17 votes

People with low BMI aren't more active, they are just less hungry and 'run hotter'

12 comments

  1. mtset
    Link
    From Cell Metabolism.

    From Cell Metabolism.

    Scientists looked at individuals with a very low BMI. Their findings reveal that these individuals are actually considerably less active than people with a BMI in the normal range, contrary to speculation that they have a metabolism that makes them naturally more active. Additionally, they eat less food than those with a normal BMI.

    9 votes
  2. [2]
    Jakobeha
    Link
    Anecdotally, people who are very fit usually have a normal or even overweight BMI, because they have muscle and muscle has weight. People who have a low BMI are usually the "don't exercise eat...

    Their findings reveal that these individuals are actually considerably less active than people with a BMI in the normal range, contrary to speculation that they have a metabolism that makes them naturally more active. Additionally, they eat less food than those with a normal BMI.

    Anecdotally, people who are very fit usually have a normal or even overweight BMI, because they have muscle and muscle has weight. People who have a low BMI are usually the "don't exercise eat less" type, so they have low fat but also low muscle mass. Sometimes these people also have health issues which make them struggle to eat or get nutrients from food.

    Many forms of exercise build muscle. You don't get an underweight BMI by exercising too much, you get it by eating too little.

    Fit != skinny. Fit also != obese or high bodyfat. It's a shame because people really, really don't know much about nutrition and health. BMI obese is basically always unhealthy (even if you're a low-BF% bodybuilder), carrying "excess" fat or low muscle mass is unhealthy, but the thin models in bodybuilding magazines and fashion shows are also unhealthy.

    8 votes
    1. hhh
      Link Parent
      i would argue carrying "excess" muscle is both effectively impossible for natural lifters, and even for geared lifters it's not that bad. the life expectancy for a professional bodybuilder is...

      carrying "excess" fat or low muscle mass is unhealthy, but the thin models in bodybuilding magazines and fashion shows are also unhealthy

      i would argue carrying "excess" muscle is both effectively impossible for natural lifters, and even for geared lifters it's not that bad. the life expectancy for a professional bodybuilder is still about 75---the same as for the average US man---despite injecting grams upon grams of anabolics and peptides for years on end (along with the stresses of competing).

      your heart adapts to the extra stress of lifting weights and carrying extra muscle. iirc there is usually benign hypertrophy of the left ventricle wall (cavity size stays the same unlike in pathological LVH) and a slight drop in heart rate/blood pressure, and combined with a higher insulin sensitivity due to muscles acting like a glucose sink means even a lean 200lb natural monster probably isn't negatively affected by having that much muscle.

      if anything the increased bone density, metabolic markers, and mobility afforded by having a higher baseline muscle (and buffer for when sarcopenia sets in) means most people would be well served by getting as big as possible when in their youth (in a healthy and sustainable way of course)

      4 votes
  3. [9]
    stu2b50
    (edited )
    Link
    I'm a bit surprised at their surprise - maybe this is an expectation that health experts have but laymen would not? This study was comparing a group with a BMI in the "underweight" category (so,...

    "We expected to find that these people are really active and to have high activity metabolic rates matched by high food intakes,"

    I'm a bit surprised at their surprise - maybe this is an expectation that health experts have but laymen would not? This study was comparing a group with a BMI in the "underweight" category (so, below 18.5) with the "healthy weight[0]" category. Of course, BMI is two numbers divided by each other so certainly it is not a perfect representation of anything as with any particular metric, but I would imagine for most configurations of humans that I, at least, would not imagine people with a <18.5 BMI to be "highly active" + "high consumption". I would think those would be in 18.5-24.9. In general, for the modal human, high exercise will push you towards that 18.5-24.9 range, from both directions for different reasons.

    Not just from the fact that muscle tissue is slightly denser than fat tissue, but also that more muscles does correlate with more physical ability. Highly active people would usually have some kind of goal for which the athleticism is the point - you want to lift more weight, run longer, play soccer, and so forth. With <18.5 for most configurations of humans, your lack of muscle mass will hamper the physical feats you can do, thereby causing you to naturally intake more calories as you physically strain yourself, develop more muscles, and increase your weight.

    [0] How NHS describes the categories, https://www.nhs.uk/common-health-questions/lifestyle/what-is-the-body-mass-index-bmi

    6 votes
    1. [7]
      NoblePath
      Link Parent
      I’ll pile on and take it up a notch. Bmi is not a good measure of health. Muscle is much denser than fat according to your source, which was my understanding. So any degree of athleticism...

