UV-B sunlight strikes a substance on the skin called 7-dehydrocholesterol converting it into vitamin D3. The vitamin D created by this reaction is then reabsorbed by the skin and enters the bloodstream. Apparently some new research indicates reabsorption may take up to 48 hours!

Melanin inhibits the above reaction, so blacks need more sun to get the same amount of vitamin D.

More details: http://www.cholesterol-and-health.com/Vitamin-D.html