likely to experience depression.
The research, published in the journal Biomolecules and Biomedicine, analyzed evidence from 66 studies across 31 countries.
The results show that when vitamin D levels drop below 30 nanomoles per liter, depression rates increase sharply. However, scientists stress that the findings do not prove that low vitamin D directly causes depression—it may only be linked with it.
Depression affects around one in twenty adults worldwide and is expected to become the leading cause of illness by 2030. Although antidepressant medications help many people, their overall benefits are often only moderate.
Because of this, researchers are exploring other possible biological factors that could be adjusted to improve mental health, and vitamin D is one of the most promising candidates.
Vitamin D is sometimes called the “sunshine vitamin” because our bodies make it when exposed to sunlight. It plays many important roles beyond maintaining healthy bones. The brain contains vitamin D receptors in areas that control mood and emotions, such as the hypothalamus and pons.
The active form of vitamin D, called 1,25-dihydroxy-vitamin D, helps the brain by reducing inflammation, supporting nerve growth, and protecting cells from damage caused by oxidative stress. These processes are all linked to how depression develops and affects the brain.
In the 46 studies that looked at people’s vitamin D levels and depression at a single point in time, lower vitamin D was strongly associated with higher depression scores or clinical depression diagnoses. The pattern was especially strong when vitamin D levels were 30 nanomoles per liter or less.
But when researchers looked at 10 studies that followed people over time, the results were mixed. Some studies found that people with low vitamin D were more likely to develop depression later, while others—especially large population studies—did not find this connection.
One challenge the researchers faced was that the studies used many different ways to measure both depression and vitamin D. They identified eight different depression questionnaires, including PHQ-9, CES-D, and BDI, and several types of lab tests for vitamin D.
Because of these differences, the researchers could not combine all the results into a single statistical analysis, so they instead summarized the evidence in a detailed narrative review.
They also noticed that not all studies controlled for important factors like sunlight exposure, body weight, and existing health conditions, which might have affected the results. Interestingly, some research suggested that the link between vitamin D and depression may be stronger in women, but more studies are needed to confirm this.
According to Vlad Dionisie, Ph.D., an assistant professor at Carol Davila University of Medicine and Pharmacy, the findings suggest a practical approach.
He said, ‘Our advice is cautious but useful: test vitamin D levels in adults with depression and correct any clear deficiency for general health reasons—while continuing to study whether this can truly prevent depression.’
The research team followed PRISMA-2020 guidelines for systematic reviews and registered their work in the PROSPERO database under number CRD42024515918. They recommend that future studies should track vitamin D levels over time, include measures of sunlight exposure, and consider genetic factors that may affect how vitamin D works in the body.
They also call for randomized clinical trials to see whether correcting low vitamin D can actually help prevent depression in people who start out healthy.
In summary, this study strengthens the evidence that vitamin D deficiency and depression often go hand in hand, though scientists are still not sure which comes first.
Low vitamin D might not directly cause depression, but maintaining healthy levels could still support better brain function and mood. Until more data are available, checking and correcting vitamin D deficiency remains a simple and safe step that may improve both physical and mental well-being.
If you care about mental health, please read studies about 6 foods you can eat to improve mental health, and B vitamins could help prevent depression and anxiety.
For more health information, please see recent studies about how dairy foods may influence depression risk, and results showing Omega-3 fats may help reduce depression.
The study is published in Biomolecules and Biomedicine.
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