[chiefalchemist]

Feel like it might be being optimistic. I believe "positive outlook" at the very least has correlated with better health, longevity, etc.

Feeling in better shape (and not actually being so) is a byproduct of optimism.

[usrusr]

"Being healthy makes people feel healthy" (and thus more optimistic/active/positive) would be the more occhamish interpretation of that correlation.

[chiefalchemist]

But I think it goes both way, with anticipation driving outcome as much as if not more than the other way around. That's the gist of the study. How you feel is more important than how healthy you actually are.