As someone who has personally recovered from depression through diet and lifestyle, I think this topic doesn't get nearly the attention it deserves.
I might add that at my lowest level, when considering suicide, I had severe digestive issues - to the point where I considered living exclusively on smoothies.
I was also taking anti-depressants at the time, which made me feel too calm, almost impervious to any emotion whatsoever.
Over the last 10 years I have radically changed my diet, paying particular attention to how my now-improved digestion has completely changed my outlook on life. After introducing fermented foods and probiotics into my daily ritual, I can honestly say that I feel the best I have ever felt. I no longer have a need for antacids or anti-depressants, and I doubt I ever will again.
Coincidence? Who knows. But I do know that this approach has radically changed my life.