8 ms·

I thought this was pretty well known already. Being “overfat” is the problem, not being overweight (though they’re often correlated). BMI is really easy to meas

I thought this was pretty well known already. Being “overfat” is the problem, not being overweight (though they’re often correlated). BMI is really easy to measure, and is mostly accurate, that’s why it’s so pervasive. However it remains a pretty rudimentary metric (and really should use the third power or your height instead of the second).

kennyadam 1mo ago
Genuine question: What is the difference between overfat and overweight? Isn’t the extra weight in an overweight person comprised of fat?
- piva00 1mo ago
  Not necessarily, a bodybuilder or very athletic person might be overweight in the sense of above average for their height but not overfat.
- arximboldi 1mo ago
  A muscular person can be "overweight" by BMI standards but not overfat at all. This is the case for many types of athletes, specially in strength disciplines but not only. It is the case in the fitness world that people ignore BMI and are intesterested in body-fat-percentage instead. This is hard to measure accurately so not so useful metric for the general population.
  - wjholden 1mo ago
    It's also possible to be overfat without being overweight! These "skinny fat" people often look normal enough but they carry very little muscle. It is a concerning condition because a doctor might not recommend a normal-weight person to hit the weights. I like to think of this as four quadrants around two axes. Low fat/low muscle is simply skinny. High fat/high muscle is the "big guy/gal" look that I associate with laborers. Low fat/high muscle is an athletic look; unhealthy in extremes (bodybuilding) but generally desirable. High fat/low muscle is skinny fat, which I associate with sedentary knowledge workers.
    - wbl 1mo ago
      Different athletes look different. A runner vs a Olympic powerlifter vs a quarterback.
  - deleted 1mo ago
    [deleted]
- hansvm 1mo ago
  The dominant discrepancies are "expected" fat (boobs, etc), and "unexpected" muscle (cyclists, body builders, hard labour careers, etc). I'm currently around 10-15lb overfat and 40-50lb overweight [0]. Only the former statistic matters. [0] And that, at least somewhat, tracks visual perceptions. Nobody looks at me with a shirt on and believes I need to lose weight, because 10-15lb isn't _that_ much extra on a tall frame. If I were 40-50lb overfat then that would be painfully obvious regardless.
  - sanderjd 1mo ago
    What is the line for "overfat" in this framework?
    - hansvm 1mo ago
      In my mind, it's equivalent to subtracting a coarse estimate of how much extra muscle by mass I have compared to the median adult male they're using to construct BMI charts. My actual methodology is pretty crude though -- when I was 15lbs lighter I had borderline visible abs and other visual indicators of being around the 15-20% body fat mark, and aside from gaining weight I don't have any reason to believe I've had any notable muscular atrophy since then, so I'm estimating the actual excess fat in that ballpark. It's be easy for numbers to be off 10lb or more, but even then it'd still indicate the overfat surplus being much less than the overweight surplus. A better methodology uses calipers and various skin measurements to estimate true body fat percentage.
      - sanderjd 28d ago
        Right, but what is the line of body fat percentage that you would consider "over fat", that's what I was curious about. I think it's implicit in your comment that it is maybe 20%.
        hansvm 28d ago
        Yep, that's right. Denominated in some fixed weight and any fixed fat percentage the delta is the same (e.g., if my weight is exactly 150lb or 200lb or 250lb and, at some concrete weight, I evaluate my real BMI at X and my naive BMI at Y, then the delta Y-X is independent of my target 20%. But my target is to waffle around 20% or so on average.
- sn9 1mo ago
  "Overfat" just refers to having too much fat. "Overweight" can mean the same thing for weight, but it also refers to those with a BMI specifically between 25 and 30, and those with BMIs greater than 30 are classified as "obese".
