Are the biosynthesis, utilisation and catabolism systems of vitamin D, especially the pathways of neural cells' vitamin D response, altered by the pathophysiological condition of major depressive disorder? If so, can antidepressant drugs restore that? (Assume the dietary uptake and ultraviolet exposure are normal.)
As someone who has been diagnosed with severe vitamin D deficiency, I can tell you from my personal experience that low vitamin D levels almost always makes you feel sad and depressed (we are talking about the chronic type). And since the levels take a lot of months to rise to normal, this lower mood levels start to become a part of your life.
Anti-depressant drugs help in most cases, although the doctor should decide more on this. Almost always as the vitamin D level starts to rise, it does take care of this.