Researchers say vitamin D deficiency could be a driver of seasonal affective disorder - a form of depression that occurs during the winter months.
The research team - led by Alan Stewart of the College of Education at the University of Georgia - publish their findings in the journal Medical Hypotheses.
Seasonal affective disorder (SAD) - a form of depression that usually begins in the fall, continuing throughout the winter months - affects up to 10% of the US population. Symptoms include feeling sad or anxious, fatigue, concentration problems, irritability and feelings of guilt and hopelessness.
Although the exact cause of SAD is unclear, numerous studies have suggested the condition may be triggered by lack of sunlight. SAD is more common among people who live at high latitudes or areas with lots of cloud.
One hypothesis behind SAD is that reduced sunlight exposure interferes with the body's biological clock that regulates mood, sleep and hormones. Another theory is that lack of sunlight causes an imbalance of neurotransmitters - such as dopamine and serotonin - which regulate mood.
A link between vitamin D deficiency and depression is 'logical'
In this latest study, Stewart and colleagues present the idea that vitamin D deficiency may be behind all of the aforementioned theories related to SAD.
- Onset of SAD is most common in early adulthood
- Around 75% of individuals with SAD are women
- Treatments for SAD include antidepressants, light therapy or a combination of both.
"We hypothesize that rather than functioning primarily as a proximal or direct sub-mechanism in the etiology of SAD, vitamin D likely functions in a more foundational and regulative role in potentiating the sub-mechanisms associated with the depressive and seasonality factors," say the researchers.
Firstly, the researchers note that vitamin D levels in the body fluctuate with the changing seasons in response to available sunlight.
"For example," says Stewart, "studies show there is a lag of about 8 weeks between the peak in intensity of ultraviolet (UV) radiation and the onset of SAD, and this correlates with the time it takes for UV radiation to be processed by the body into vitamin D."
Co-author Michael Kimlin, of the Queensland University of Technology in Australia, says that vitamin D also plays a part in the synthesis of both dopamine and serotonin, noting that past research has associated low levels of these neurotransmitters with depression.
"Therefore," he adds, "it is logical that there may be a relationship between low levels of vitamin D and depressive symptoms. Studies have also found depressed patients commonly had lower levels of vitamin D."
The researchers also believe there is a link between skin pigmentation and vitamin D levels, which may affect an individual's risk for SAD. They explain that studies have shown that people with darker skin pigmentation are at greater risk of vitamin D deficiency, and if such individuals relocate to high-latitude regions, they may have a higher chance of developing SAD.Commenting on their theories, Kimlin says:
"What we know now is that there are strong indications that maintaining adequate levels of vitamin D are also important for good mental health. A few minutes of sunlight exposure each day should be enough for most people to maintain an adequate vitamin D status."
Vitamin D and schizophrenia
This is not the first study to associate vitamin D deficiency with mental health conditions. In July, Medical News Today reported on a study suggesting low vitamin D levels may increase the risk of schizophrenia.
Ahmad Esmaillzadeh, PhD, of the Isfahan University of Medical Sciences in Iran, and colleagues found that study participants with vitamin D deficiency were 2.16 times more likely to have schizophrenia than participants with normal vitamin D levels.
"Our findings support the theory that vitamin D may have a significant impact on psychiatric health," said Esmaillzadeh. "More research is needed to determine how the growing problem of vitamin D deficiency may be affecting our overall health."