A nice article about vitamin D in the NY Times.
Studies indicate that the effects of a vitamin D deficiency include an elevated risk of developing (and dying from) cancers of the colon, breast and prostate; high blood pressure and cardiovascular disease; osteoarthritis; and immune-system abnormalities that can result in infections and autoimmune disorders like multiple sclerosis, Type 1 diabetes and rheumatoid arthritis.
Most people in the modern world have lifestyles that prevent them from acquiring the levels of vitamin D that evolution intended us to have.
I have taken 5000IU of vitamin D3 daily for the past year and a half. This is significantly above normal dietary recommendations, but has been suggested to be safe in one study cited below. I do not recommend taking such doses without a doctor’s supervision and checking blood levels.