Today I read a study published in the American Journal of Cardiology in which the authors looked at vitamin D deficiency, supplementation and cardiovascular disease risk factors like high blood pressure and diabetes. The researchers (not surprisingly) found an association of vitamin D deficiency with an increased risk of cardiovascular disease and also with decreased survival time. So they looked a little closer at the patients who were deficient in vitamin D levels to see if taking vitamin D supplements had any impact on survival times. Turns out the vitamin D deficient-subjects who were taking vitamin D supplements tended to have increased survival times. This was an observational study and not a randomized control trial (which provides the highest form of evidence), so there was no specific dosage used and the subjects weren't required to take any vitamin D. So there was a wide range of dosages reported, from 1,000 International Units per day up to prescription forms of vitamin D taken at 50,000 International Units bi-weekly.
Interestingly, subjects who had cardiovascular disease but were not deficient in vitamin D according to blood tests, but took vitamin D supplements anyway didn't have the same increased survival times. We still have a lot to learn about any therapeutic benefits of vitamin D.
You don't usually get much vitamin D from the foods you eat, although milk is usually fortified with it, you're body makes vitamin D after your skin is exposed to sunlight.
Those of us who live north of a line drawn on a map from the northern border of California in the west to Boston in the east can't even get vitamin D from the sun during the colder months because the sun's rays just aren't strong enough to give us the UV exposure we need. So it's a good idea to take vitamin D supplements during the winter (or any time of the year if you avoid sun exposure).