Comment by Aurornis
1 day 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.
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.
Body fat percentage can't conflict with BMI because BMI is not an estimate of body fat percentage.
And I agree that it's never caused me problems, but that's because I and, as you suggest, doctors who look at the number, have understood that it's not that big of a problem in my case. That's what I mean by not looking at the BMI number naively and ignoring other physiological states.
I don't think anything you said conflicts with anything I said, and it sounds like we mostly agree.
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.
Average sumo wrestler bodyfat% is ~25–27%