Comment by strongpigeon
1 day ago
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).
> 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.
At a population level, it's great. For individuals, it can fail in all kinds of ways. For me: it is not very good because my body has very strange proportions. I'm 6'2", but my waist is barely higher than my 5'1" wife, and my torso is nearly the same length as my 6'7" brother.
In other words, for my height, I have a very long torso and very short legs. I think it should be relatively obvious that an inch of leg weighs significantly less than an inch of torso, so at a given level of body fat percentage, I'm going to weigh quite a bit more than someone my same height with more typical proportions, and thus my BMI reads me as more overweight than it otherwise would.
My point is not that BMI is bad or useless or anything else. My point is that it was designed as a population statistic and that it can be fraught when one tries to apply it to any individual with no nuance. A high BMI should cause one to consider and examine your health and weight. But it should not over-ride specific details about your physiology that point in the other direction.
It's actually still useful. It doesn't function as well as a measure of fatness for you as an individual, but that was only ever a proxy anyway.
It still functions well as a measure of load on your circulatory system, joints, you name it.
Your body is working harder than a typical 6'2" person, even if you have a great waist-to-hip ratio.
(But you'd still be better off with a smaller WTH ratio!)
Do you have a concrete example, though? Like an actual body fat percentage measurement that conflicts with what BMI explains for you?
I was also in the "overweight" category at one point while being low body fat. It was never a problem for anything, anywhere. I wasn't going to doctors who looked only at the BMI sheet and then told me to lose weight, because everyone could see that I was not overweight.
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The serious backwoods rock climbing cohort calls that "goon coefficient". I (also) have a high goon coefficient.
> screening tool
People tend to think in absolutes. Even perfectly rational people fall logically foul to not considering outliers.
> I’ve been in the “overweight” BMI range with low body fat before. It’s not too hard to get there
Did you do that as a natural lifter? It’s hard for me to imagine most guys being able to get into the “overweight” range at < 15% body fat without some assistance.
Yes, sorry, I meant as someone lifting weights and working on it deliberately.
For someone not working out or working a job that builds muscle, getting into the overweight category probably does mean they should take a look at their body weight.
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.
What percentage of any random western country population do you think has reached three plates on squat, and therefore has to consider this?
IIRC, the farther you are from an "ideal" 19th century Frenchman, the less BMI says about you.
Yeah, people don't want to understand that your body structure messing with BMI is quite real but only a few points. It's unquestionably "wrong" in scoring me higher than my wife despite the fact that she's the one with a few extra pounds. But it scores us both as normal, the difference doesn't matter.
Anything to pretend they aren't too heavy.
>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 sumo wrestlers.
The line seems to be drawn at (effortful) activity level more than anything else. And BMI doesn't say anything about that.
I think the vexation comes from the focus on an attribute that is not directly mutable, per one's agency, because that's easier to sell things related to. Versus action that you have a lot more direct control over (social or physical circumstances notwithstanding).
> The lifestyle has a negative effect on their health, with sumo wrestlers having a much lower life expectancy than the average Japanese man.
(from https://en.wikipedia.org/w/index.php?title=Sumo&oldid=136533...), so I'm not sure this is a counterexample.
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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)
> Many active gym people have pretty high BMIs but fairly low fat
I think this is greatly exhaggerated.
Yeah you can put on as much muscle as Arnold and have a FFMI of about 27 then maybe have a BMI of 34 if you have 20% body fat.
But a very gifted natty might cap out at maybe 24 with a BMI of 30 at 20% bodyfat. But let's be realistic. This is not "many active gym people". This is the guy winning the local strongman meet.
Yes a couple of years training for most people can add a couple of points to BMI but I think people exaggerate how much this is. Go look at a 20lb brisket at costco - you don't see people with that much extra muscle all that often.
I do agree to some extent, but really the overweight threshold for BMI is 25. And 30 for obese.
At 6 feet / 183 cm that it takes 184 lbs / 83 kg to be overweight. From what I could find, for regular gymgoers the typical weight for people around 6feet is 180-190lbs, which put many people around the overweight threshold. I am myself at 24.9, and while not skinny I am definitely in the skinnier half of the people at my local gym.
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I very much disagree. Overweight starts at 25 BMI, and obese at 30. Most regular gym goers in my social circles maintain <20% body fat, and are classified as overweight based on BMI. In the US weightlifting is quite popular, and it’s not hard to gain enough muscle mass to classify as overweight. I’ve been “overweight” at 27 BMI going on >10 years now, with actual body fat fluctuating ~12-18%. I’ve had doctors tell me I need to lose weight because my BMI is too high, while I had visible abs. It’s a common theme among other casual weightlifters I know. I don’t think BMI is particularly useful for anyone that has done more than a year of basic barbell strength training cycle.
My completely made up rule is you get an extra point for every plate you bench (or squat etc). Its not a dramatic difference. If you bench 225 you’re not overwreight till 27 BMI, which iirc used to be the actual cutoff until they narrowed it.
Obese is hard. Overweight is easy if you walk, take the stairs, do the occasional pushup, and play something like hockey once week. Source: my entire life.
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> Many active gym people have pretty high BMIs but fairly low fat (because muscle is dense),
I’ve been in the overweight BMI category with low body fat by being active at the gym and focusing on diet. It takes some work to get there.
If I check the calculators for the obese BMI range, there was no way I could get there without either gaining a lot of fat on top of the muscle. The amount of muscle required to have an obese BMI with healthy amount of fat is absurd.
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.
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.
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Do you have a citation for that? I've had thoughts in a similar direction (specifically around the upper/lower intervention points) but have had no luck finding data.
Its not just absolute amount of fat though you also have to account for intra abdominal fat being much worse metabolically than subcutaneous fat.
Does that have to do with sizes of the heart and few other organs apparently being largely constant regardless of height?
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?
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.
If bmi says you're obese, you very likely (~95%) are. The problem is that bmi only catches about half of people who are obese.
https://www.nature.com/articles/s41598-020-69498-7
I haven't been down in the normal BMI range since 8th grade. And I was measured at 6% body fat my senior year in high school! BMI has always run several points high for me. For that reason, I've always liked waist to height ratio better.
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How about other categories, in either direction? I seem to remember it to be a lot less clean cut at the normal/overweight boundary.
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.
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.
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.
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.
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.
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.
If that's true, you're claiming that it only accurately measures about 27% of the population yet somehow returns 66% accuracy.
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.
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.
Genuine question: What is the difference between overfat and overweight? Isn’t the extra weight in an overweight person comprised of fat?
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.
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.
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Not necessarily, a bodybuilder or very athletic person might be overweight in the sense of above average for their height but not overfat.
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.
What is the line for "overfat" in this framework?
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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.
"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".
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.