Low vitamin D levels have been linked to depression, manifested, in part, by low serotonin levels. The connection has been made through years of survey and research data which have shown that low or deficient vitamin D levels are directly correlated with both the presence and the severity of depression.
One small study showed that women with moderate to severe depression who took vitamin D supplements for eight to 12 weeks reported substantially improved symptoms after taking vitamin D. More studies are being undertaken to examine the effect of vitamin D supplementation on those with depression.