There seems to be more and more evidence that the Vitamin D levels considered "normal" may in fact be suboptimal, especially for those who live at more Northern and Southern latitudes who may be chronically deficienct during the winter months especially.

And Vitiamin D does have a lot to do with the functioning of the immune system, besides its necessary functions of allowing calcium/magnesium/phosophorous to be regulated in the body and be taken up into bone. People with low Vitamin D are at risk for osteopenia/osteoporosis.

Unfortunately, there are not a lot of threads here about this issue, but some excellent ones on the Braintalk Vitamin/Mineral forum. (We keep trying to get some of the researchers over there, such as halsgluten and Ted Hutchinson, to come over here and share their material.)