over 10%.

    My BMI is 26.7 at last test. HMMMMM

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