Now a new meta-analysis of 41 previous studies suggests that taking vitamin D supplements can relieve depressive symptoms in people already diagnosed with depression, opening up a potential alternative option for treatment.
As well as controlling levels of calcium and phosphate in the body, it's thought that vitamin D helps to regulate various functions in the central nervous system – and earlier research on animals suggests it could even contribute to the control of chemical balances in the brain, which may explain the association between vitamin D and mental health.
"These findings will encourage new, high-level clinical trials in patients with depression in order to shed more light on the possible role of vitamin D supplementation in the treatment of depression," says Tuomas Mikola, doctoral researcher and lead author at the University of Eastern Finland.
The new meta-analysis covered a total of 53,235 study participants from 41 studies, including those with and without depression, people taking vitamin D supplements and people taking placebos, and individuals with a variety of physical conditions.
While the doses used varied, the typical vitamin D supplement was 50-100 micrograms a day. In the participants with depression, vitamin D supplements were shown to be more effective than placebos at alleviating depressive symptoms.
Vitamin D supplements seemed to be most effective in shorter bursts (under 12 weeks), the researchers report. However, in healthy individuals, it was placebos that had a slightly greater impact on depressive symptoms.
"Our results suggest that vitamin D supplementation has beneficial effects in both individuals with major depressive disorder as well as in those with milder, clinically significant depressive symptoms," write the researchers in their published paper.
With depression now recognized as the leading cause of disability worldwide – affecting over 280 million of us every year – and antidepressants not effective for everyone, more treatment options need to be explored urgently.
However, before we get ahead of ourselves, the data we have so far isn't enough to prove that low vitamin D levels cause depression, or that supplements are an effective treatment. Even though this new meta-analysis shows a link, previous research hasn't been quite so conclusive.
While a meta-analysis like this is helpful in comparing results across a large number of people, the different approaches and factors in each individual study make it more difficult to draw broad conclusions – even though a lot of work is done to correlate information across the studies as a whole.
Yet more statistical crunching will be required to know what the story is for sure: via studies of larger general and clinical populations, and by observing different dose amounts and different treatment durations, for example.
"Despite the broad scope of this meta-analysis, the certainty of evidence remains low due to the heterogeneity of the populations studied and the due to the risk of bias associated with a large number of studies," says Mikola.
The research has been published in Critical Reviews in Food Science and Nutrition.