Fluoride is not a nutrient. It is added to drinking water for the same reason it is present in toothpaste: to convert tooth enamel from hydroxyapatite to harder and more acid-resistant fluoroapatite. For details, see https://en.wikipedia.org/wiki/Fluoride_therapy http://books.nap.edu/catalog.php?record_id=11571 http://www.cdc.gov/mmwr/preview/mmwrhtml/00056796.htm.