Thoughts on alternative formulations of BMI (adjusted for frame size, sex, height, etc)?

Zodikosis
Zodikosis Posts: 149 Member
in Health and Weight Loss
My friend and I were recently discussing the BMI system and how we've never really fit in with its predictions for us. For the record, I'm a 5'1" female and he's a 6'4" male, so we're on two opposite ends of the height spectrum. Back when I was 120 lbs, I was a US size 0 and had 17% body fat. I have another 5'1" friend who, at the time, was the same size as me, probably a similar BF %, but was 105 lbs at the time. Clearly, the same weight looks different on different people of the same height. I assume this is due to frame size, as well as many other factors.

However, I was always prodded by my family doctor to try to get to somewhere in the range of 100 lbs to 110 lbs, since at 120 - 125 lbs I was "at risk" for slipping into the overweight zone according to her. Furthermore, there's also been talk in the medical community for the last 20 years about how the BMI scale may be a bit too generous for people on the shorter end of the height scale, and that shorter people may be fatter than they think. This, in addition to existing discussions about just how useful the BMI scale in assessing an individual vs. assessing a population, lead me to explore alternative formulations of the BMI scale -- partially out of curiosity, partially because I'm a numbers freak and my personal data keeps me motivated, so I like to try to get my data as accurate as possible (I realize that, given the current state of research regarding nutrition and weight, I'm mostly groping in the dark for a solid answer at this point, but humor me).

I came across this calculator after our conversation and found that the results predicted for me (5'1", 6" wrist circumference) made a lot of sense given my past experience. However, since I'm not a medical researcher or a health professional, I find it hard to tell which formulations are just woo to make us feel good and which ones might have a more solid basis, so I wanted to open this discussion up to the community. What's your take on BMI taking into account frame size? Any other interesting adjusted BMI formulations you want to bring into the spotlight?
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Replies

  • Lounmoun
    Lounmoun Posts: 8,423 Member
    Based on my own experience of being a non-athletic 5'4" woman weighing everything between 100-180 lbs as an adult my judgement is that the bmi range is a good guide. My frame size is probably medium and I felt my best in the middle to the top of the bmi range. So I guess that makes sense.
    As clothing size is also not consistantly applied across brands or over time it is a terrible way to compare size. I know there are people the exact same weight wearing different clothing sizes.
  • VintageFeline
    VintageFeline Posts: 6,771 Member
    I think BMI is good enough for most. Your doctor likely just didn't know/care about taking into account your BF% as that's where the real risk assessment should come from.
  • Zodikosis
    Zodikosis Posts: 149 Member
    edited February 2018
    @Lounmoun You're right, I probably should've specified it in a US sewing size or measurements, which is standardized. I was 2 (Misses' size 2011) at the time, about 34-26-36 (inches).

    @Packerjohn You're right that it's not needed, but it's my twisted idea of fun to discuss. :)
  • lemurcat12
    lemurcat12 Posts: 30,886 Member
    edited February 2018
    I think BMI as formulated does take into account frame size, that's why such a range.

    I also think that anything based just on height and weight is inherently imperfect as a way to determine if an individual is overweight or underweight -- bodyfat is what really matters for that, but it's hard to determine accurately without a more expensive test than most want or need.

    Good judgment (and visual observation) plus other measures (waist to height, waist to hip) and general fitness/health should come into play, and IME they normally do, although I know others have had bad experiences.

    Re: "However, I was always prodded by my family doctor to try to get to somewhere in the range of 100 lbs to 110 lbs, since at 120 - 125 lbs I was "at risk" for slipping into the overweight zone according to her." This seems really silly given the other information given and the fact that 120 is about 22.7 BMI, so perfectly within the normal range -- the higher end of the normal range is FOR people with bigger frames. (Also, being a bit over 24 or 25 is not actually a big health risk, or at least not if you aren't also dealing with excess body fat, which can be a risk at 23 or 24 too.)

    Basically, I don't see a way to have an improved scale based just on height and weight and I think giving everyone a DEXA would be cool but is not medically necessary or justifiable. So given that I think BMI plus the other measures is fine.
  • rybo
    rybo Posts: 5,424 Member
    BMI is just one marker that statistically accounts for a significant portion of the population. Yes there are outliers, but not as many as people would like to think. And yes it does show muscular people to be overweight, or close to it, despite having a lower body fat %. Like others have said take all the other factors into account and decide from there. It has a pretty large range to account for frame sizes and builds.
  • JoRocka
    JoRocka Posts: 17,525 Member
    Packerjohn wrote: »
    There is a large range in the categories to take frame sizes and differences in muscle mass for the vast majority of people. No need IMO to complicate it.

