Do you frequently feel fatigued, or experience muscle soreness, difficulty sleeping or concentrating, and feelings of apathy - especially during these winter months? If you answered yes to these questions, then you may be deficient in vitamin D. Vitamin D has long been known as the "sunshine vitamin," and for its role in maintaining strong bones. However, many people who live in northern climates don't realize that they are likely deficient in vitamin D.

Vitamin D does more than promote calcium absorption - it is involved in nerve signaling, immunity, and neuromuscular function. In other words, it helps the cells in your body communicate with each other. In addition, vitamin D may help protect against certain cancers and autoimmune diseases, regulate mood and reduce the effects of inflammatory diseases such as arthritis, heart disease, diabetes, and hypertension.

Vitamin D may be obtained from food or sunlight, though there are very few significant food sources (small amounts may be found in fatty fish, egg yolks, some cheese and mushrooms, and fortified cereals and dairy).

When skin is exposed to UVB rays, vitamin D is manufactured, with the help of cholesterol, and converted to its active form in the body by the kidneys. Vitamin D can stored in the body, but when the sun's rays are blocked out for months at a time - as they are in North Idaho during the winter - the body's stores can become severely depleted. In addition, since excess body fat causes vitamin D to become "locked up" in fat cells, reducing fat stores through exercise can increase availability of vitamin D.

Although in general, dietitians prefer people get their nutrients from food first, in the case of vitamin D, we take exception. People living in the Northwest or climates where sunshine is limited should consider taking a vitamin D supplement during the winter months. Taking vitamin D in the winter may help reduce Seasonal Affective Disorder and provide more energy and vitality.

The recommendations for vitamin D by the National Institutes of Health are 600 International Units for adults and 800 International Units for adults over 70. To put that into perspective, 10 minutes of sun exposure without sunscreen can produce over 10,000 IUs, a three-ounce portion of salmon contains 450 IUs, and a glass of vitamin D-fortified milk provides about 100 IUs. Many vitamin D experts deem the current recommendation too low, and suggest supplementation of 1,000-2,000 IUs/day or more. Vitamin D supplements in the form of D2 (ergocaliferol) or D3 (cholecaliferol) are equally effective; however, be sure to speak to your doctor before beginning supplementation if you are pregnant, on any long-term medications, or have a history of kidney stones. Supplementation up to 4,000 IUs/day is considered safe, though individuals who are extremely deficient will require more for several months. The only way to be certain of your own vitamin D status is to get your blood levels checked by a health-care provider, who can then discuss proper doses for you.

What can you do in the meantime to beat the moody blues on those cold, dreary winter days? Consider taking a vitamin D supplement in addition to eating plenty of vitamin D foods, and be sure to exercise frequently and practice stress relief.

Dr. SeAnne Safaii, Ph.D., RD, LD, is an assistant professor at the University of Idaho.