from Dr. Graveline's web site: http://www.spacedoc.net/statins_vitaminD.html

"...Our most important hormones depend upon adequate reserves of cholesterol for their production and nowhere is this more important than as the precursor substance for the synthesis of Vitamin D, known also as calcitrol.
"...A vitamin D precursor is synthesized in the skin from cholesterol in response to absorbing UVB rays. It then gets converted in the liver to an intermediate form. In the kidneys it joins with an important enzyme for conversion into its active hormonal form.
Many factors potentially interfere with the UVB conversion. People having darker skins are much more likely to have vitamin D deficiency. The aged skin of the elderly impairs cholesterol conversion as does the presence of obesity. Our present day emphasis on protecting our skin from the sun, using sun-screens and blockers, also cuts down on the ability of UVB to convert cholesterol to vitamin D. Last but not least, one needs UVB exposure.

Without any sun exposure you need about 4,000 units of vitamin D a day. In the absence of other supplements you would need 40 glasses of milk or ten multi-vitamins capsules daily to supply your vitamin D needs. Most of us make about 20,000 units of vitamin D after 20 minutes of summer sun due to UVB conversion of cholesterol. Numerous studies document that the majority of our society falls short of meeting either their dietary or UVB conversion needs for vitamin D..."