There are two main types of people in this, the healthists, and the rest of us who aren't. Some of those healthists are fat and some aren't. Some fat healthists are to be found within FA, some not. Some non healthists are also within and outside fat acceptance.
I will not stoop to calling myself a 'bad fattie' even in jest because it validates the healthist premise, if you validate it, live it. If you cannot/ will not, you either don't believe it or need to question your belief, either way, why define yourself in terms that you don't speak to you? I am not a 'bad fattie' because I don't romaticise the fact that I just don't believe in it, it's not applicable to me, mostly. It's like I wouldn't call myself a 'bad moralist' for not achieving the sanctified 'moral ideal' of thinness either.
Healthism in essence = A, B and C are 'healthful behaviours', if you do them that means, you are healthy. You are literally doing and creating health. I can see why it's attracts in this era of self actualisation.
At some point, a conjunction between diet believers trying to prop up their defunct credo switched to 'it's all about health' and healthists making thinness=health. They like to keep up with the latest health trends and dictates and the medical professions, doctors especially are hugely influential. This means that when the medical professions bought into the conflation of thinness with health- some say it was aimed at healthists or the 'worried well' of which they are a part- they went along with it.
Meanwhile fat healthists, who connected with the same healthy eating and exercise messages as everyone else, did A+B+C and liked it. They didn't become thin they remained fat, but they liked doing the behaviours and bought into the idea in it's original form-A+B+C = healthy.
The clash in short is A+B+C= healthy versus A+B+C= thin-healthy.
There are other definitions of health, the original medical one, the absence of disease. A lot of people feel that's too negative but it's hard to beat for making sense.
There is another one within medicine itself where if you have a chronic medical condition and doctors feel you are complying with their instructions and if your body is responding well, they'll call you healthy.
There's also the growth of the ideal of preventative medicine, which is represented by numbers. That is mainly blood pressure, cholesterol level, etc. These are said to be proven by medical studies to be linked with health outcomes, both good and ill at certain levels or numbers. There is conflicting evidence about this though.
If your numbers are said to be within the healthy ranges, then this is another way you can say you are healthy, although again, those who believe fatness is incompatible with health, dispute that this is a false when it comes to fat people who are a(nother) disease waiting to happen, thereby undermining the fundamental basis of these as indicators of health, as a whole.
I got the message too about what was defined as a healthful diet would =slimness and/therefore health. Health was like a big overlord telling me that the massive distance between me and it represented the true depths of my inner corruptness. There's me scrambling desperately flailing arms outstretched trying to get to 'Health-thin'. Couldn't manage it. Felt bad about it.
I could however state that I was healthy in the past medical sense of no apparent diseases, I saw this as an inheritance I was desperate not to 'squander' it due to my fatitude. I wasn't worried at all because I knew that no matter that I fell off the horse every day, every day I got back on and would tame the beast eventually by grinding down it's will.
Instead I ground down my ability to keep repeating the error.
It so happens that past the age of early childhood infections, I have had a pretty sound underlying physicality, though any possible complacency has been punctured by the endless 'obesity related' list and it's constant repetition. That has ended any sense of security that many haters wish ferverently to cling to. And the fact that I saw weight loss-that I didn't achieve- as part of my healthy quest.
The upshot of all this is I would never say, 'fat is healthy', because it's outside a meaningful understanding of health that refers to my understanding of it. It's refering to a position I don't operate from. It's not that I don't think there are things that tend to toward and against promoting health or more specifically well being, it's just that I don't feel it describes how I feel on any real level, mostly. My experiences have alienated me from it.
So I leave it to healthists-fat and thin- to slug this one out, they can relate.