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Comment by cmiller1 | Hacker News Reader

Note that body fat percentage is notoriously difficult to accurately measure. The linked calculator uses the navy circumference method which gives readings ±3-4 points for most people and even further off for some. It gives me 3.5% which would be insanely low (I'm probably somewhere around 8-10 right now) because I have an unusually wide neck. To get more accurate readings you'd have to pay for a DEXA scan or a bodpod but even those can be off by a couple points.

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My favourite comment on this is the following joke exchange:

Doctor: Your BMI is high I'd like you make some dietary adjustments.

Patient: BMI doesn't measure fat accurately for example professional Rugby players are marked as obese but aren't.

Doctor: Are you a Rugby player?

Patrient: No.

It's a good guideline. Don't let edge cases control your perception and let professionals do their thing with the tools they have -- they're aware of limitations that's why their professionals.

While that joke is accurate (although some football players are also obese [1] depending on their position -- Aaron Gibson was 400lbs), BMI was never meant to be applied to individuals. It was designed to be applied to populations. It does a good job of that. Also note that guidelines have historically been created against a very limited selection of race and have not appropriately adjusted for height (although BMI 2.0 corrects the height issues).

If we're just relying on the professional's opinion then they don't really need the BMI, right. They can just look at you and go "huh, you're looking a little thicc today."

BMI is a good tool for population health, a bad tool for individual health, and if it just so happens to correlate to your thiccness then you probably already know that.

I agree with parent that body composition analysis via DEXA or air displacement plethysmography is a far better metric.

I have no doubt that a carpenter can bang a nail in with a screwdriver 90% of the time, that's why they're professionals after all, but when I see it, I can't help but think "there's gotta be a better way mate."

[1] https://www.nytimes.com/2019/01/17/sports/football/the-nfls-...

Why would race matter?

Melanin doesn't effect fat comp... does it?

Body composition varies by race.

https://pmc.ncbi.nlm.nih.gov/articles/PMC4968570/

https://www.sciencedirect.com/science/article/pii/S000291652...

They don't need to be causally connected, just be correlated.

> Why would race matter?

Because social matter

It's really not, and it's always weird when people jump to that conclusion.

Different genetics lead to different health outcomes for the same body fat percentages - or the same BMI on a population level. Just as it's not a "social matter" that the prevalence of sickle cell is much higher in African Americans. It doesn't make sense to use health outcomes standardized against a BMI range for one population racial mix against individuals of a race not represented -- in at least two separate ways.

Hong Kong for instance sets the obesity cutoff at a BMI of 25. China and Singapore use 27.5 I believe. The WHO and various other organizations generally endorse lower cut-offs for people of Asian descent. [1]

[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC10108164

Sickle cell isn’t affected by social matters, unlike body fat.

I'm not sure if you're following what I am saying. The same body fat percentage leads to different health outcomes for members of different races in aggregate because of different genetic predispositions. Someone of one race with BMI X is likely to have a different risk profile than someone of a different race with BMI X no matter what X is, high or low. This is not a social matter. We're just talking about heath statistics, not aesthetics.

That's not to say there aren't social aspects to obesity as well (obviously) but that's not what we're talking about right now, and parent dragging that in is just a distraction.

From [1] in the comment you replied:

> Access to medications and surgery should be improved, in part by updating US indications for therapies to reflect race‐specific obesity thresholds and through inclusion of Asian American people of all subtypes with lower BMI values in clinical trials.

It is bad all around, the inaccuracies start waaaay before you get to something you could call an "edge case".

I'm 5'11, 180 lb male, that counts as overweight.

I'm 18% bodyfat. You wouldn't even note me as being particularly athletic looking if you walked by, I'm right in the middle of the bell curve for "guy who works out sometimes". There's no way it should be flagging someone like me as overweight.

Two months ago I was in a similar situation as you. I'm a few inches shorter but had the same body fat percentage, and my history of lifting still showed.

I went to a new primary care doctor and who said I was a bit overweight. I had the same initial response of "oh come on!" and general exasperation with the state of medical care.

The thing is, he really wasn't wrong. From an epidemiology perspective it is simply better to be leaner if possible. The fact that I didn't "look fat" isn't really relevant; I was still carrying excess adipose tissue.

