When we go to the beach, one of the things we all do is catch a bit of sun and work on a tan. Usually a few minutes means you have recieved a good amount of vitamin D. It will vary from person to person and skin type. Laura-S loves working on her tan while in Australia, because we have so many sunny days. Of course she can't resist tanning nude. If you haven't tried it then you won't know why this is the case, until you do try it.
