This essential vitamin is involved in calcium absorption and plays a part in the development and maintenance of strong bones. However, it has many other non-bone-related functions which are important to your health. In fact, studies conducted by the NIH have shown that a large percentage of the population is vitamin D deficient, and that deficiency is associated with mood disorders and with cognitive dysfunction.
Now, the best source of vitamin D is the sun (your skin synthesizes the vitamin in the presence of solar radiation); however, to make enough vitamin D, you need to be getting about 20 to 30 minutes of direct sunlight several times per week, more if you are darker skinned. And hold the sunblock, which blocks out helpful rays along with the harmful ones. If you fail to get enough sunlight, you may find your vitamin D levels to be less than optimal, since food sources of this essential vitamin are scarce (fatty fish, fortified cereals and other vitamin-enriched foods about round out dietary sources).
You may consider getting your levels checked. Vitamin D deficiency is defined as a 25OHD level less than 24 ng/mL. Optimal levels are between 25 ng/mL and 80 ng/mL. If you can't get to the doc and believe your levels may be low, it is a good idea to go ahead and take a supplement. Make sure to take vitamin D3, which is also called cholecalciferol. Aim for 2,000 IUs per day. Don't worry about taking too much. Toxicity is not seen in levels up to 10,000 IUs per day, which is much much more than you'll get in your standard vitamin tablet.
Vegans should keep in mind that most supplemental D3 is derived from animal sources, most often cold-water fish (plant-based D2 is available but may not be as effective). If you are against ingesting the small amount of fish derivative found in the capsule, opt for plant-based D3 products, which are harder to find but available.
And take note that the health benefits of vitamin D extend far beyond the realms of bone and mood. The vitamin is a powerful immunomodulator that may be beneficial in the prevention and treatment of a variety of conditions, from autoimmune disorders to cancer. In a recent article published by the American Journal of Clinical Nutrition evaluating the prevalence of breast cancer in Saudi Arabian women, subjects with lower levels had the highest rates of breast cancer, supporting the cancer-protective benefit of this very important nutrient. And in another study, higher concentrations of vitamin D were associated with lower risk of death from heart disease, lung disease, and cancer. So D up!