The body needs about 5-30 minutes worth of exposure to Ultra Violet B rays ( UVB rays) from the sun at least twice a week in order to produce enough vitamin D. The body then uses it to promote healthy bones and teeth, regulate cell growth, hormonal balance, and a healthy immune system. The UVB rays are weaker in the winter and in the northern part of the country are not sufficient to produce enough Vitamin D.
If you cannot receive adequate amounts of vitamin D from the sun, nature has provided an answer with vitamin D naturally occurring in certain foods. The top three are cod liver oil, raw herring, and dried shiitake mushrooms. Other great sources are button mushrooms, salmon, and other fatty fish. The Scandinavian culture eats an abundance of fatty fish and is able to avoid vitamin D deficiency.
To avoid the winter blues, remember to get plenty of vitamin D by either getting a few hours of sun exposure a week or by eating food rich in the vitamin.
Please note that this information is not meant to substitute for medical advice. Please consult with your doctor.