The researchers noted that vitamin D supplementation had an overall positive effect on the 1,116 subjects diagnosed with major depressive disorder (MDD), also known as clinical depression.

Vitamin D supplements also had a positive effect on depressive symptoms among 407 people with perinatal depressionTrusted Source.

The researchers further found that patients who used antidepressants alongside vitamin D supplements experienced small yet statistically significant positive effects.

However, they found placebo was slightly more beneficial than vitamin D supplements in a subgroup of healthy individuals without a diagnosis of clinical depression.

They noted this could be because a reduction in depression symptoms was more challenging to detect when the baseline symptoms levels were already very low in this subgroup.

Vitamin D supplements also had no significant effect on seasonal affective disorder (SAD) among a subgroup of three studies and did not seem to affect depressive symptoms in older adults.

The researchers also acknowledged that their results carry a “very low certainty,” as 36.6% of studies had a high risk of bias, and overall results were very diverse.

When asked how vitamin D supplementation may be linked to inflammation and depression, Dr. Monique Aucoin, ND, MSc, naturopathic doctor and senior research fellow at the Canadian College of Naturopathic Medicine, not involved in the study, told Medical News Today:

“The exact mechanism of how vitamin D impacts the brain and mental health is not entirely clear. There is evidence that vitamin D may modulate, or balance, parts of the immune system that are involved in inflammation.”

Dr. Aucoin added that an emerging body of evidence suggests that mental health disorders may be related to increased inflammation in the body and that a reduction of inflammation levels may have a therapeutic benefit.

Tuomas Mikola, a doctoral researcher at the Institute of Clinical Medicine at the University of Eastern Finland and lead author of the study, explained to MNT:

“Depression has been associated with low-grade inflammation in the form of elevated cytokine levels. The body’s innate and adaptive immune responses have been shown to be partially dependent on circulating vitamin D levels. Hyperactivity of the hypothalamic-pituitary-adrenal axis (HPA axis) and hypersecretion of the stress hormone cortisol are common physiological abnormalities in patients with depression. Vitamin D may regulate the secretion of cytokines in the immune system and normalize the secretion of cortisol.”

The researchers concluded that further studies should investigate the possible benefits of augmenting standard treatment options for depression with vitamin D supplements.

When asked about the study’s limitations, Dr. Aucoin said that one limitation of the new research is that among the studies with a low risk of bias, no antidepressant effect was observed.

“While these studies make up a small sub-section of the research, it suggests that more high quality studies are needed before clinical recommendations can be made,” she noted.

“The authors of this meta-analysis looked at the data in many different ways to identify which factors (like participant diagnosis, vitamin D status, or vitamin D dosage) impacted the results. However, because most of the studies had unique combinations of these different factors, it was very hard for the researchers to clearly pinpoint who is most likely to benefit or which dose of vitamin D treatment might be most effective.”

According to Dr. Mikola, another limitation of the new research is that their meta-analyses “focused on change in depressive symptom scores rather than examining the associations between serum vitamin D levels and depressive symptoms.”

When asked whether vitamin D supplementation could be used alongside other treatments for depression or as an alternative, Dr. Aucoin noted that it is hard to draw any conclusions due to a lack of current research available.

She added, however, that “there are no concerns about providing a vitamin D supplement to a person who is also receiving other treatments for depression.”

“In this new meta-analysis, I was surprised to see a smaller antidepressant effect from vitamin D supplementation in studies where participants were also taking antidepressants. However, as the authors suggest, this is likely due to other characteristics of the participants in those studies.”