"Why not just take the supplements and forget about the sun?

Vitamin D made in the skin lasts at least twice as long in the blood as vitamin D ingested from the diet.  When you are exposed to sunlight, you make not only vitamin D but also at least five and up to ten additional photoproducts that you would never get from dietary sources or from a supplement.

Old Mother Nature is pretty parsimonious with her creations, and I suspect she wouldn’t have five to ten photoproducts circulating around if they didn’t do something good for us.  Just because we aren’t advanced enough yet to figure out what it is they do, doesn’t mean they don’t do something.  Thus Dr. Holick’s recommendation to hit the sun if at all possible instead of the supplement bottle.

Plus, there are some downsides to indiscriminately throwing back the supplements without monitoring your 25 (OH)D levels.  See here and here, for example."

http://www.proteinpower.com/drmike/supplements/sunshine-superman/#more-4165