Ok, so this might not be the most medically sound question, since I am a mathematician by training, but please bear with me.
The question originates from an observation I made from being confronted with many advocates for different types of diets (vegan, vegetarian, carnivore, paleo, gluten-free, sugar-free etc) and what baffles me is that no matter what the special type of diet is, there seem to be individuals that have very good success with less depression, less overweight, more vitality, less pills to take and so forth. These individuals are often presented by the respective advocates as good rolemodels.
This really struck me, since it indicates that it might not be the actual diet itself, but maybe something else. The only thing I could make out to be present across all individuals is the fact that these individuals got their life together at least with respect to their food intake. So could the correlation between a consequent and dedicated change of diet and the overall improvement of the individual be caused by psychological phenomena that stem from the new approach / mindest these successful individuals all share?
Is there any research on that? How could one even figure out something like this?