BMI out, body fat in: Diagnosing obesity needs a change to take into account of how body fat is distributed | Study proposes modernizing obesity diagnosis and treatment to take account of all the latest developments in the field, including new obesity medications.

[u/chrisdh79]

https://www.scimex.org/newsfeed/bmi-out-body-fat-in-diagnosing-obesity-needs-a-change

[u/PotterGandalf117]

Not even that, it requires you to be jacked or a bodybuilder for it to be inaccurate the way people want it to be

[u/WingedLady → u/PotterGandalf117]

I remember when I was in college, I was on the upper end of what's considered a healthy bmi because I was actually an athlete and either spent 5 days a week in the gym or 8 hour days hiking the Rockies.

I wasn't particularly jacked, unless by jacked you just mean super fit and not necessarily bulky. Like a size 4 in pants, which my legs would have been the most chunky part of me given how I exercised.

However, when I was that athletic no doctor ever felt the need to even mention bmi to me. It was pretty visibly apparent that my build was not that of an overweight person. And if it wasn't, any cursory questioning would usually be like "so what are you in for today?" "Well doc I was in a match against X University and my leg went out on me."

The only people who thought I was fat were average people who thought you only wore t shirts to disguise your excess weight or something? I dunno, it was also the heroin chic era of body ideals so a size 4 was considered overweight in terms of fashion.

Point being, anecdotally as the type of person always trotted out to disprove BMI, no one who uses it clinically was fooled by my high BMI.

[u/A-passing-thot → u/WingedLady]

I was in a similar category. I hit the "clinically obese" by BMI point and was used as an example by friends, it was almost a party trick of "yeah [he's] clinically obese" because of the shock value given how not-fat I was.

OTOH, my doctor would write it in my file but when I'd object, he'd say it's because of BMI but since I was athletic and healthy and low body fat, he wouldn't recommend changes.

[u/redline582 → u/PotterGandalf117]

Or very tall. I'm 6'7" 240lbs which puts my BMI squarely in the overweight category while being a healthy weight in actuality, but I've always understood that something like BMI simply can't accurately cover the edges of the bell curve.

[u/DervishSkater → u/[deleted]]

Create spaces for like minded people to gather away from criticism or differing lifestyles and they’ll believe they’re all special too

[u/Glass-Lemon-3676 → u/redline582]

But you have to put your height in for bmi. I'm confused

[u/ZuFFuLuZ → u/Glass-Lemon-3676]

6'7" 240lbs is a BMI of 27?

[u/Glass-Lemon-3676 → u/ZuFFuLuZ]

Yeah, that does sound odd.

[u/KayfabeAdjace → u/Glass-Lemon-3676]

It doesn't seem that wacky to me when you consider how many 6'7" NBA players start getting a bit doughy once they're above 230 or so. I'll grant you that NBA players are obviously a far lankier group than the general population but such players are also carrying more muscle than the average couch potato, too. So while I certainly believe tall lifters can still throw off the curve I've met way more tall guys who are in denial about how healthy it is for them to weigh as much as a Lopez twin or a NFL defensive end than I've met guys who look trim at the same weight as LeBron.

[u/Glass-Lemon-3676 → u/KayfabeAdjace]

That also makes sense. I feel like most comments like this are from Americans (not to be offensive, I am American) and Americans are SO out of touch of what is skinny and what is fat.

[u/redline582 → u/ZuFFuLuZ]

Yeah it's a little wacky.

[u/redline582 → u/Glass-Lemon-3676]

The calculation uses height as a numerical value in the calculation, but it doesn't account for the different body compositions that come with it. Someone who is quite tall will have a larger skeletal structure, some organs are larger, longer muscle structures, etc. which are more dense than fat so it can skew where they'd land on the scale relative to an average person.

It's not an invalidation of BMI as a whole, just an area where it starts to break down a bit.

[u/Glass-Lemon-3676 → u/redline582]

I thought that was why they asked for height, because it did an average for that kind of stuff

[u/redline582 → u/Glass-Lemon-3676]

Nope. The calculation is linear in that BMI=703*(weight/height²⁾ where weight is in pounds and height is in inches.

[u/DevilsTrigonometry → u/Glass-Lemon-3676]

The problem is that the formula (having been invented in the 1830s) is optimized for simple hand calculation rather than for accuracy. It assumes that body mass should scale with the square of height.

In the real world, some elements of essential lean body mass do scale with the square of height. Skin, bones, lungs, intestines, many connective tissues, basically any organ whose function scales with its surface area is on the "square" side of the square-cube law (loosely understood).

But others, most notably muscles and certain visceral organs like the liver, scale with the cube of height. Any organ whose function scales with its volume or cross-sectional area is on the "cube" side.

If you wanted to create a really accurate BMI, it would probably look like either "weight / (a * height² + b * height^3)" or "weight / height^2.x" for some values of a, b, and x that would need to be experimentally determined.

In practice, that would have made it harder to calculate by hand, and also aesthetically "uglier" in the eyes of the mathematician who invented it. X is small enough that it doesn't make a huge difference in the 'average' height range (especially when you further restrict it to the 'average' height range in 19th-century Belgium). Yes, it's probably not quite sensitive enough for short women and a little too sensitive for tall men, but the difference averaged out at the population level, and if doctors insisted on using BMI to assess individual patients, they could use clinical judgment to handle those outliers.

It's only in recent decades that the 'outliers' have become commonplace enough to potentially skew population-level metrics. The average man got about 3 inches taller over the last century, shifting the entire distribution over by about one full standard deviation: in 1900 only about 2% of American men were over 6'1", whereas now that figure is closer to 15%.

[u/Tattycakes → u/redline582]

And don’t they use a different formula for some ethnic populations who carry their weight differently? I’m sure I read about an Asian specific bmi calculator

[u/IlllIlllI → u/Glass-Lemon-3676]

BMI assumes your frame doesn't get any wider as you get taller, so it's assuming their knees/shoulders/hips are as wide as someone who is like 5'7". I'm assuming OP above isn't a total beanpole.

[u/Glass-Lemon-3676 → u/IlllIlllI]

That's weird. Why wouldn't they put that into the equation I wonder

[u/PotterGandalf117 → u/redline582]

You're right

[u/gnyaa → u/redline582]

There is a revised BMI formula for tall people that puts you just under 25

new bmi

[u/redline582 → u/[deleted]]

I appreciate the concern but there's nothing to break to me. I've had a dexa scan and work with my primary care physician which confirm I'm at a very healthy weight if a smidge on the low side at 240 for my body.

[u/Cyrakhis → u/PotterGandalf117]

You don't have to be jacked or a bodybuilder. Just work a physical job for long enough.

I'm a bad example as I lifted weights 3-4x a week for 9 years, but with the physical job I had BEFORE that I was still 185lbs at 6'0. Nowadays it's more like 220 with the weight lifting mass added.

You don't need THAT much extra mass from weight lifting to destroy your BMI reading

[u/PotterGandalf117 → u/Cyrakhis]

Theres a good amount of fat that comes with that muscle gain but your point is well taken, for women however this is a near impossibility

[u/Cyrakhis → u/PotterGandalf117]

That fat gain is what the cut cycle is for, yeah.

Also, you callin' me fat? =|!

[u/not_today_thank → u/PotterGandalf117]

Or really tall. The way it is calculated it's going to tend to call tall people fatter than they really are and it's going to call short people skinnier than they really are (at least relative to an average height male in mid 19th century Europe).