Vitamin D is an essential nutrient proven to be important for bone health. It has other physiological functions, and there are plausible reasons for investigating vitamin D in depressive disorders. Some cross-sectional clinical and epidemiologic studies, but not all studies, have found that low levels of vitamin D are significantly associated with higher levels of depressive symptoms or with a depression diagnosis. However, cross-sectional studies cannot establish causality, and the methodology of these studies has been criticized. Due to the poor quality of the treatment studies, the effectiveness of vitamin D for depression cannot be adequately assessed. Current evidence does not definitively demonstrate that vitamin D deficiency is a cause of or risk for developing depression or that vitamin D is an effective therapy for depression.

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