They also barely move their bodies.

I think this is the elephant in the room that is generationally hard to address. At least in America, all demographics have gotten more sedentary over time, but Millennials in particular. To your point, it doesn't really matter what you eat if you live a sedentary lifestyle with more screentime and less physical activity than is ideal. To put it in a neutral way, I would not be surprised if many emergent cultural wars nominally about body acceptance are actually more a societal reaction to this root cause.

Psychosomatic, and(/or) psychogenic. There comes a point where the influence of the mind actually makes things physiologically wrong, which becomes even harder to untangle.

Some humour/sarcasm to illustrate your point:

"No better way to sum up the generations"

https://www.reddit.com/r/GenX/comments/s4nbru/no_better_way_...