This is pure anecdotal, but since getting C-Diff a few years ago, whenever I get sick my gut feels much better (same with my skin). When my gut starts hurting again, and my eczema flares back up, I know that I'm almost over it and should slow down the vitamin C. And of course, my anxiety/stress also flares up.
... in mice.
I used to struggle with anxiety and depression (it's very likely was stress-induced depression), and nothing seemed to help. But then I read a similar research article about how gut health could affect mental health. So I started eating more fiber and multiple probiotics. And guess what? My mood improved! I felt less anxious and depressed and more energized. It's crazy to think that something as simple as gut health could make such a difference in a brain.