[PuppyTailWags]

I think I'm a little confused by your definition of obesity. As far as I know, obesity is defined by BMI measure which is defined by a specific ratio of weight and height, so I don't understand how being tall would affect your being obese or not. [Unless you're saying the measurement doesn't work for tall people?]

[salty_biscuits]

The usual argument is that the scaling of the power wrt height is wrong (people aren't spherical basically). I don't buy it though. So many people say they have a high BMI but "they just lift weights". If you aren't on PEDs it is very unlikely that you can be BMI > 30 with low body fat.

[BrandoElFollito]

My son is 15, 182 cm and weights 81 kg. According to BMI charts he is in the 94th percentile and it's categorized as obese.

But he is an athlete with almost no fat (he is not aiming at muscles so he does not have a diet that minimizes fat, he is looking for sheer power and explosivity (gym, martial arts)). He is very built up (muscular) though.

So I wonder how the BMI can give any kind of reasonable indication when the fat level is not taken into account.

[salty_biscuits]

He is only BMI = 24 though? What crazy redefinition of obese is going on there.

[BrandoElFollito]

If you input these values at https://www.nhs.uk/live-well/healthy-weight/bmi-calculator/ you get the "obese" suggestion.

Without any consideration for fat BMI doe snot make much sense I guess.

[0xB31B1B]

182 cm isn't even that tall. I'm 193 CM and athletic build and at my recommended weight range by body composition measurements had me at like 26 BMI or "overweight".

[Volundr]

Ehhh I'm not sure I'd go that far, but the person who has >30 BMI with muscle mass is going to look completely different than the person doing it with body fat.

Sure BMI can be more or less accurate depending on the person, but for those few who have actually rendered it inapplicable to them it's going to be blatantly obvious. No one will confuse them with being obese.

[jjj123]

It seems like you’re moving the goalposts a bit by talking about “low” body fat. I don’t have “low” body fat but I know my bmi would be significantly lower if it weren’t for the 30+lbs of extra muscle mass I’ve put on in the last 5 years.

I wouldn’t have been obese before, but I technically am now (with reduced bodyfat %).

[yread]

Ronnie Coleman has BMI of 38.9 while having less than 1% body fat. Bit of an extreme example, but "light weight baby!"

[TheCoelacanth]

He also lifted weights to the point that he put himself into a wheelchair in his 50s. I also highly doubt the no PEDs part.

[evol262]

Ronnie was never less than 1% body fat, or even 1%. Stage ready bodybuilders are more like 3-5%. People are just poor at estimating bodyfat, and the media throwing out nonsense numbers doesn't help.

[afiori]

Obesity implies high BMI, but not vice versa.

Fat is not the only way to be heavy, body builders have very high BMI value yet have very different risk profiles.

[PuppyTailWags]

The poster above states outright that you don't have to be a bodybuilding to have high BMI though. You just have to be tall and lift occasionally? Thus my confusion.

Also, I'm still confused. My previous understanding was that bodybuilders are classified as obese without having excess body fat because they fit into that ratio because the ratio doesn't account for exceptions. Not that obesity actually incorporates more than just a ratio.

[soco]

As other commenters noted, BMI is just a low fruit metric to open the door for more investigations (or not). And when you see the person I bet you'll know immediately whether it applies or not. The word "obesity" existed well before BMI.

[afiori]

BMI uses an exponent of 2, for physical reasons it needs to be around that value, but the "correct" value might be different; it appears that GP believes it should be higher.

The point about body builders is that BMI as a metric assumes a fixed muscular profile, so doing (an healthy amount of) muscle training can wildly affect your BMI independently of your fatness.

If is still likely a good measure at a population level as very few people that have BMI > 30 are also fit. But you need to contextualize it to your own case.

[0xB31B1B]

To be more precise, my problem with this study is when you select for people with BMI>30 and filter for those who do not have obesity related health issues, you’re going to have a lot more of the pathological case, like body builders, tall folks who are overweight but not obese, etc.