      I’ll pile on and take it up a notch. Bmi is not a good measure of health.

      Muscle is much denser than fat according to your source, which was my understanding. So any degree of athleticism increases bmi inappropriately. Plus, it ignores genetics of folks who naturally have more tissue or thicker frames.

      Waist to height ratio might be a better measure, or the one where they measure the width under your arm. Olds like me will remember “pinch an inch”.

      10 votes
      1. [6]
        Rez
        Link Parent
        BMI has its place - I feel like there’s overall too much pushback against the metric in recent years. For most people, their BMI will be a good-enough reflection of their situation, because it’s a...

        BMI has its place - I feel like there’s overall too much pushback against the metric in recent years. For most people, their BMI will be a good-enough reflection of their situation, because it’s a population level statistic, so it will apply for most people. If you’re an exception, you already know it by virtue of being 6’6” or a bodybuilder working out most days of the week or a marathon runner. Most people are not an exception. If your BMI puts you at a barely overweight category, then yeah you can mention mitigating circumstances, but if your BMI is over 30 (obese) then you almost certainly do have some fat to lose, even if you run some every week or hit the gym 2-3x a week. Over a third of Americans are obese.

        19 votes
        1. NoblePath
          Link Parent
          I guess i’m sore because my life insurance is higher because i have a high bmi, but am very fit (I ride my bike a lot). Others like me ger adverse decisions and there’s no way to show we lie...

          I guess i’m sore because my life insurance is higher because i have a high bmi, but am very fit (I ride my bike a lot). Others like me ger adverse decisions and there’s no way to show we lie outside the norms.

          7 votes
        2. [4]
          NaraVara
          Link Parent
          For most people BMI will not. It was designed as a population level health metric, not an individual one.

          For most people BMI will not. It was designed as a population level health metric, not an individual one.

          7 votes
          1. [3]
            hhh
            Link Parent
            yes, but most complicating factors --- height, muscularity, bone density --- fall along a normalish distribution, so BMI is accurate for most people still

            yes, but most complicating factors --- height, muscularity, bone density --- fall along a normalish distribution, so BMI is accurate for most people still

            9 votes
            1. [2]
              NaraVara
              Link Parent
              Not really. Just picking on height, for example, only about 50% of the population will be in the fat part of the curve (less than 1 standard deviation from the mean). Another half of all people...

              Not really. Just picking on height, for example, only about 50% of the population will be in the fat part of the curve (less than 1 standard deviation from the mean). Another half of all people will be at least 1 or more SDs above or below it. Then you add the other measures and it’ll be surprising if you’ve got more than 40% of people for whom it’s reliable since bodies can vary along so many dimensions.

              3 votes
              1. Greg
                Link Parent
                You piqued my interest on how significant those outliers might actually be, and I found this study (N=13,601): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2877506/ It looks like at the top end...

                You piqued my interest on how significant those outliers might actually be, and I found this study (N=13,601): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2877506/

                It looks like at the top end (BMI >= 30) it's incredibly likely that a given person is indeed obese (95% in men and 99% in women), but that actually significantly under-predicts the obesity levels in the cohort as measured by body fat: "BMI-defined obesity (≥ 30 kg/m2) was present in 21% of men and 31% of women, while BF %-defined obesity was present in 50% and 62%, respectively.".

                At the intermediate overweight BMI range (25-30) it seems a lot fuzzier - basically a toss up between overpredicting lean mass and underpredicting very high body fat in a way that drags both closer to an average that doesn't quite fit either.

                Seems like it's a decent first-pass filter: a BMI of under 20 or over 30 is sufficient to suggest health problems on its own. Within the 20-30 range it's less reliable, and 25-30 particularly suggests that a further follow up measure would be valuable.

                1 vote
    2. vektor
      Link Parent
      Yeah, there's precious few sports that routinely produce very skinny athletes. Endurance running I think? But beyond that, yeah, nah. Honestly, I initially would attribute the quote you're looking...

      Yeah, there's precious few sports that routinely produce very skinny athletes. Endurance running I think? But beyond that, yeah, nah.

      Honestly, I initially would attribute the quote you're looking at to mediocre academic writing. Motivations and introduction of studies are something people often write really uninspiredly. Like, you'll see the most generic of phrases to explain why XYZ phenomenon is interesting to study, that kind of thing. However, the quote is from a presumably english-as-first-language professor...

      2 votes