If health is an 'outcome', for example a pants size, we run into a problem with natural differences between people making access to that outcome uneven. There is a lot of material on really unhealthy skinny people, people who smoke, people who do cocaine, people who's labs are way off...etc. There is simultaneously a lot of good science about the heritability of weight problems. There is also a strong link between poverty and obesity, I would argue a causal link.
Is there a difference between actual measures of health like V̇O2 max, and pants size? If I have a poor V̇O2 max is it immutable? Am I permanently in a class of people who are going to die younger, and have a shorter health-span? The reality may be that due to genetics, or economics, or what county you live in, some specific metric of health may in fact be largely immutable.
However, as a person with a poor V̇O2 max, starting to gently do aerobic training, can _vastly_ improve the situation for health and longevity, miles and miles and miles before the actual outcome (the V̇O2 max value itself) moves at all. If I have to shop at the big and tall store, but I also really try to control my blood sugar with good nutrition and meal timing, I am in fact healthy (or healthier), in the sense that I am doing what I can with the body I was born with. I am miles and miles and miles healthier even before the scale starts to move.
This is health as a behavior, in the moment, working with the cards you have been dealt, doing your best, maintaining hope. This is the argument for the concept of health regardless of size.
Where I acknowledge that Health at Every Size has gone off the rails -- and keep in mind I was deeply immersed in the originator of the movement, Lindo Bacon's work, as well as the work of Sandra Aamodt a few years ago -- is this idea that you are automatically healthy at every size.
Let's prove this by looking at the extreme: If you take a heavy person who smokes, and a heavy person who doesn't, one is healthy, for that specific metric of smoking or not smoking, and one is not. Extend this logic to how you eat with the resources you have, and how you move within your abilities, and you can clearly see that there are healthy (or healthier) ways to be heavy, and unhealthy ways.
The high order bit is that you can move the needle on your health, long before the needle on the scale moves. I would even go so far as to say something as radical as you can embody health as a behavior, even if the scale doesn't move at all.
