Per the Wall Street Journal:
Microorganisms in the gut have been linked to diseases beyond depression including obesity, arthritis, diabetes and several cancers. Researchers said more work is necessary to confirm the associations and explore possible treatments and tests based on the microbes.
I do recommend fermented foods, including kimchi (check your local grocer) and kefir (yemoos.com among many others).
For Kimchi I recommend buying “broccoli slaw” in the bagged-greens area of the produce section. It’s just broccoli stalks cut into long thin spears. Take a small Tupperware container and fill it 2/3 of the way with the slaw (again there are no liquids, just raw broccoli). Then fork 3 or four scoops of kimchi on top. Close the lid and shake it all together. Now pour it on a plate and eat. You have a great tasting nutritious lunch, and your expensive kimchi will last all week. Broccoli has fiber that the probiotics need. Plus it’s extremely good for you, yes even the stalks!
There may be a paywall — Wondering Why You’re Depressed? Check Your Gut - WSJ