Vitamin D is an important vitamin your body needs to stay healthy. Most people don’t get enough vitamin D, because our primary way of making it is through exposure to sunshine (without sunscreen). Lack of of vitamin D — vitamin D deficiency — has been implicated in numerous health problems, including mood disorders like depression.

What is the relationship between vitamin D and depression? Can a simple vitamin D deficiency be the cause of my depressed mood? It’s more complicated than it looks.

The Mixed Evidence on Vitamin D & Mood

There have been more than a few research studies that have examined the impact of vitamin D on depression and other mood disorders. Observational studies have generally found a correlation, but could not determine which way the relationship went (e.g., does depression contribute to low vitamin D levels in the body, or does low vitamin D levels contribute to depression?).

For instance, one set of researchers conducted a systematic review and meta-analysis in 2013 (Anglin et al.). They looked at one case-control study, ten cross-sectional studies and three cohort studies. (Notice the lack of randomized-controlled trials (RCTs) versus observational studies?) “Our analyses are consistent with the hypothesis that low vitamin D concentration is associated with depression,” but acknowledged their findings weren’t based on any RCTs.

Randomized-controlled trials (RCTs) are the gold standard in drug and supplements research. They compare the effectiveness of the drug or supplement with a sugar pill, what researchers call a placebo.

Earlier this year, a study was published that did look at the findings of RCTs in connection with depression and vitamin D. This study examined 10 randomized trials (nine were randomized placebo-controlled trials [RCTs]; one was a randomized blinded comparison trial) and 20 observational (cross-sectional and prospective) studies (Okereke & Singh, 2016). What did the researchers find?

In 13 of the observational studies, they found a correlation between vitamin D deficiency and mood (e.g., depression). But in the placebo-controlled, randomized trials — the gold standard of drug and supplemental research — they found something very different.

“Results from all but one of the RCTs showed no statistically significant differences in depression outcomes between vitamin D and placebo groups.” In other words, the group of people who received vitamin D supplements didn’t differ significantly from the group of people who received a sugar pill (placebo) on their depression scores. This suggests that vitamin D supplements don’t really help much, if at all.

Another big study published in 2014 — the Pro V.A. Study — also examined vitamin D concentration levels in 1,039 women and 636 men aged 65 and older (Toffanello et al., 2014). Their findings were also not good. “Although an independent inverse association between 25OHD levels and GDS scores emerged for women on cross-sectional analysis, vitamin D deficiency showed no direct effect on the onset of late-life depressive symptoms in our prospectively studied population.” In other words, while they found a small effect in women (a one point difference in depression scores), the differences overall were not significant.

What This Means for Depression & Vitamin D

Contrary to conventional wisdom, it appears that the association between depression and vitamin D is a small, tenuous one at best. The most recent studies seem to suggest that the believed connection between vitamin D deficiency and depressive mood either doesn’t exist, or is simply a small correlation.

Regardless, vitamin D is important to your overall health. There are other studies demonstrating its impact on reducing blood pressure, hypertension, risk of MS, and even Type 1 diabetes (Webb, 2015). It also appears important for bone health in general, and long-term deficiency in vitamin D has been linked to osteoporosis (Webb, 2015).

You can get a lot of your vitamin D by just spending a little time each day outdoors in most places. However, in colder seasons or climates, that may not always be possible. Vitamin D supplements can be obtained over-the-counter and are a safe way to increase your vitamin D serum levels.

However, according to the latest research, taking vitamin D supplements alone aren’t likely to change your mood. If you expect it to work like an antidepressant drug, you may be in for a surprise.