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Comment by mrerrormessage | Hacker News Reader

I wonder if anyone has run the data using body fat percentage instead of BMI. BMI is a very coarse metric that will sometimes label short and/or muscular people as overweight/obese when they are quite healthy (think football players or weight lifters).

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BMI seems to only be a useful metric if you're interested in estimating the general health of, say, the entire human population of the state of Wisconsin.

For assessing individuals (or even small groups), I can't imagine it being very helpful.

I came to post more or less this exact same comment. BMI does not take muscle mass into account at all. The Rock has a BMI of 31 (260lbs at 6'5") which is classified as obese. Hafþór Björnsson has a BMI of 41 (419lb at 6'9").

I agree with some of the other commenters too in that a full nutritional/lifestyle analysis over a long period of time is next to impossible because it relies on self-reporting which is known to be flawed [1][2]. People under-report their intake and overreport their exercise frequency and duration.

[1]http://www.ncbi.nlm.nih.gov/pubmed/2082216 [2]http://ajcn.nutrition.org/content/76/4/766.full

Uf you don't fit into any of the categories of people that would normally measure their own Body fat percentages (athlete of some description, normally), your BMI is probably a good estimate of your "shape". Sure it's not perfect, but it's a perfectly good estimate for a large number of people.

I wouldn't call it perfectly good. It assumes that weight should increase as the square of height when really you should be using something like the 2.6th power. That means that taller people will have a systematically higher BMI than they ought to which can be misleading when populations get taller due to better nutrition, say.

Speaking as someone who is a tad over 6ft tall, and not a body builder, it fits me quite well. My bmi when I started paying attention was roughly 26, which I was shocked at, and I got defensive about very quickly saying "it's incorrect for tall people" but the fact of the matter was I was about 20 pounds (10kg) overweight. It also quite well describes my partner, who is about 5ft5-6

> The Rock has a BMI of 31 (260lbs at 6'5") which is classified as obese

Why shouldn't he be classified as obese? If he was that heavy because he was working out then that would be one thing, but the reason he's that heavy is due to using steroids to put on low quality muscle mass. I don't see why that would be any less unhealthy than just sitting around eating doughnuts or whatever.

If it were as unhealthy as eating donuts (which you would need to prove), it would most likely be unhealthy by a different mechanism. So it would make sense to distinguish this if you want to find out what causes illnesses.

I also don't understand what you mean by "low quality muscle mass".

> I also don't understand what you mean by "low quality muscle mass".

Muscle with a low weight to power ratio, and/or with an unfavorable type I to type IIa/b ratio. E.g. you can quickly bulk up by lifting 3 x 3 at a high weight (or whatever Starting Strength recommends), but you're going to develop much lower quality muscle than if you just do 6 x 10 or whatever in terms of power. And similarly if you're not doing cardio in at least 90 minute increments then on a regular basis then your type II muscles aren't going to be able to properly utilize fat, because they'll never get past the point of just relying on stored glycogen or whatever.

Would you mind explaining "low quality muscle mass"? That doesn't seem like a real thing. Do you know how steroids typically work? They allow you to work out more and recover quicker. You're putting on "real" muscle, just faster than you could un-aided.

> They allow you to work out more and recover quicker.

They also act as anabolics, so you could argue that the muscle mass gained due to their anabolic effect is "low quality" (that's not what I'm doing, though).

That there are people like The Rock, Hafþór Björnsson who have a high BMI but are otherwise healthy* does not take anything away from the fact that a high BMI is a pretty good indicator that you're overweight or obese. It's an imperfect measure but realistically if you're at those Obese or above levels you're either overweight and you know it, or an outlier and you know it.

* = I suspect that a good number of guys like this are taking substances that would damage them in other ways that could lead you to describe them as unhealthy, but as another commenter said that's maybe not for BMI to measure.

But are they really outliers?

As of this moment I'm 5'9", 187, and borderline ripped. People often compliment me on my physique.

But my BMI is 27.6, which makes me "overweight."

Yes. As are you. Put it this way, BMI is used to calculate that 35% of people in the USA are obese and 34% are overweight. Is that because there are lots of people who are borderline ripped and BMI is flawed, or is it because there are lots of overweight people? Which do you see more of when you walk down your average street. If it's the former then I will concede that I am incorrect and that you're statistically normal, but we both know it's the latter and it's to your credit that you're likely to be very healthy.

Note: I'm making an assumption you're in a western country like the USA or the UK

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