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Comment by Clubber | Hacker News Reader

Using the BMI to gauge obeseness of a population is very lazy and disingenuous.

My understanding is this is precisely backwards: using BMI to gauge your own health is inaccurate, but using it to analyze populations is the intended purpose.

As the country gets more fit, and that fat turns into muscle, what happens to the BMI? It either gets higher or stays the same, right? Certainly a good measure to perpetuate a crisis.

Let's cross that bridge when we get there.

In the meantime, I more regularly see the people whose BMI is highly correlated with their body fat percentage; and, one of those requires a scale and a number that doesn't change after your early 20s. The other one requires a tape measure, calipers or an immersion pool.

Accurate body fat tests either require expensive equipment or training to use. The inaccurate method those cheap bathroom scales use would probably be good enough to detect trends but if we're just going to detect trends: why not stick with BMI that's accurate enough for that purpose and way cheaper/easier to use? There aren't that many people with overweight BMIs that aren't overweight. Maybe it could be slightly better to change methods, but it's not like it would be orders of magnitude better.

-Someone with a BMI of 28 (smack in the middle of overweight) and a 6pack

>Accurate body fat tests either require expensive equipment or training to use.

I agree, that's why I think it's lazy and disingenuous. Look how much we fund global warming research. But when it comes to calling something a "national health crisis," we don't even bother and take something invented around the time of Andrew Jackson's administration. Good enough, right?

>There aren't that many people with overweight BMIs that aren't overweight.

How many people would you estimate have overweight BMI's aren't overweight? How are you even defining overweight? See how it's a soft metric? Countries basing taxation and policy on this stuff.

You are overweight and part of the national health crisis, period. No discussion because the BMI doesn't allow it.

Oh come on. I'm overweight by BMI behind lifting, but take a trip outside a coastal city and you will find that the number of people in similar positions amounts to rounding error in the general population.

It seems correct to a first order of approximation for the general population. What is your objection?

I know people who lift weights regularly can have high BMI but low body fat. But until those people make up a large share of the population, I would think BMI is still an OK general measure.

I disagree.

Using BMI to gauge the obesity of an individual is lazy, but for a population, its quite okay.

How many people are more than 6'5" in a population? How many athletes? And even athletes sometime are overweight! A 6'5", 29 BMI NFL LB might be healthy, but the 32 BMI lineman with the same size will have the same problem as obese people once he's 35, discounting injuries.

I would say it is useful only to gauge obesity in a large population (where only a marginal percentage has such a high muscle mass that a high BMI does not represent obesity).

But how many people does having both muscle and fat push them from the overweight category to the obese category? At 5'-9" the overweight category starts a little over 169 lbs and the obese category starts just above 203 lbs. Given that as a teenager I was well into the overweight category with a six-pack and very little fat from an active lifestyle and playing a lot of basketball multiple times a week, I could easily see how just eating more might put be closer to obese while still having less fat than one would expect from that designation.

That's not to deny Americans have a problem with weight. I just think it's counter-productive to use metrics that people can easily see are problematic when trying to make a valid point. Using controversial measurements because the are easily available is lazy.

The level of fitness you describe would put you in the top 2% of people I have ever met.

I think that margin of error is acceptable compared to the ease of use of the BMI.

I did not feel overly fit. I was definitely not as fit as the people that played a sport competitively. I hung out with friends, ate (a lot of) fast food, and didn't do much outside of school. I did have a "weight training" class a few times a week, where we ran a mile and then I played basketball the whole time. A total of about five hours of exercise a week, and not being sedentary, definitely not an outlier for many teenagers. Yet still considered overweight?

The problem is that the BMI probably matches fairly well to a specific slice of the population. Maybe males 25-40. It likely loses accuracy of meaning the farther in age either direction you go, as well as the more different you are from the optimal subgroup. This is only barely noted in most cases (it's alluded to here in this article that Asian people might see problems at different BMI numbers). What about women vs men? What about the young and elderly? As a metric, BMI is poorly understood by the public, and this poor understanding is targeted and capitalized on by different studies. All it does is cloud our view of what's really going on.

BMI is generally fine as a statistical tool. Obviously if you're looking at a whole country of olympic athletes it'll break down a bit, but in practice that sort of extreme edge case doesn't exist on a national level.

For personal use, it can be an indicator, but only that.

Why? It can be inaccurate for individuals because it can show lean but muscular people as overweight, but those people are a tiny fraction of the population.

Yeah, I just worked backwards from the metrics they provided and at my height I would have to be under 165 lbs to not be considered overweight. In high school I weighed about 180, and was not overweight in the slightest. I had a six-pack, and was fairly active. Yet my BMI suggests I was half-way through the overweight range on my way to obese? That seems somewhat ridiculous.

Is an average way to determine it but a good and quick estimation even if +-10%

Yeah I came here to say the same thing.

I’m 6’8” and if I were within the suggested range, I would be 227 pounds. I’ve been that weight and when I look at pictures of myself at that weight, I always think I look dangerously thin.

BMI is a far too simple a measure to be helpful.

Your height puts you at the 99.986 percentile [1]. You are an extreme outlier, so can't possibly expect general metrics to fit well.

[1] https://tall.life/height-percentile-calculator-age-country/

BMI is an absurd measurement for people whose height is outside a small range. See, for example, https://people.maths.ox.ac.uk/trefethen/bmi.html

dangerously thin hahahha omg

The irony of the BMI is the more I workout to stay in the healthy range, the more difficult it is to stay in the healthy range. Best bet is to sit around not eating anything to be 'healthy' according to BMI.

True, I started weightlifting for three months and my BMI went into overweight territory. I had an athletic build both before and after. Probably good for a lot of people but weightlifting really puts it into question.

It's not like weightlifting is something only few men can do; most men can weightlift. Then they end up having healthy body fat levels, but end up with BMI in the overweight category. If something as simple and straightforward as weightlifting messes up a measure, then it's not a good measure.

Lifting heavy weight doesn't mean you're fit. You can still be very overweight and bloatmaxxed

This guy is strong as hell but he isn't healthy at all https://youtu.be/1LsIQr_4iSY

BMI in no way gauges fitness or health. Another reason it's lazy and disingenuous.

The question is not whether most men can weightlift, but whether most men do weightlift. They don't.

The question is what criteria would you want a measure to have for it to be called a good measure.

You are arguing that if it empirically works for most people, then it’s a good measure. I’m saying that a good measure should in principle work for most people.

You don’t lose weight lifting things up and putting them back down. You certainly don’t put any appreciable strain on the CV system.

I was doing a lot of cardio training for an ironman and had really low body fat, but still borderline overweight according to BMI.

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