A couple of URLs well worth reading:
Robert P Heaney
Lessons for nutritional science from vitamin D
American Journal of Clinical Nutrition,
Vol. 69, No. 5, 825-826, May 1999
http://www.ajcn.org/cgi/content/full/69/5/825 (free)
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R.P. Heaney (John A. Creighton University Professor, Creighton
University, Omaha, Nebraska, United States).
The roles of calcium and vitamin D in skeletal health: an evolutionary
perspective
http://www.fao.org/docrep/W7336T/w7336t03.htm