How does flourinated water help teeth?

Does it help because you ingest the flouride or is it that the flouride comes into direct contact with teeth when you drink the water? I am asking because about a year ago I switched from tap water to almost exclusively bottled spring water and my teeth have started to stain. I have a hunch that it is because bottled spring water is not flourinated (to my knowledge). I brush my teeth everyday and dont drink coffee and have never had this problem before. I have also heard that distilled water can leech minerals from your body if you drink it too much? It seems to me that spring water wouldn't do this since its not distilled and I would imagine has more minerals in it than tap water...