Researchers have found that the production of previtamin D3 in your skin varies depending on several factors, which include skin type, weather conditions, and sunscreen use.
During the winter at altitudes above 35 degrees, there is minimal previtamin D3 production in the skin. Darker skin pigmentation, application of sunscreen, aging and clothing can also have a dramatic effect on previtamin D3 production.
However, at the other end of the scale, excessive exposure to sunlight does not result in vitamin D overdoes, because previtamin D3 and vitamin D3 are photolyzed to biologically inert chemicals before they can build up to dangerous levels.
* Eurekalert February 20, 2008