Low vitamin D levels linked to depression

Major depressive disorder affects nearly one in 10 adults in the U.S.  UT Southwestern researchers examined the results of almost 12,600 participants from late 2006 to late 2010.   Low vitamin D levels were associated with depressive symptoms, particularly those with a history of depression.  The researchers did not look into causation, whether depression is a cause of low vitamin D levels or vice versa.

http://www.utsouthwestern.edu/newsroom/news-releases/year-2012/vitamin-d-brown.html

The existence of conditions such as Seasonal Affective Disorder provide grounds for suspicion that lack of Vitamin D is a cause of depression.  And for the purposes of urban design the importance of creating attractive, safe outdoor environments in which people will spend time and achieve the exposure to daylight necessary to maintain Vitamin D levels.