A new study has just been published in JAMA (the Journal of the American Medical Association) that seeks to answer this question definitively. This subject has been somewhat controversial, because observational studies have tended to show a positive effect, while randomised clinical trials have failed to show any effect. Usually you would trust the trial data more than the observational data, but these trials were all relatively short (generally a year or less), had relatively small study groups, and often used quite low doses of vitamin D.
That is why this new study is so interesting. It took the form of a double-blind randomized clinical trial and it included 18,353 participants, a nice big number that should definitely be large enough to show effect. 9,181 individuals were randomized to receive 2,000 IU of vitamin D daily, and 9,172 were randomized to placebo. 2,000 IU is a good size dose that should definitely be enough to show effect (the recommended daily allowance (RDA) for older adults is 800 IU). Participants were followed for an average of five years, and were contacted once a year and asked to answer questions about whether they had been diagnosed with depression during the preceding year, and also asked to fill out a form evaluating current symptoms of depression.
The participants were all over 50. This is a bit odd, since younger people get depressed too. My guess as to why they chose to limit it to this age group would be that this trial was done as part of a bigger trial to see the effects of vitamin D and omega 3 on cancer and heart disease, and older people are much more likely to develop these diseases, so it’s easier to see a statistically significant effect in a study that only includes older people. A problem with doing this is, of course, that the data can’t be generalized to young people with absolute certainty (although I think it’s likely that young and old people respond similarly to vitamin D).
The study was financed by the NIH (National Institutes of Health), which generally means the data are much more reliable than if the study had been financed by a supplement manufacturer. Some of the authors had financial ties to companies that make supplements, which would normally make a study suspect in my eyes, but in this case I think the results can actually be trusted, for reasons that will become clear later in this article.
Ok, on to the results. In the vitamin D group, 6.6% of participants developed depression over the five year period. In the placebo group, 6.8% of participants developed depression. This 0.2% absolute difference was not statistically significant. Basically, there was no difference whatsoever in rates of depression between the groups.
After the main analysis, a sub-group analysis was performed to see if there were smaller groups within the study who did benefit from vitamin D. The most important sub-group, where you would most expect to see a difference, is those who were vitamin D deficient at the beginning of the study. Among people with a deficiency at the beginning of the study, 7.7% developed depression in the vitamin D group, compared with 6.3% in the placebo group. This marginal 1.4% difference in favour of placebo was not statistically significant. Similar results were found for all sub-groups. There was no statistically significant difference between the treatment and placebo arm in any of the sub-groups analyzed.
Can the results be trusted? In my view, yes. This is the highest quality study looking at using vitamin D to prevent depression yet carried out. It involved a large number of patients and followed them over an extended period, and the dose of vitamin D given was high enough that a response could have been expected. The fact that the results were negative in spite of the fact that some of the authors had a clear financial incentive in getting a positive result, also strengthens the case. Basically, this was a definitive study.
Conclusion: vitamin D supplementation does not have any role to play in the prevention of depression. It does however have other roles. For example, check out my article on vitamin D to prevent respiratory infections.