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Low vitamin D levels are associated with the onset of depression even in young people who have otherwise happy lives. It's not like the doctor will run a blood test and say, "Your vitamin D levels are low. You must be depressed." However, supplemental vitamin D seems to be one way to treat depression, without all the side effects.
Dr. David Kerr of the School of Psychological Science of the College of Liberal Arts at Oregon State University in Corvallis ran a series of assessments of 185 "apparently healthy" young women. He found that about one-third of the women actually displayed some symptoms of depression. One-half of the women in the study had deficiency of vitamin D. The women who had the lowest vitamin D levels were the most likely to display some signs of depression, and to be at risk of a major depressive episode.
"Winter Blues" May Not Be the Real Cause of Depression
Medical researchers first started looking for a link between vitamin D and depression as a way to explain a condition called seasonal affective disorder. As days get gloomier, people get gloomier, especially at high latitudes, including locations like Oregon. Less exposure to sunlight results in lower production of vitamin D, so maybe lack of vitamin D causes depression. The fact that depression also increases during the summer in extremely hot climates where people spend most of the daytime hours indoors in air conditioned places seems to reinforce the theory.
Kerr measured vitamin C and vitamin D levels and gave his 185 volunteers the Center for Epidemiologic Studies Depression (CES-D) scale, a standard psychological test for depression, once a week for four weeks. Since he conducted the study during the fall, the women's vitamin D levels fell throughout the study, and measurements of clinically significant depression rose throughout the study. When he took into account body mass (fat "traps" vitamin D and keeps it from getting into the bloodstream), race (people who have darker skin make less vitamin D), outdoor activity (time in sunshine), and exercise, he found that only two variables were associated with depression, the use of antidepressants (meaning a doctor had diagnosed depression), and vitamin D.
Treat Depression With Vitamin D
This doesn't mean that if you take vitamin D you can throw your antidepressants away. Vitamin D probably won't do the whole job of preventing or treating depression. It takes several weeks or months for vitamin D to accumulate in your body, so the effects may not be immediate. You may need to take supplemental vitamin D all year round even to notice a difference with seasonal affective disorder (the winter blues) in the fall and winter. Vitamin D is not enough. However, taking 1000 IU per day probably will help.