Vitamin D has long been known as the "sunshine" vitamin since your body naturally absorbs the vitamin from the sun's rays. It helps promote bone growth and regulate neuromuscular and immune systems.

Now, researchers from the The Irish Longitudinal Study on Ageing (TILDA) at Trinity College Dublin revealed through their findings that vitamin D can also help stave off depression.

TILDA followed a group of Irish senior citizens for four years and discovered that those with a vitamin D deficiency had a 75 percent increased risk of developing depression. They even took into account participants who were taking anti-depressant medication and vitamin D supplements, and the results did not change.

“This is highly relevant for Ireland as our previous research has shown that one in eight older adults are deficient in the summer and one in four during the winter. Moreover, only around eight percent of older Irish adults report taking a vitamin D supplement," wrote the study's senior author, Dr. Eamon Laird. "Given that vitamin D is safe in the recommended intakes and is relatively cheap, this study adds to the growing evidence on the benefits of vitamin D for health."

What about people with a history of depression?

A 2012 study out of the University of Texas Southwest’s Medical Center and The Cooper Institute in Dallas examined the vitamin D test results of nearly 12,600 participants from late 2006 to late 2010. Vitamin D testing is now a regular part of physical exams.

The results found that high levels of vitamin D were associated with lower chances of developing depression, especially among those patients who had a prior history of depression.

Low levels of vitamin D in study participants were associated with depressive symptoms, and even more so among those participants with a history of depression.

"Our findings suggest that screening for vitamin D levels in depressed patients — and perhaps screening for depression in people with low vitamin D levels — might be useful," said Dr. E. Sherwood Brown, professor of psychiatry at UT Southwestern and senior author of the study.

The link between vitamin D and depression remains elusive. Brown and his team say they are not sure if depression causes lower vitamin D levels or if lowered vitamin D levels cause depression.

“We don't have enough information yet to recommend going out and taking supplements,” he said.

But the vitamin D may affect neurotransmitters and other factors associated with depression.

Vitamin D levels have been connected to a host of other medical problems including autoimmune disease, obesity, diabetes and neurological disorders.

The study was published in The Mayo Clinic Proceedings.

Editor's note: This article has been updated since it was originally published in January 2012.

Vitamin D deficiency linked to depression
Researchers discover that an overwhelming number of older adults with low vitamin D levels suffer from depression.