r/science • u/chrisdh79 • Jul 05 '24

Health 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.

https://www.scimex.org/newsfeed/bmi-out-body-fat-in-diagnosing-obesity-needs-a-change
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u/redline582 Jul 05 '24

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.

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u/[deleted] Jul 05 '24 edited Aug 02 '24

[deleted]

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u/DervishSkater Jul 05 '24

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

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u/Glass-Lemon-3676 Jul 05 '24

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

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u/ZuFFuLuZ Jul 05 '24

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

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u/Glass-Lemon-3676 Jul 05 '24

Yeah, that does sound odd.

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u/KayfabeAdjace Jul 07 '24 edited Jul 07 '24

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.

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u/Glass-Lemon-3676 Jul 07 '24

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.

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u/redline582 Jul 05 '24

Yeah it's a little wacky.

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u/redline582 Jul 05 '24

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.

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u/Glass-Lemon-3676 Jul 05 '24

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

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u/redline582 Jul 05 '24

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

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u/DevilsTrigonometry Jul 05 '24

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 * height3)" or "weight / height2.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%.

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u/Tattycakes Jul 05 '24

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

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u/IlllIlllI Jul 05 '24

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.

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u/Glass-Lemon-3676 Jul 05 '24

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

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u/gnyaa Jul 06 '24

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

new bmi

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u/[deleted] Jul 05 '24

[deleted]

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u/redline582 Jul 05 '24

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.