Vitamin D is fat soluble. Yes, some is made from sunlight, but oral supplements appear to have a more measurable intake on blood parameters than does exposure to UV-B wavelengths of light.

Vitamin D is a steroid hormone, derived from cholesterol.

Yes, the RDA is defined as that level of intake that prevents *overt* deficiency symptoms in 97.5% of normal healthy people. That means that 1 in 40 normal healthy people are still obviously deficient at that level of intake. Moreover, no allowances are made for people of poor health.

Here are two references for that 4000 IU level. http://www.ncbi.nlm.nih.gov/entrez/q..._uids=15260882
http://www.ncbi.nlm.nih.gov/entrez/q..._uids=11157326

And, just for context, here's one using 100,000 IU dosing. (There are others, but here's a recent example.) http://www.ncbi.nlm.nih.gov/entrez/q..._uids=12609940

I want to say that I am not advocating taking 4000 IU/day. Under controlled medical observation, no adverse effects were noted, at that dose. In fact, there was a linear dose-response effect. I see no reason to limit oneself to the RDA, however. In fact, rural Norwegians who eat cod livers as part of their diet exceed the RDA 50-fold as a matter of routine. Your mama trying to feed you cod liver oil as a child is probably based in good and healthy cultural wisdom.

Lar