- zdragnar 1mo ago
  I've been considered borderline or obese by the standard BMI index for 20 years at least, but for a lot of that had very little to no visible extra weight. A lot of that was just plain having more muscle than the average the standards are built around, but I also suspect BMI gets to be really inaccurate for tall people. I would look anorexic if I had to drop enough weight to be considered not overweight.
AStrangeMorrow 1mo ago
Yes, it is a pretty high level metric with good correlation a bunch of diseases, but really many of these is because it is ALSO correlated to percent of fat, which is often the more relevant metric. But as you said BMI is so much simpler to measure. Many active gym people have pretty high BMIs but fairly low fat (because muscle is dense), and unsurprisingly have better outcomes than the average person (if you ignore the share that uses/overuses anabolic steroids and co)
- adam_arthur 1mo ago
  I believe evidence points towards absolute level of fat being more important than % for overall health. E.g. 20% bodyfat at 250lbs is still a lot of fat. Of course it's difficult to ever get very high on absolute fat if at 15% or below.
  - mathgeek 1mo ago
    Not sure I follow the logic here, but happy to be proven wrong if I'm misunderstanding. It's not intuitive to me that a 300 lb. person at 20% body fat is generally at the same risk as a 200 lb. person at 30% body fat.
    - adam_arthur 1mo ago
      Fat is inherently unhealthy for you beyond some low baseline level. Additional muscle is positive for health, but only up to some reasonable threshold. There is no health benefit to having very high levels of muscle, and in fact it may be negative for your health at extreme levels. E.g. many bodybuilders have trouble breathing, sleep apnea etc. Both people in the example have 60lbs of fat. 1lb of muscle doesn't cancel out the negative health effect of 1lb of fat
      - sarchertech 1mo ago
        Do short people have much lower risk of heart disease?
ColdStream 1mo ago
Yeah BMI is sort of a 'good enough' technique but also has some very obvious areas that it can miss. If you fall outside the typical fat to muscle ratio is a good example.
- XorNot 1mo ago
  Which for the majority of the population isn't relevant. The people most obsessed with "BMI isn't accurate" are overweight people making excuses for themselves.
folkrav 1mo ago
How accurate is it, in practice, for a given individual? I'm not that out of the ordinary proportion wise. I have a slightly longer torso and arms versus my legs, a somewhat muscular-ish baseline and broad shoulders, but I accumulate fat almost exclusively abdominally. My teenage self, lifetime peak of my fitness, no visible body fat, hyperactive football player, qualified as solidly overweight. If I was to listen to it, I'd be called obese before I noticeably start to show body fat. I often wonder far from the median I am in this regard. I was under the impression that it was pretty accurate for assessing populations, but fell apart very quickly at the individual level. How many "normal"/otherwise healthy people do fall outside BMI's numbers?
- kevin_thibedeau 1mo ago
  It isn't accurate. It is a statistical tool meant for easily measuring populations of people. It does not apply in any meaningful way to individuals.
  - folkrav 1mo ago
    That was my impression as well. I'm just wondering if there are actual numbers to gauge by how much. I'd find it almost concerning anyone would draw any conclusions from it if it's not significantly any better at predicting health outcomes than a coin toss.
- conception 1mo ago
  It’s a tool not the tool. If you have obese BMI you can… look in the mirror and see how accurate it is. If you aren’t sure take one of the dozens of other tests. People focus too much on it being a one stop shop - it’s not. It’s the start of a dialogue with your pcp and self on assessing your health that’s highly correlated with poor health outcomes.
- plorkyeran 1mo ago