    Look at BMI, bodyfat% and height to waist measurement. Then look at yourself naked in a mirror. For most people all 5 will correlate.

    SHUSH its' Friday- and I have nothing to do. Let's have the conversation.
  • JoRocka
    JoRocka Posts: 17,525 Member
    Although- I have to agree with @Packerjohn if you sort of take all the makers into consideration- you're likely to get damn close.

    I'm chronically overweight by that standard. I DO feel that I'm overweight for my size- I could stand to lose some weight- but I don't feel I'm remotely close to obese.
  • combsshan
    combsshan Posts: 47 Member
    I think most of us can benefit from trying to stay inside the BMI range, but there are always exceptions. My best friends daughter is a gymnast and is nothing but solid muscle, not a 1/2 inch of fat on her anywhere, but her BMI says she is on the borderline of obese for her height. It really upset her when she found that out (she's only 14 and you know how sensitive you are at that age), but her doctor told her to ignore it. That BMI was meant for people of normal activity levels. She's in the gym training 4 hours a day 5 days a week and in meets on the weekends so she, by no means, gets a "normal" amount of activity. However, very few of us are that active, so I still think BMI is a good goal.
  • bogiepower
    bogiepower Posts: 2 Member
    I am 5'5" 170 and considered overweight to obese by the bmi calcs. Laughable

    I am now 9% body fat. (Up from 7%)
  • MarkusDarwath
    MarkusDarwath Posts: 393 Member
    Johnd2000 wrote: »
    An internationally renowned scientist once told me that there is no significant mortality risk increase until BMI 30-35. His view was that BMI 25 is unnecessarily low as bar for “overweight” and is counterproductive.

    The 25 came about when WHO got access to the numbers from several nations that they didn't previously have. Several of those, most notably Japan, were primarily genetically homogeneous cultures that tend toward slighter builds. Those averages really should not have been combined. For the Japanese, BMI 25 is already in the elevated risk zone. For the rest of us, there's a slight (but statistically significant) risk increase between BMI 27 and 30, but the major uptick doesn't hit until 30 and above. The whole "overweight" category really is counterproductive as it's actuallly borderline obese for typical Asians, but pretty meaningless for everyone else.

    For individual assessment, body fat percentage is the superior choice. Yes, the at-home methods of determining BF% (Navy composition formula, skin-fold calipers, BIA, etc.) are prone to error, but even the typical error margin on those methods provides a more accurate picture than BMI.



  • MarkusDarwath
    MarkusDarwath Posts: 393 Member
    amusedmonkey wrote: »
    the whole "BMI is not accurate" argument is based on a very small athletic group,

    This is the common assertion, but it is simply not true. BMI does not scale accurately with height, such that tall people, particularly those with large frames, are more likely to be above BMI 25 at a healthy body fat percentage, and they absolutely do not have to be very muscular or athletic for this to be the case.
    On the other end, you have short people, especially those of small frame size, who show up as underweight by BMI when they get toward the lower half of healthy BF%, without having to be in the athletic range.

    Then there's also the inactive "skinny fat" people, especially among the elderly, who are fine according to BMI yet are at a higher risk BF%.

    The group of people for whom BMI is a non-productive assessment is far larger than the sub-set of "outliers" claimed by BMI proponents.

  • amusedmonkey
    amusedmonkey Posts: 10,330 Member
    MarkusDarwath wrote: »
    amusedmonkey wrote: »
    the whole "BMI is not accurate" argument is based on a very small athletic group,

    This is the common assertion, but it is simply not true. BMI does not scale accurately with height, such that tall people, particularly those with large frames, are more likely to be above BMI 25 at a healthy body fat percentage, and they absolutely do not have to be very muscular or athletic for this to be the case.
    On the other end, you have short people, especially those of small frame size, who show up as underweight by BMI when they get toward the lower half of healthy BF%, without having to be in the athletic range.

    Then there's also the inactive "skinny fat" people, especially among the elderly, who are fine according to BMI yet are at a higher risk BF%.

