It feels way too complex to draw clear conclusions, but perhaps we can try a more transitive approach: - It's well established now that physical exercice has a

It feels way too complex to draw clear conclusions, but perhaps we can try a more transitive approach: - It's well established now that physical exercice has an effect on clinical depression and specific mental issues - Gut flora directly affects your physical state, in particular the amount of energy you'll have to consume that day (both in what nutrients it can extract, and how effectively it does so) Gut flora impacting your physical state, impacting your mental health would look like a decent link to me.