For me, it was taking vitamin B12 injections. Seriously.
If you have gut problems, chances are you have vitamin B12 deficiency and depression is a symptom of that. Why you have to figure that out for yourself after you get diagnosed is beyond me.
See also:
This is no joke. When I downloaded Slackware in '95 or '96, it was over 100 3.5" floppies of 1.44 MB each. And there were still more available, those were just the ones I thought I’d need. And before you could even begin installing, each of those had to be downloaded, written and verified because floppies were not terribly reliable.