[Symmetry]

Yes. BMI is your weight divided by height squared. Which is really weird when you consider that humans are 3D objects! So you should naturally expect that a taller person will have a higher BMI than a shorter person even with the exact same proportions. In practice humans who are taller tend to be skinnier so we'd ideally use something close to the 2.5th power but that math would have been impractical for the 19th century clerks who were the first consumers of BMI information.

[sorenjan]

A few years ago I read a paper where they checked the correlation between CT measured body fat percentage and a lot of easily measured and calculated figures, in the same style of BMI (w/h²). I haven't been able to find it again, but if I remember correctly BMI was actually pretty good, although worse for women than for men, and the best measurement included waist circumference.

[scoofy]

Why do we assume that fat is bad for you, but muscle is always good for you. The blood necessary to maintain excessive muscle is still excessive to normal humans, and may put strain on the heart. Only our skin grows with our muscles, other organs do not grow in tandem.

I've always thought that while BMI is crude, it still is reasonable for measuring metabolic syndrome, considering that many of the things associated with heart problems involve the needing a powerful heart, when adding any mass whatsoever will necessarily increase the work the heart must do.

[jhanschoo]

Do you have anecdotes or other evidence of someone experiencing heart issues because they were too muscular?

It seems to me that in the usual case where a person develops muscle mass by working out, this isn't a problem because whatever "blood necessary to maintain excessive muscle" is negligible in the face of the body adapting to improve the cardiovascular system in order to serve muscular oxygen needs during workout. Of course, heart problems due to an unhealthy person ramping up exercise too quickly is a concern, but this is separate from your concern.

One also observes that athletes in general have lower heart rates than the average person; whatever additional burden their muscles impose, their cardiovascular system is way more adapted to handle that due to exercise.

[scoofy]

My argument is that we should not assume that extra mass from muscle does not impose similar health problems from fat. If we are going to say that, it should be demonstrated. A fit person doesn't need to weight 30+lbs more than standard overweight section of the BMI. If they do, they will be a very non-standard human.

Here are some things I found in a brief glance at google scholar. There appears to be a condition called Athlete's heart, but I dunno:

https://pubmed.ncbi.nlm.nih.gov/26187713/

https://www.degruyter.com/document/doi/10.1515/jom-2020-0046...

[jhanschoo]

The first link describes heart problems perhaps arising from intense exercise, which is a different thing that heart problems arising from muscle mass.

The second describes the same thing:

> Several middle-aged and older men trying to improve their physical fitness by weightlifting have presented for repair of severe mitral regurgitation, some of whom stated that they remembered feeling a pop in the chest while weightlifting, which was the start of their dyspnea.

> Arterial BP increases most during weightlifting that is accompanied by mechanical compression of blood vessels (such as when bent forward at the waist or squatting) and when accompanied by a Valsalva maneuver.

Neither of these papers link them to muscle mass but to the stress of high-intensity exercise itself, which I have already addressed.

I agree that we should not assume things, but in the lack of cursory evidence, I would still stand by my argument that in the normal case where muscle mass is formed from working out, such workout stresses and modifies the cardiovascular system to such an extent the result is a body that is more than capable of handling whatever stress that muscle at rest imposes on the heart.

Once again I reiterate my cursory evidence to my argument, that athletes' cardiovascular system are generally so much more efficient and less taxed than the general population that their hearts beat at 3/4 the average heart rate.

It seems like athlete's heart syndrome is a benign condition.

[Buttons840]

Actually, it appears BMI is more accurate for women than for men. I'm guessing because men have more variability in their muscle mass.

Overall, I think BMI looks like a pretty useful metric considering how easy it is to measure. That would explain why it sticks around despite being frequently criticized.

See charts 2.A and 2.B in: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2877506/

[edflsafoiewq]

Waist-to-height ratio is surprisingly good: your waist should not be more than half your height. Simple to measure too, since you only need a piece of string.

[meew0]

Humans are 3D objects, but fat is not distributed equally throughout the body. Most fat is subcutaneous fat, i.e. located right below the skin. Its abundance can therefore be approximated by body surface area, which is proportional to height squared.

[naniwaduni]

This might be a nice explanation of it didn't fail to agree empirically with reality, where in fact there is a significant cubic component which gives rise to the "something close to the 2.5th power" that GP suggests.