Vitamin D deficiency causes depression Yay, vitamin D deficiency is far from the most obvious problem. And depression is not a made-up disease. Vitamin D deficiency can provoke the development of this mental disorder as it affects the production of serotonin. That’s proved not only by the pathogenesis of the disease, but also by the fact that people suffering from depression sometimes report a certain reduction of symptoms after taking vitamin D. Surely, depression is not always caused by vitamin D deficiency, but it is possible.