    The group of people for whom BMI is a non-productive assessment is far larger than the sub-set of "outliers" claimed by BMI proponents.

    What other measure that can be done reliably within seconds do you propose that would weed out the outliers?
  • BitofaState
    BitofaState Posts: 75 Member
    BMI was never intended to be applied to individuals, only to populations and the formula was developed in a time before electronic calculators so had to be "simple".

    What it shows in population studies is that the mortality curve for the population as a whole bottoms out around a BMI of 24, this is where the WHO derive their recommendations for populations, that is the focus on either issues around malnutrition in a population or weight management.

    There is some evidence that other methodologies such as waist to hip ratios are better predictors for individuals, but that has some issues with measurement repeatability and reliability.

    It seems that most folks try and use the "BMI isn't very good argument" to justify being heavy. Problem is that the data says BMI is poor at identifying those at risk who are in the overweight category, but carry obese levels of BF, missing 64% of men and 51% of women.

    A study of the NHANES III data with 13,601 subjects showed that BMI defined obesity was present in 21% of men and 31% of women, but BF% defined obesity (>25% in men and >35% in women) was present in 50% of men and 61% of women. So for most american's there is a high risk of carrying excessive BF without tripping the BMI trigger (and for a small number of of around 5% of men and 1% of women with a BMI of >30 of identifying obesity where it isn't present)

    https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2877506/

    Now this also gets into the BF% measurement issues (they used Bioelectrical impedance analysis), the distinction between visceral and subcutaneous fat and how we get these highly precise, but not necessarily very accurate measures of BF% from the various techniques available. Of course the problem with BF measurement is that the only way to validate the results of any test is to then sacrifice the subject and perform an ex-vivo separation of adipose tissue and whilst I'm really interested in measurement of BF% I can't see myself volunteering for any such study, can you?

    Easily available techniques such as skin fold and Bioelectric impedance have significant repeatability and reproducibilty issues. BIA actually measures FFM and then determines FM by subtracting that from total body weight. It is highly influenced by water content and the electrolytes contained therein, stomach contents and skin impedance variations, so getting a consistent reading is problematic, let alone being able to correlate that back to actual BF%. Of the "body scan" techniques avaliabel (CT, DEX, Air displacement & Ultrasound being the others) it's actually the poorest at predicting visceral adipose tissue.

    https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3473928/

    Skin fold only measures sub-cutaneous fat, not the visceral fat and the correlation between these is poor. The measurement process is highly variable depending on both the person performing the measurement, the person being measured and the specific selection of measurement sites. Throw in the variability of density in SAT and it's getting close to think of a number.

    Finally throw in individual genetics and the response to body fat. We've all met the skinny fat folks, those with low subcutaneous fat, but a belly stretched like a drum who's BMI will be normal, BF highish, but OK, but who's visceral fat will be well into the danger zone.

    TL:DR - there is no good measure that works for individuals, any anyway there is no universal right answer for all of us.
  • Tacklewasher
    Tacklewasher Posts: 7,122 Member
    amusedmonkey wrote: »
    MarkusDarwath wrote: »
    amusedmonkey wrote: »
    the whole "BMI is not accurate" argument is based on a very small athletic group,

    This is the common assertion, but it is simply not true. BMI does not scale accurately with height, such that tall people, particularly those with large frames, are more likely to be above BMI 25 at a healthy body fat percentage, and they absolutely do not have to be very muscular or athletic for this to be the case.
    On the other end, you have short people, especially those of small frame size, who show up as underweight by BMI when they get toward the lower half of healthy BF%, without having to be in the athletic range.

    Then there's also the inactive "skinny fat" people, especially among the elderly, who are fine according to BMI yet are at a higher risk BF%.

    The group of people for whom BMI is a non-productive assessment is far larger than the sub-set of "outliers" claimed by BMI proponents.

    What other measure that can be done reliably within seconds do you propose that would weed out the outliers?

    The "New BMI" seems a bit better, as it accounts for height differently.

    https://people.maths.ox.ac.uk/trefethen/bmi_calc.html

    But there isn't one that will work uniformly. Our bodies are just not that homogeneous. The problem comes when insurance providers rely strictly on the BMI when deciding on rates etc.

