I have had an interesting thing happen. After having been on and off SSRIs for years and, before that, tricyclics (with varying and inconsistent results), now that I have been taking 2,000-4,000 IU/day of D3 for several months I am almost free of symptoms (depression and anxiety). We'll see if the effect lasts. Depression is complex and multifactorial in causation so there will never be one cure-all for everyone. But I may have hit on mine by accident.
If you’re interested in doing a little reading; this site has some free academic articles. http://highwire.stanford.edu/. Just type in vitamin d deficiency (or any topic you’re interested in).
Your statement...”Depression is complex and multifactorial in causation so there will never be one cure-all for everyone.”
This is very true. Depression is a symptom and yes, there can be many causes. I have found that the actual causes are almost always psychological, based upon perception. Many times this is at the emotional subconscious level and when a person begins to access it, the anxiety and panic attacks serve to block the recovery of the emotional issue. Even the biochemistry of the body changes based upon perception of the environment.