On of my first story submissions [0] (way before covid, 2 points only) was about how fastfood could prime the immune system for an overreaction.

Reduction or even prevention of systemic/chronic inflammation should be the prime goal of any diet and all the rest ((mental) health, weight, ...) will follow automatically. I guess a lot of research is still needed but a mediterranean diet is at least a small step in the right direction.

[0] https://news.ycombinator.com/item?id=16133976