It turns out that vitamin D receptors are present in most if not all cells in the body. Additionally, experiments using cultured cells have demonstrated that vitamin D has potent effects on the growth and differentiation of many types of cells. These findings suggest that vitamin D has physiologic effects much broader that a role in mineral homeostasis and bone function. As one example, many immune cells not only express vitamin D receptors, but are capable of synthesizing active vitamin D, and deficiency in vitamin D has been associated with increased incidence of autoimmune disease and susceptibility to disease.
10. Cannel, MD, John Jacob, "The Truth About Vitamin D Toxicity," http:///. Published September 4, 2003. Accessed May 24, 2006. 11. Rosenfeld, Louis, "Vitamine - vitamin. The early years of discovery," Clinical Chemistry, Vol. 43 No. 4 (1997) 680-685. 12. DeLuca, Hector F., "Historical Overview," in Feldman et al., eds., Vitamin D, San Diego: Academic Press (1997) 3-11. 13. Dobs, et al., "Effects of pravastatin, a new HMG-CoA reductase inhibitor, on vitamin D synthesis in man," Metabolism, Vol. 40 No. 5 (1991) 524-8. 14. Folkers, et al., "Lovastatin decreases coenzyme Q levels in humans," Proc Natl Acad Sci USA Vol. 87 No. 22 (1990) 8931-4. 15. Heaney, Robert P., "The Vitamin D requirement in health and disease," Journal of Steroid Biochemistry & Molecular Biology, 97 (2005)13-19. 16. Cannel, MD, John Jacob, "The Vitamin D Newsletter: April 2006 -- Dr. Cannell Answers Readers' Questions," Vitamin D Council: http:///PDFs/April2006-. Published April 1, 2006. Accessed May 25, 2006. 17. Masterjohn, Chris, "Vitamin A on Trial: Does Vitamin A Cause Osteoporosis?" Wise Traditions , Spring 2006.
Vitamin D is very important for strong bones. Calcium and phosphorus are essential for developing the structure and strength of your bones, and you need vitamin D to absorb these minerals. Even if you eat foods that contain a lot of calcium and phosphorus, without enough vitamin D, you can’t absorb them into your body. Vitamin D is important for general good health, and researchers now are discovering that vitamin D may be important for many other reasons outside of good bone health. Some of the functions of the body that vitamin D helps with include:
Calcium metabolism appears to underlie neuronal cell death via excitotoxicity,     and hormonally active vitamin D confers a protective effect in vitro at physiologically relevant concentrations up to 100nM but not above.  This mechanism of protection appears to be mediated via a downregulation of L-type voltage-sensitive Ca 2+ ion channels,  an effect which has also been seen in bone cells.   These L-type channels have been implicated in excitotoxicity.  
The effects of vitamin D supplementation on health are uncertain.   A 2013 review did not find any effect from supplementation on the rates of disease, other than a tentative decrease in mortality in the elderly.  Vitamin D supplements do not alter the outcomes for myocardial infarction , stroke or cerebrovascular disease , cancer, bone fractures or knee osteoarthritis .   Low vitamin D levels may result from disease rather than cause disease. 
Dr. Cross is a researcher at the Medical University of Vienna. Recently, she has made the case that the reason people who eat soy products have lower rates of cancer is because one of the components of soy, genistein, blocks the enzyme that degrades vitamin D and thus raises tissue levels of activated vitamin D. She makes the case that cancer could not only be prevented, but possibly treated, not only by taking vitamin D, but by manipulating the enzymes that metabolize (build up) and catabolize (break down) vitamin D. It’s important to lay persons and non-medical professionals because the substances necessary to do all this (vitamin D and genistein) are available without a prescription. I should add that Dr. Cross does not recommend lay people do this on their own, she is making the case for further research. However, if you are fighting a serious cancer, the relative risk of taking 10,000 units of vitamin D and 1,000 mg of genistein every day is quite small compared to your impending death. 19
Calcium metabolism appears to underlie neuronal cell death via excitotoxicity,     and hormonally active vitamin D confers a protective effect in vitro at physiologically relevant concentrations up to 100nM but not above.  This mechanism of protection appears to be mediated via a downregulation of L-type voltage-sensitive Ca 2+ ion channels,  an effect which has also been seen in bone cells.   These L-type channels have been implicated in excitotoxicity.