[Ragnar54]

Actually, you need a lot more than 1000 IU per day.

Immunologic Effects of Vitamin D on Human Health and Disease

“Although most of the biologic effects of vitamin D have been related to its active metabolite there continues to be intriguing evidence that vitamin D itself may have its own biologic actions independent of its metabolism. Our hunter gatherer forefathers likely maintained serum vitamin D levels in the range of 10–50 ng/mL (25–125 nmol/L). This is supported by the observation that Maasai herders and Hadza tribesmen maintained serum 25(OH)D in the range of 40–60 ng/mL (100–150 nmol/L) [12,13]. To maintain these blood levels, a person would require ingesting approximately 4000–6000 IUs daily."

[RayChuang88]

I kind of don't encourage people to take that much Vitamin D because high levels of the vitamin can be toxic, since Vitamin D is a fat-soluable vitamin. 1,000 IU per day is enough for the vast majority of people out there.