Winter weather got you down? You're not alone. Research shows that up to 10 percent of Americans experience a condition called seasonal affective disorder, a form of depression that often begins in the fall and continues through the winter months. Shortened days and limited access to daylight are often listed as the causes for this condition. But now researchers think they may have found another culprit: low levels of vitamin D.

A new study led by researchers from the University of Georgia, the University of Pittsburgh, and Queensland University of Technology in Australia found that people with low vitamin D levels were at greater risk for developing seasonal affective disorder, or SAD

Previous research has linked SAD to a lack of sunlight in the winter months. That's why the condition is often called the winter blues. One theory is that reduced sunlight exposure diminishes the ability of the body's internal clock to regulate mood, sleep and hormones. Other research has suggested that limited sunlight causes an imbalance of the neurotransmitters dopamine and serotonin, chemicals in the brain that regulate mood.

But this new study suggests that it may be a lack of vitamin D that's the real culprit behind the increase in depression. 

Alan Stewart, a professor at the University of Georgia and a lead researcher for the study noted, "studies show there is a lag of about eight 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."

Vitamin D is also known to play a role in the production of dopamine and serotonin. So it seems likely that decreased levels of the vitamin could lead to symptoms of depression. 

Low levels of vitamin D have also been linked to an increased risk of asthma, cancer and chronic pain, among other conditions. So if you are feeling low this winter, it's worth talking to your health care provider about your vitamin D levels and what you can do to make sure you get enough.

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