Vitamin D.
To get it from the sun you need 10 mins exposure on your hands face and arms for every 1,000 units.
How do I know this, here in Australia there was a story about people not getting enough Vit.D. As they lived inside the cities and used sunscreen. The best time is early morning or after 3 in the afternoon. I take no Vit D as my levels are high from just going out side early morning to hang washing on the line or just collectring the mail.
Kathy