  It's not really intended to be applied to individuals at all, but if you're in the "healthy" BMI range then you are probably fine just not thinking any further about your weight and doing something like measuring your body fat percentage would just be a waste of time and a source of stress for some people. If you're outside that range, then it is worth doing some of the more involved things to judge if your weight is healthy.
Aurornis 1mo ago
> BMI is really easy to measure, and is mostly accurate, that’s why it’s so pervasive. BMI is easily misunderstood by people who know just enough to see that it’s imperfect, but not enough to understand why it’s still a valuable screening tool. I’ve been in the “overweight” BMI range with low body fat before. It’s not too hard to get there if you’re lifting weights and paying attention to your diet consistently for years, but it takes a lot of work to get there. It doesn’t happen accidentally except for people who win some genetic lottery to build a lot of muscle and keep body fat low without trying. Getting all the way to the obese BMI range while having healthy body fat is only happening for people with an extreme dedication to body building and diet (and let’s be honest, a lot of the people in this category are manipulating hormones too). Yet whenever BMI comes up some people try to dismiss it as too flawed based on these possible edge cases. The edge cases for BMI exist, but that doesn’t mean it’s not useful. It’s a good general purpose screening tool with numbers that are available. If someone has more precise measurements available, those should be used instead. BMI is a really good first pass screener to determine if a closer look should happen.
- sscaryterry 1mo ago
  > screening tool People tend to think in absolutes. Even perfectly rational people fall logically foul to not considering outliers.
- galangalalgol 1mo ago
  The farther from average height you are the worse it is. If you are from some part of the world where the average height distribution is near the point BMI based itself you will perceive it to be so much better. I am only 6' and only an intermediate lifter. I'm overweight, but BMI makes me look obese. It really only takes a normal person a year or so to get to three plates on squat with no supplements other than chicken broccoli and rice if they don't skip workouts. Assuming no injuries. That level of strength easily distorts bmi wildly if you are even a little taller than average.
  - underlipton 1mo ago
    IIRC, the farther you are from an "ideal" 19th century Frenchman, the less BMI says about you.
sn9 1mo ago
But it's actually not just being "overfat". It's specifically a high level of visceral body fat. Genetics determines which parts of the body gain fat first as you gain fat overall, and some people have the unfortunate genetics to gain it first viscerally and some people have the fortune to gain it there last (and everything in between). This means that you can have different people with the same body fat percentage but drastically different disease risks. But yes this was also known already. That's why it's been recommended to take a waist measurement alongside BMI to get a much more informative screening tool. Waist-to-height ratios are another alternative.
diydsp 1mo ago
BMI is at best 66% accurate, so "mostly" is correct, but what is mostly good enough for? BMI has known biases in gender, age, and race. It misclassified Asians, women, elderly w sarcopenia, and people with high body fat to lean tissue ratio.
- Wolfenstein98k 1mo ago
  If that's true, you're claiming that it only accurately measures about 27% of the population yet somehow returns 66% accuracy.
contubernio 1mo ago
Another common misconception is that one is still healthy with high BMI that comes from having lots of muscle. It's not clear that lots of muscle is healthy.
- toasty228 1mo ago
  Very very few people are in that category, and usually they have undiagnosed mental disorders. I don't even think it's possible to be leanish and have a BMI in the obese category from muscles without taking steroids or other products.

