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OK, now I really don’t believe BMI

One online calculator seems to have me at BMI of 24.8. Which is absurd – I’ve got a proper gut on me and feel bloated at times.

I would be fit at 5 to 8 kg less.

Which, give it a couple of months and I will be – moving, more exercise in the new place etc.

But it does make me think that BMI is a pretty feeble measure.

21 thoughts on “OK, now I really don’t believe BMI”

  1. Bloke in the Fourth Reich

    You have to measure different things based on the available technology. It’s neither possible nor desirable to measure everyone’s fat and muscle mass every month.

    BMI is just a ratio, with some broad brush upper and lower limits of desirable, and as with all medical measurements, “normal” is defined by roughly two standard deviations around the mean.

    These things become dangerous in the hands of midwit head-girl types who get safe jobs doing simple rule-based work that involve little cognitive work (and doesn’t even require understanding of what a mean is less a standard deviation), and therefore send out letters to parents when an over- or underdeveloped child exceeds one end of the limit, while ignoring flabby Timmy who is within two standard deviations.

    If you feel fat and unfit you know what to do, regardless of what some measurement tells you.

  2. Just divide your weight by your height and then by your height again, that’s your BMI. If you’re 92kg and 1,83m tall, your BMI is 27.5.

  3. “DocBud

    Just divide your weight by your height and then by your height again, that’s your BMI. ”

    Okey dokey.

    170lbs, 69 inches tall.

    Gives me a BMI of 0.036.

  4. BMI was intended for use in *populations*, not individuals.

    E.g. the Japanese population’s BMI has gone up over time (probably). Why? Well, they eat more now.

    For individuals, at most it answers “are you average?” It has nothing to do with fitness as such.

    The US Navy has or had a fitness grading system for recruits where they measured height, weight, neck and waist size. It sounds a bit more useful. But it would require more work on the part of the doctor, so it isn’t used.

    Just so you know where I’m coming from: BMI of 34. But I’m also able to deadlift 420 pounds. So I’m not particularly “unfit”.

  5. My BMI has been increasing because, following family tradition, I’ve been shrinking. Using my peak height, my BMI is 30. Must be all the cabbage, cauli, broccoli I eat that’s putting on the flab.

  6. In Iraq, one of the Royal Marine officers was nicknamed “Bluto” because he was basically about six foot three both vertically and horizontally, and was strong. I mean, “strong” in the sense that when he walked into the gym and started putting on boxing gloves, the punchbag began sobbing… before he hit it, and it swung back so far it hit the ceiling. I have no idea what he was lifting but it was a lot.

    (I was just watching, impressed, while doing some basic cardio, being of a weedy disposition myself)

    A civilian medical drone tried to write him up as “obese” and require a weight loss plan because he had a BMI of thirty-odd.

    This caused some hilarity, and earned a robust rebuff from his chain of command who pointed out that for as long as Major Bluto was smashing out “outstandings” on his fitness test (because as well as being ungodly strong, he also had all the endurance you’d expect a wearer of the RM green lid to possess) no action need be taken.

    But then, there was a pushback in the Army on “gym culture” and soldiers trying to “get ripped” (nicknamed Op MASSIVE) – fitness and strength are good things, but a lesson of several campaigns has been that when the going gets really tough, the racing snakes and buff bodies with zero body fat go over surprisingly quickly with a combination of exposure, exertion and short rations; while the “rugby player” types with a bit of padding over the muscle, have some reserve to burn through.

  7. I’ve just read that overweights (25.0 to 29.9) live longer than normal weights (18.5 to 24.9), but that the optimal range to be in is 22.5 to 24.9.
    Population level stuff though. And happiness or number of healthy life years not considered.
    How could that be so : J-curve effect, adding a bit of BMI above optimal reduces your longevity slightly, going under the other way reduces your longevity a lot.

  8. BMI is a flawed measure. It compares your weight (or mass) which is proportional to height cubed, with your height squared. For two equally proportioned people of different heights, the taller will have a higher BMI than the shorter.

  9. I just checked mine and I come in at “overweight” at 27.5. That’s a load of bollocks, I do weights 4 times a week, badminton twice a week, can run a 5k Parkrun and at 54 still play the odd game of rugby. Losing enough weight to get to “healthy” would make me what I’d consider skeletal.

  10. Bloke in the Fourth Reich


    The expected volume (thus mass) of a human of certain fatness is actually very close to the square of the height, not the cube. The bits that tend to get proportionally longer (legs) are actually of lower mass per unit area than the bits that don’t, cancelling out any “broadening” effect with increased height. So as it affects expected mass, taller humans do not get proportionally wider.

    Of course, insert usual caveats.

  11. 70kg and 1.77m puts me at a BMI of 22.34 using DocBud’s calculation. So I stray into the territory for premature death due to being underweight.

  12. ISTR, some many years ago, reading that Linford Christie, the 1988 Olympic Gold-Medallist sprinter, was technically “obese” because of his BMI.

    I decided at that point that the concept of BMI was essentially bollocks, and have had no reason since to change my mind.

  13. >”M
    >February 29, 2024 at 1:32 pm
    >. . .
    >The US Navy has or had a fitness grading system for recruits where they measured height, >weight, neck and waist size. It sounds a bit more useful. But it would require more work on >the part of the doctor, so it isn’t used.”

    That system we used (I was in the USN) is still pegged to BMI. They measure your neck and waist circumference, subtract, compared the answer to a list (based on BMI) to get a derived body fat percentage. Then, using BMI, there are arbitrary ‘body fat percentage’ maximums you’re allowed based on sex. Or gender. I don’t know anymore;)

    Its not your actual body fat percentage. It has little bearing to reality – especially on the outer edges of human size. For example, I’m 73 inches (1.85m) – the Navy expected me to weigh 180lbs. Its certainly not impossible – I joined at 72 inches and 133 pounds but I had a sub-5% body fat percentage at 18 and basically no muscle.

  14. As the joke says …
    “BMI. I’m a bit overweight, but as long as I weigh less than a small plane I’ll be fine”

  15. Agammamon:

    6’1″ and 133 pounds sounds like a famine victim. Even 180 pounds sounds fairly underweight. Perhaps it’s okay if you have a desk job, but not if you’re lifting anything.

  16. My weight these days is pretty constant and my BMI is 24 exactly.

    I am 5’10 and a bit these days (180cm) where I used to be 5’11 1/2. I lost masses of weight (between 20 lbs and 2 stone ) when I was poorly and felt awful. I gained it back as my health improved, it is up to what I consider my natural level and I feel great.

    I think BMI personally is tosh, but the lazy land whales who inhabit my local surgery seem to believe in it.

  17. Frank H beat me to it – BMI is dimensionally unsound.
    @ BiFR – No, you have misread or misremembered. The *original* version was weight/height^2.5 because tall people are less than proportionally broad. Various lazy people found that/felt that calculating any height (apart from 144cm, 169cm, 196cm) to the power of 2.5 required too much effort and/or intelligence and chose to use weight/height^2 instead.
    As several people have mentioned this is intended for populations not individuals, but even then the choice of ^2 instead of ^2.5 makes it an utter nonsense

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