  • kimny72
    kimny72 Posts: 16,011 Member
    Tacklewasher wrote: »
    amusedmonkey wrote: »
    MarkusDarwath wrote: »
    amusedmonkey wrote: »
    the whole "BMI is not accurate" argument is based on a very small athletic group,

    This is the common assertion, but it is simply not true. BMI does not scale accurately with height, such that tall people, particularly those with large frames, are more likely to be above BMI 25 at a healthy body fat percentage, and they absolutely do not have to be very muscular or athletic for this to be the case.
    On the other end, you have short people, especially those of small frame size, who show up as underweight by BMI when they get toward the lower half of healthy BF%, without having to be in the athletic range.

    Then there's also the inactive "skinny fat" people, especially among the elderly, who are fine according to BMI yet are at a higher risk BF%.

    The group of people for whom BMI is a non-productive assessment is far larger than the sub-set of "outliers" claimed by BMI proponents.

    What other measure that can be done reliably within seconds do you propose that would weed out the outliers?

    The "New BMI" seems a bit better, as it accounts for height differently.

    https://people.maths.ox.ac.uk/trefethen/bmi_calc.html

    But there isn't one that will work uniformly. Our bodies are just not that homogeneous. The problem comes when insurance providers rely strictly on the BMI when deciding on rates etc.

    I think the bolded is the key, which is why I think the logical response is - start with BMI as a general guide and once you get close to a healthy BMI, start adding in other measurements. Like waist measurement, waist to hip ratio, maybe spring for a BF% test if the numbers don't feel right to you.

    Since no indicator tells the 100% whole story, don't throw them all out. Just add them all up, consult your doctor, and come to a consensus.

    And I agree, the fact that the insurance industry has come to rely on BMI stinks.
  • MarkusDarwath
    MarkusDarwath Posts: 393 Member
    amusedmonkey wrote: »
    What other measure that can be done reliably within seconds do you propose that would weed out the outliers?

    Good quality BIA devices are accurate enough to at least place most people in the right general range. So can calipers and the Navy calculation, but those take longer than "seconds" due to the need to take multiple measurements.

    Pretty much any means to analyze actual fat composition is going to be a better health measure than proxy data with no actual composition component.

  • amusedmonkey
    amusedmonkey Posts: 10,330 Member
    edited February 2018
    "MarkusDarwath wrote: »
    "amusedmonkey wrote: »
    What other measure that can be done reliably within seconds do you propose that would weed out the outliers?

    Good quality BIA devices are accurate enough to at least place most people in the right general range. So can calipers and the Navy calculation, but those take longer than "seconds" due to the need to take multiple measurements.

    Pretty much any means to analyze actual fat composition is going to be a better health measure than proxy data with no actual composition component.

    I personally have issues with these methods as they don't tell you how much of that fat is visceral. In fact in some cases with calipers its better to have a higher body fat than a lower percentage with visceral fat since can't grab your visceral fat in calipers, and BIA doesn't tell the whole story. Short waisted people also get the short end of the stick in calculations that rely on waist circumference because they can be structurally thicker in that area without necessarily being fat. See? No one measure is perfect and all simple methods have more outliers than more expensive methods. I do like the modified BMI equation above. The difference for tall people isn't pronounced, but shorter people get a more fair estimation. As for insurance and BMI, yeah, that's too simplistic if they really do want to get a risk assessment based on weight, but that's probably a cost benefit kind of thing for them. I think that's the reason people are against BMI but not other methods that also have shortcomings, because it's actually used to determine important things in real life so whether a person is a true outlier, delusional, or simply wants to be overweight because they have assessed their risk as low, it's frustrating when they are pushed externally into not making choice.
  • MarkusDarwath
    MarkusDarwath Posts: 393 Member
    kimny72 wrote: »
    I think the bolded is the key, which is why I think the logical response is - start with BMI as a general guide and once you get close to a healthy BMI, start adding in other measurements.

    The issue with that approach is, how do you define "close"? According to the BMI charts, I hit 25 at 189 pounds (I'm presently obese by any measure, no arguing against that). But according to my BIA scale and the Navy formula, which both also happen to line up well with BMI at my current size, my Lean Body Mass is around 173 pounds. To get under a 25 BMI, I'd have to either drop to less than 10% BF or shed a significant amount of lean mass. I'm trying to avoid loss of lean mass. I can say pretty confidently that while I'm pretty strong, I'm not carrying an overly large amount of muscle in comparison to my height and frame that would qualify me as a genuine outlier.
    At my present LBM, a max healthy BF% of 25 would put me at 230#. 15% would be 204#. So again, if I were using BMI as a starting point, how close do I have to get to the chart range before I start examining actual body composition? 15 pounds, 40 pounds?

