I Don't Believe in Imaginary Property
writes "Members of this community may want to venture out of the basement more often, because Dr. Harald Dobnig and his team have found that vitamin D deficiency leads to increased mortality. These results still hold when they take into account such factors as exercise and heart disease. Low vitamin D status has 'other significant negative effects in terms of incidence of cancer, stroke, sudden cardiac death and death of heart failure,' Dr. Dobnig said. The evidence of ill effects from low vitamin D 'is just becoming overwhelming at this point.' Vitamin D3 is usually produced by exposure to the UV-B in sunlight, but in high latitudes, especially in the fall and winter, insufficient UV-B gets through the atmosphere to produce enough vitamin D3, even with hours of exposure. The researchers are recommending that people at risk for deficiency take 800 IU of vitamin D3 daily. Just don't go overboard — as a fat-soluble vitamin, D3 is more capable of causing adverse effects at unnaturally high dosages. The human body tops out at producing about 10,000 IU per day."
According to the Wikipedia entry linked above, the D2 (ergocalciferol) version -- available as a vegan product -- works approximately as well to supply humans with their needed vitamin D.