I just finished reading an interesting article in the September 2006 Reader's Digest about vitamin D. I won't elaborate on the importance of vitamin D except to say that researchers are finding out that it is more important than they realized, and the MDA of 400 IU isn't enough. The article is available online at The Miracle Vitamin.
The article discussed the importance of getting vitamin D from the sun as well as from supplements, and it suggested that we spend 10-15 minutes in the sun each day before we apply sun screen. Fair skinned people may need to spend less time in the sun. When I run, it takes me 10-15 minutes to warm up, and I thought I'd try running without sunscreen for that time. After 10-15 minutes of running, I'll stop and apply sunscreen to my arms and legs. When I run, I wear a wide brimmed hat that shades my face, and I don't put sunscreen on my face.