In a large-scale study published in JAMA, researchers found that high-dose, long-term vitamin D supplementation did not result in improvements to late-life depression risk or mood.
 
The clinical trial, which enrolled 18,353 men and women aged 50 years and older in the VITAL-DEP (Vitamin D and Omega-3 Trial-Depression Endpoint Prevention) study were evaluated for a number of health outcomes including cardiovascular disease and cancer prevention.
 
Of the participants, 16,657 had no previous depression history, whereas, 1,696 participants had a depression history with no treatment for depression within at least two years.
 
The randomized, double-blind, placebo-controlled study included a group that was administered 2,000 IU of vitamin D daily and fish oil, another group took the vitamin D dosage only, and groups were randomized to matching placebos in a 2×2 factorial design.
 
Researchers evaluated depression outcomes, as well as difference in mood scores based on the eight-item Patient Health Questionnaire depression scale, which was given on a yearly basis. Outcomes were assessed over an average period of 5.3 years.
 
“Risk of depression or clinically relevant depressive symptoms was not significantly different between the Vitamin D3 group (609 depression or clinically relevant depressive symptom events; 13.3/1000 person-years) […] there were no significant differences between groups in depression incidence or recurrence,” the authors of the study concluded. Additionally, “no significant differences were observed between treatment groups for change in mood scores over time; mean change in PHQ-8 score was not significantly different from zero.”
 
For these reasons, the authors of this study believe that their findings do not support the use of vitamin D3 in adults to prevent depression.
 
Several studies prior to JAMA’s publication concluded that vitamin D resulted in significant improvements to mood and depression, though most of these studies evaluated short-term outcomes following high doses who were deficient in vitamin D. Further, correlations with vitamin D deficiency and depression have been established in numerous clinical trials.
 
It has also been theorized, and the subject of a small study, that seasonal affective disorder, a mental illness in which depressive symptoms manifest seasonally (during the winter months), could be related to reduced exposures to UV radiation, as vitamin D is synthesized endogenously through exposure to sunlight.