The exact connection between vitamin D and depression is not fully understood. There seems to be a correlation between the two, if not a direct causation. People with clinical depression have been found to have vitamin D deficiencies, though these could both be symptoms of a different root cause. Seasonal affective disorder (SAD) is a condition that is thought to be linked with the declining exposure to bright light during the winter months, and a lack of sunlight can also create a vitamin D deficiency. Research suggests there may be a causative relationship between vitamin D and depression, since positive effects have been achieved through the administration of the compound to people suffering from depressive conditions.
There appears to be a link of some kind between vitamin D and depression. The human body is capable of synthesizing vitamin D if sufficient sunlight is available, so a deficiency is typically linked to a lack of natural light or a defect of some kind. A lack of sunlight can also cause symptoms of depression in some people, as seen in seasonal affective disorder. It is thought that the lack of sunlight has a negative effect on brain chemistry, though it is also possible that low levels of vitamin D production could contribute to this disorder and other forms of depression.
The human body depends on a type of light known as ultraviolet-b (UVB) to produce vitamin D. Studies and anecdotal evidence suggest that light therapy with UV filters can improve mood, suggesting that conditions such as SAD are not entirely caused by a lack of vitamin D or improved by its presence. Other studies, however, suggest that vitamin D can have a similar or complimentary affect on mood. One double blind study conducted in Australia suggested a causative relationship between vitamin D and depression. This study involved the administration of vitamin D3 and found positive results one standard deviation over the placebo.
Anecdotal evidence also suggestions a relationship between vitamin D and depression. Increased depression in recent history has coincided with a number of trends that have reduced human exposure to sunlight, particularly to the UVB necessary for the production of vitamin D. Urbanization and time spent working indoors, particularly in enclosed office environments, can dramatically cut down sunlight exposure. The potentially harmful effects of ultraviolet radiation have also resulted in the wide use of sunscreens, which can have the unintended effect of reducing vitamin D production. None of this necessarily proves that low levels of vitamin D cause depression, though a strong relationship can be seen nonetheless.