The basis of RDA vitamins/supplements is the amount that will keep you from getting a deficiency disease. Thus usually 100% of RDA is still too little to be healthy on. Many studies on vitamins have been done and the ROD (Recommended Optimum Dose) is usually many times greater than the RDA.

Robert
It's interesting that there is a crisis of vitamin D deficiency in England and other northern countries. Further, you need 4000 IU of vitamin D from all sources, including the sun to be healthy. Too, they've found that winter sun above the 33 paralell is not adequate for vitamin D production and that summer sun in these places is not strong enough to ensure adequate vitamin D either.

Also it's a fallacy and one of those scientific lies about vitamin D toxicity that just gathers strength and becomes truth the more it gets repeated. Man-made vitamin D, D2 has been found toxic at 20,000 IU/day where natural vitamin D, D3 has been found safe at doses of 50,000 IU/day. That safety was proven with a study that covered four years.

Coincidently, man-made vitamin A has also been found toxic at 20,000 units/day while natural vitamin A has been found safe at 35,000 units/day. Further, it's been found that vitamin D buffers vitamin A potency so that a larger amount is safer when taken in conjunction with vitamin D.