According to the Vitamin and Supplement guide:

Can you get enough vitamin D from sunlight alone to avoid vitamin D deficiency symptoms?
If you live north of 40 degrees north latitude (about the latitude of New York City) or south of 40 degrees south latitude (about the latitude of Wellington, New Zealand), for at least half of the year, you can’t.
In Boston, Massachusetts, for instance, there is insufficient ultraviolet radiation in the sunlight for skin to make adequate amounts of vitamin D from early November to early March. On the South Island of New Zealand, vitamin D deficiency is a real risk from early May to early September. In Calgary, Alberta, residents don’t get enough vitamin D even if they get daily sun exposure from October until nearly April. And in northern Europe, Alaska, and Russia, the problem is even worse.

Go ahead and do a Google search on Vitamin D and you’ll notice a lot of new research that points to D being important on a number of levels and indications that it can help safeguard against many maladies (fatigue, chronic pain, cancer, MS, obesity, stroke, etc.) can be found all over the net.

All that said, I am taking supplements because I went to a doctor and was tested and prescribed 5,000 IU, daily. DO NOT take my word for it, especially if you’re on any medication. However, if you find yourself down in the dumps, getting frequent colds or just want to make sure you’re as healthy as you can be, go to your doctor and get your vitamin D levels checked. I’ve only been on the D for a few days and I’ll let you know how I feel in a couple of weeks!