I already had a DEXA scan booked and was planning to do a cut before that exam. I lost about 10 pounds over the next 8 weeks through basic caloric deficit and moderate exercise. It wasn't exactly fun but it also wasn't that difficult, and the results were far more impactful than I expected.

Yes, I looked noticably lean with more muscle definition. I also went down a pants size and felt better in general. My blood work, especially lipid panel, was better than ever.

All this is to say that, while BMI charts certainly have flaws, and all individuals are different, most of us can still improve our health and longevity prospects by shedding excess fat. And that probably is leaner than we might expect.

> It's a good guideline.

100% accurate. Caveat Emptor - the world does not understand even the smallest nuance that needs to take into consideration - in this case "guideline".

For example, the US DoD used BMI (DoDI 1308.03) to disqualify a friend of mine from joining. his actual body fat by DEXA? 7%, he was just short and very fit.

There are plenty of people who do weightlifting or have high muscle density. Probably more than 10% of people and certainly not edge cases. Doctors are basically taught like undergrads are taught, not much critical thinking involved, they just recite whatever is in the relevant document they are supposed to recite from.

I really doubt more than 10% of people do weightlifting and have high muscle density. It's likely closer to 0.1% which is why BMI is a good guideline over a group.

being within an "ideal" or "average" bodyfat range will easily put you as overweight on BMI.

it's a useless statistic.

Doctors are much more likely to let obesity slide than they are to nag you about BMI when you're clearly in shape in my experience.

The problem is not so much what the doctor will tell people in person but that these same people will end up in stats and should be separated into their own category. Having a high BMI and that excludes strength trainers would make high BMI stats worse than it is already.

Those people do skew the stats by a bit, but they are outnumbered by the people who are "skinny fat", i.e. people who are very sedentary and who have high body fat but still have normal BMIs due to having very low lean body mass.

If we're going to use imperfect measures for the average Joe, then wouldn't a cheap bioelectrical impedence body fat test be at least a better approximation than jamming height and weight into a formula?

GP is not questioning the usefulness of body fat percentage as a statistic (that's its own question), but the crude method of measurement.

It says mine is 1.6%, which I'd say is at least 5% out.

I've done a couple DEXA scans recently and they have in fact confirmed that the BMI calculation for me is spot on.

I also just did a DEXA scan last week and the calculator is within ~2% so pretty good considering it's free and takes 1 minute.

> DEXA

Fat calipers would be less accurate than DEXA but much more accurate than Navy circumference method, yet a lot cheaper than DEXA.

Yeah, there are a couple inexpensive midground options here. Calipers is one method, bioelectrical impedance is another.

Yeah, 3.5% would make you leaner than many Mr. Olympia competitors! And nearly at the point of negative health effects.

It's a shame when people use those electronic home scales and believe the results. Someone was so happy to tell me they were at 10% body fat, I could only smile and nod...

Most healthy people would have difficulty dropping below 10-12% without very deliberate effort. And beyond that would probably be kinda freaked out when they start to see all the veins and striations appearing.

I got down to about 6% for a bodybuilding show. It was incredible to look at, but also literally painful to bump into any hard surface, to sit on any hard surface, etc. The amount of bony protrusions you have when the body fat is gone is eye opening. I walk around at 10-13% typically, then lean down for summer and shows.

Most people at 30+ lbs further away from 'having abs' than they think. I see it time and time again.

When I dip below around 8%ish my appetite goes through the roof, all self control goes out the window to the all consuming signaling in my brain to eat eat eat

I had a DEXA scan done and for me, it showed higher body fat about five percentage points higher than my scale at home did. But I used that as a calibration point for my home scale. I've found as long as I use the home scale under similar conditions each time, mostly in the morning after voiding my bladder but before showering, that there's not much fluctuation from day to day. If I ever hit my goal on the scale, I plan to go back and get a second DEXA scan.

Two interesting things that also came from getting the scan done was that it showed that I have poor bone density so that's an issue that needed addressing (and hopefully the second scan will show improvement) and the scan was part of a package where they also did a 3d scan of my body exterior, which I was able to download and 3d print. I plan on making a half-and-half model of my body's change upon hitting my goal body fat percentage.

Nearly? 3.5% bf is unhealthy for men and probably dead for women.

Also, when using the Navy method, be sure you read the instructions for how to measure. The “waist” measurement is unfortunately not the number on your trouser size. :)

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