kennyadam 1mo ago

Genuine question: What is the difference between overfat and overweight? Isn’t the extra weight in an overweight person comprised of fat?

piva00 1mo ago
Not necessarily, a bodybuilder or very athletic person might be overweight in the sense of above average for their height but not overfat.
arximboldi 1mo ago
A muscular person can be "overweight" by BMI standards but not overfat at all. This is the case for many types of athletes, specially in strength disciplines but not only. It is the case in the fitness world that people ignore BMI and are intesterested in body-fat-percentage instead. This is hard to measure accurately so not so useful metric for the general population.
- wjholden 1mo ago
  It's also possible to be overfat without being overweight! These "skinny fat" people often look normal enough but they carry very little muscle. It is a concerning condition because a doctor might not recommend a normal-weight person to hit the weights. I like to think of this as four quadrants around two axes. Low fat/low muscle is simply skinny. High fat/high muscle is the "big guy/gal" look that I associate with laborers. Low fat/high muscle is an athletic look; unhealthy in extremes (bodybuilding) but generally desirable. High fat/low muscle is skinny fat, which I associate with sedentary knowledge workers.
  - wbl 1mo ago
    Different athletes look different. A runner vs a Olympic powerlifter vs a quarterback.
- deleted 1mo ago
  [deleted]
hansvm 1mo ago
The dominant discrepancies are "expected" fat (boobs, etc), and "unexpected" muscle (cyclists, body builders, hard labour careers, etc). I'm currently around 10-15lb overfat and 40-50lb overweight [0]. Only the former statistic matters. [0] And that, at least somewhat, tracks visual perceptions. Nobody looks at me with a shirt on and believes I need to lose weight, because 10-15lb isn't _that_ much extra on a tall frame. If I were 40-50lb overfat then that would be painfully obvious regardless.
- sanderjd 1mo ago
  What is the line for "overfat" in this framework?
  - hansvm 1mo ago
    In my mind, it's equivalent to subtracting a coarse estimate of how much extra muscle by mass I have compared to the median adult male they're using to construct BMI charts. My actual methodology is pretty crude though -- when I was 15lbs lighter I had borderline visible abs and other visual indicators of being around the 15-20% body fat mark, and aside from gaining weight I don't have any reason to believe I've had any notable muscular atrophy since then, so I'm estimating the actual excess fat in that ballpark. It's be easy for numbers to be off 10lb or more, but even then it'd still indicate the overfat surplus being much less than the overweight surplus. A better methodology uses calipers and various skin measurements to estimate true body fat percentage.
    - sanderjd 28d ago
      Right, but what is the line of body fat percentage that you would consider "over fat", that's what I was curious about. I think it's implicit in your comment that it is maybe 20%.
      - hansvm 28d ago
        Yep, that's right. Denominated in some fixed weight and any fixed fat percentage the delta is the same (e.g., if my weight is exactly 150lb or 200lb or 250lb and, at some concrete weight, I evaluate my real BMI at X and my naive BMI at Y, then the delta Y-X is independent of my target 20%. But my target is to waffle around 20% or so on average.
sn9 1mo ago
"Overfat" just refers to having too much fat. "Overweight" can mean the same thing for weight, but it also refers to those with a BMI specifically between 25 and 30, and those with BMIs greater than 30 are classified as "obese".
zdragnar 1mo ago
I've been considered borderline or obese by the standard BMI index for 20 years at least, but for a lot of that had very little to no visible extra weight. A lot of that was just plain having more muscle than the average the standards are built around, but I also suspect BMI gets to be really inaccurate for tall people. I would look anorexic if I had to drop enough weight to be considered not overweight.

AStrangeMorrow 1mo ago

Yes, it is a pretty high level metric with good correlation a bunch of diseases, but really many of these is because it is ALSO correlated to percent of fat, which is often the more relevant metric. But as you said BMI is so much simpler to measure. Many active gym people have pretty high BMIs but fairly low fat (because muscle is dense), and unsurprisingly have better outcomes than the average person (if you ignore the share that uses/overuses anabolic steroids and co)

adam_arthur 1mo ago
I believe evidence points towards absolute level of fat being more important than % for overall health. E.g. 20% bodyfat at 250lbs is still a lot of fat. Of course it's difficult to ever get very high on absolute fat if at 15% or below.
- mathgeek 1mo ago
  Not sure I follow the logic here, but happy to be proven wrong if I'm misunderstanding. It's not intuitive to me that a 300 lb. person at 20% body fat is generally at the same risk as a 200 lb. person at 30% body fat.
  - adam_arthur 1mo ago
    Fat is inherently unhealthy for you beyond some low baseline level. Additional muscle is positive for health, but only up to some reasonable threshold. There is no health benefit to having very high levels of muscle, and in fact it may be negative for your health at extreme levels. E.g. many bodybuilders have trouble breathing, sleep apnea etc. Both people in the example have 60lbs of fat. 1lb of muscle doesn't cancel out the negative health effect of 1lb of fat
    - sarchertech 1mo ago
      Do short people have much lower risk of heart disease?

ColdStream 1mo ago

Yeah BMI is sort of a 'good enough' technique but also has some very obvious areas that it can miss. If you fall outside the typical fat to muscle ratio is a good example.

XorNot 1mo ago
Which for the majority of the population isn't relevant. The people most obsessed with "BMI isn't accurate" are overweight people making excuses for themselves.

folkrav 1mo ago

How accurate is it, in practice, for a given individual? I'm not that out of the ordinary proportion wise. I have a slightly longer torso and arms versus my legs, a somewhat muscular-ish baseline and broad shoulders, but I accumulate fat almost exclusively abdominally. My teenage self, lifetime peak of my fitness, no visible body fat, hyperactive football player, qualified as solidly overweight. If I was to listen to it, I'd be called obese before I noticeably start to show body fat. I often wonder far from the median I am in this regard. I was under the impression that it was pretty accurate for assessing populations, but fell apart very quickly at the individual level. How many "normal"/otherwise healthy people do fall outside BMI's numbers?