  • MarkusDarwath
    MarkusDarwath Posts: 393 Member
    amusedmonkey wrote: »
    I personally have issues with these methods as they don't tell you how much of that fat is visceral.

    True, though BMI doesn't even tell you if it's fat at all. Visceral fat does have some pretty specific indicators, like a large, round, yet taught belly. (I have much visceral fat. I need to lose probably another 10 pounds to get my blood sugar and cholesterol in line, then quite a lot after that to get into an actual healthy range.)

  • kimny72
    kimny72 Posts: 16,011 Member
    MarkusDarwath wrote: »
    kimny72 wrote: »
    I think the bolded is the key, which is why I think the logical response is - start with BMI as a general guide and once you get close to a healthy BMI, start adding in other measurements.

    The issue with that approach is, how do you define "close"? According to the BMI charts, I hit 25 at 189 pounds (I'm presently obese by any measure, no arguing against that). But according to my BIA scale and the Navy formula, which both also happen to line up well with BMI at my current size, my Lean Body Mass is around 173 pounds. To get under a 25 BMI, I'd have to either drop to less than 10% BF or shed a significant amount of lean mass. I'm trying to avoid loss of lean mass. I can say pretty confidently that while I'm pretty strong, I'm not carrying an overly large amount of muscle in comparison to my height and frame that would qualify me as a genuine outlier.
    At my present LBM, a max healthy BF% of 25 would put me at 230#. 15% would be 204#. So again, if I were using BMI as a starting point, how close do I have to get to the chart range before I start examining actual body composition? 15 pounds, 40 pounds?

    I have no data to back this up, but my gut is telling me that if you would need to get down to 10% BF to get into the healthy BMI range, you are a way out there outlier. And the fact that it doesn't work for extreme outliers doesn't make it useless for everyone. But I could be wrong.

    And I've never heard anyone say BIA devices are accurate for anything, just possibly useful to determine a general trend.

    I haven't done nearly the homework you have though!
  • TR0berts
    TR0berts Posts: 7,739 Member
    MarkusDarwath wrote: »
    amusedmonkey wrote: »
    What other measure that can be done reliably within seconds do you propose that would weed out the outliers?

    Good quality BIA devices are accurate enough to at least place most people in the right general range. So can calipers and the Navy calculation, but those take longer than "seconds" due to the need to take multiple measurements.

    Pretty much any means to analyze actual fat composition is going to be a better health measure than proxy data with no actual composition component.

    The Navy calc puts me at 13.8% BF. I'm around 20%. So, that's not a particularly good one to go by.

    Even "good quality BIA devices" are affected by various factors, so those aren't particularly good, either.
  • MarkusDarwath
    MarkusDarwath Posts: 393 Member
    kimny72 wrote: »
    I have no data to back this up, but my gut is telling me that if you would need to get down to 10% BF to get into the healthy BMI range, you are a way out there outlier.

    I'm 6'1" and broad shouldered/large frame (my fingers don't meet when wrapped around the opposite wrist). If I weren't fat I'd be built like a viking. I'm definitely above average for overall size, but my musculature in comparison to my build is nothing extraordinary. Usually, when someone suggests that only 'outliers' don't conform to BMI, they are talking about people who are highly athletic and approaching body-builder degrees of muscle mass. But the truth is that just being genetically large can put a person in that supposed 'outlier' range without possessing extraordinary musculature, because the math formula doesn't scale correctly for height.
    And again, this math flaw doesn't present an issue in population studies, because in those cases one is looking at the average for a large group. People at either end of the bell curve for size who will be flagged inappropriately as either underweight or overweight will effectively cancel each other out and not throw off the average.
  • MarkusDarwath
    MarkusDarwath Posts: 393 Member
    TR0berts wrote: »
    The Navy calc puts me at 13.8% BF. I'm around 20%. So, that's not a particularly good one to go by.

    Even "good quality BIA devices" are affected by various factors, so those aren't particularly good, either.

    Just curious, you're around 20% as measured by....?

    I find it interesting that people who support using BMI vs BF% don't wring their hands over their bathroom scale possibly being 3 to 5 pounds off, yet if one suggests tracking body fat then suddenly a similar margin of error is unacceptable.

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