Mercola has a calculator in his current newsletter for amount of time of sun exposure under conditions you plug in to achieve manufacture of vitamin D to the tune of 1,000 IU. For where I live and my skin color I have to be in the sun for about an hour. So you see, we indeed would have to run around naked all day to get enough D, apparently! The calculator is easy to use and he gives reference to finding all the values you need to plug in. It only took me a few minutes.
http://articles.mercola.com/sites/a...-You-Think.aspx