kevin_thibedeau 1mo ago
It isn't accurate. It is a statistical tool meant for easily measuring populations of people. It does not apply in any meaningful way to individuals.
- folkrav 1mo ago
  That was my impression as well. I'm just wondering if there are actual numbers to gauge by how much. I'd find it almost concerning anyone would draw any conclusions from it if it's not significantly any better at predicting health outcomes than a coin toss.
conception 1mo ago
It’s a tool not the tool. If you have obese BMI you can… look in the mirror and see how accurate it is. If you aren’t sure take one of the dozens of other tests. People focus too much on it being a one stop shop - it’s not. It’s the start of a dialogue with your pcp and self on assessing your health that’s highly correlated with poor health outcomes.
plorkyeran 1mo ago
It's not really intended to be applied to individuals at all, but if you're in the "healthy" BMI range then you are probably fine just not thinking any further about your weight and doing something like measuring your body fat percentage would just be a waste of time and a source of stress for some people. If you're outside that range, then it is worth doing some of the more involved things to judge if your weight is healthy.

Aurornis 1mo ago

> BMI is really easy to measure, and is mostly accurate, that’s why it’s so pervasive. BMI is easily misunderstood by people who know just enough to see that it’s imperfect, but not enough to understand why it’s still a valuable screening tool. I’ve been in the “overweight” BMI range with low body fat before. It’s not too hard to get there if you’re lifting weights and paying attention to your diet consistently for years, but it takes a lot of work to get there. It doesn’t happen accidentally except for people who win some genetic lottery to build a lot of muscle and keep body fat low without trying. Getting all the way to the obese BMI range while having healthy body fat is only happening for people with an extreme dedication to body building and diet (and let’s be honest, a lot of the people in this category are manipulating hormones too). Yet whenever BMI comes up some people try to dismiss it as too flawed based on these possible edge cases. The edge cases for BMI exist, but that doesn’t mean it’s not useful. It’s a good general purpose screening tool with numbers that are available. If someone has more precise measurements available, those should be used instead. BMI is a really good first pass screener to determine if a closer look should happen.

sscaryterry 1mo ago
> screening tool People tend to think in absolutes. Even perfectly rational people fall logically foul to not considering outliers.
galangalalgol 1mo ago
The farther from average height you are the worse it is. If you are from some part of the world where the average height distribution is near the point BMI based itself you will perceive it to be so much better. I am only 6' and only an intermediate lifter. I'm overweight, but BMI makes me look obese. It really only takes a normal person a year or so to get to three plates on squat with no supplements other than chicken broccoli and rice if they don't skip workouts. Assuming no injuries. That level of strength easily distorts bmi wildly if you are even a little taller than average.
- underlipton 1mo ago
  IIRC, the farther you are from an "ideal" 19th century Frenchman, the less BMI says about you.

sn9 1mo ago

But it's actually not just being "overfat". It's specifically a high level of visceral body fat. Genetics determines which parts of the body gain fat first as you gain fat overall, and some people have the unfortunate genetics to gain it first viscerally and some people have the fortune to gain it there last (and everything in between). This means that you can have different people with the same body fat percentage but drastically different disease risks. But yes this was also known already. That's why it's been recommended to take a waist measurement alongside BMI to get a much more informative screening tool. Waist-to-height ratios are another alternative.

diydsp 1mo ago

BMI is at best 66% accurate, so "mostly" is correct, but what is mostly good enough for? BMI has known biases in gender, age, and race. It misclassified Asians, women, elderly w sarcopenia, and people with high body fat to lean tissue ratio.

Wolfenstein98k 1mo ago
If that's true, you're claiming that it only accurately measures about 27% of the population yet somehow returns 66% accuracy.

contubernio 1mo ago

Another common misconception is that one is still healthy with high BMI that comes from having lots of muscle. It's not clear that lots of muscle is healthy.

toasty228 1mo ago
Very very few people are in that category, and usually they have undiagnosed mental disorders. I don't even think it's possible to be leanish and have a BMI in the obese category from muscles without taking steroids or other products.