New research suggests that a lack of sunlight may adversely impact mental health.

An international team of researchers, led by the University of Georgia found that vitamin D deficiency could have a regulative role in the development of seasonal affective disorder (SAD).

An international research partnership between the University of Georgia, the University of Pittsburgh and the Queensland University of Technology in Australia conducted a review of more than 100 leading articles and found a relationship between vitamin D and seasonal depression.

"Seasonal affective disorder is believed to affect up to 10 percent of the population, depending upon geographical location, and is a type of depression related to changes in season," Alan Stewart, an associate professor in the department of counseling and human development services. "People with SAD have the same symptoms every year, starting in fall and continuing through the winter months."

Stewart said, based on the team's investigations, vitamin D was likely to be a contributing factor in seasonal depression.

"We believe there are several reasons for this, including that vitamin D levels fluctuate in the body seasonally, in direct relation to seasonally available sunlight," he said.

Vitamin D is also involved in the synthesis of serotonin and dopamine within the brain, both chemicals linked to depression, according to the researchers.

"Evidence exists that low levels of dopamine and serotonin are linked to depression, therefore it is logical that there may be a relationship between low levels of vitamin D and depressive symptoms," said Michael Kimlin, a Cancer Council Queensland Professor of Cancer Prevention Research. "Studies have also found depressed patients commonly had lower levels of vitamin D."

Vitamin D levels varied according to the pigmentation of the skin. People with dark skin often record lower levels of vitamin D, according to the researchers.

"Therefore it is suggested that persons with greater skin pigmentation may experience not only higher risks of vitamin D deficiency, but also be at greater risk of psychological and psychiatric conditions," he said.

Researchers said adequate levels of vitamin D were essential in maintaining bone health, with deficiency causing osteomalacia in adults and rickets in children. Vitamin D levels of more than 50 nanomoles per liter are recommended by the U.S. Institute of Medicine.

"What we know now is that there are strong indications that maintaining adequate levels of vitamin D are also important for good mental health," Kimlin said. "A few minutes of sunlight exposure each day should be enough for most people to maintain an adequate vitamin D status."

The findings are detailed in the journal Medical Hypotheses.