There's this sort of vitamin d precipice: if you get enough sun to make your own, your skin can stand up to the amount of sun you need to make your own vitamin d, but at some point, at least for me, that drops off into a hole wherein I burn badly _and_ have to take vitamin d, and it's hard to get out of that. (short-sleeve shirts and lots of bicycling and just enduring arm burns, pretty much.)