How Fluoride Works
Archeologists know that tooth decay was rare in human beings until the Renaissance, when refined sugar became available to wealthy people. The problem became epidemic during the industrial revolution as sugar consumption increased among the entire population of industrialized societies. Queen Elizabeth I was known to have a fondness for sugar, and suffered greatly from tooth decay.
Until 1757, medical practitioners believed cavities were caused by worms. In 1556 Pope John XXI recommended inhalation of smoke from burning henbane seeds to kill the worms and assuage the pain. When dentistry became a recognized profession, researchers zeroed in on two potential causes for tooth decay: 1) bacteria, and 2) diet. It turns out both causes are correct.
The bacterium Streptococcus mutans colonizes tooth surfaces, forming plaque. They metabolize and ferment sugar, releasing acid in the process. The acid dissolves the mineral apatite which comprises tooth enamel, forming a cavity.
Fluoride forms a complex with the apatite mineral of the tooth enamel. The fluoro-apatite complex is much more acid-resistant than normal apatite, and forms a protective veneer on the teeth. Fluoro-apatite forms much more quickly than the body can naturally re-mineralize the teeth.
The original fluoride researchers felt the protective mechanism was entirely systemic (within the body). This has proved to be wrong. The primary protection mechanism is now known to be topical (on the surface of the tooth), but evidence for systemic protection continues to be revealed by research. An October 2010 article appearing in the American Journal of Public Health reports a strong relationship between fluoride levels in a resident’s county at the time of their birth, with tooth loss as an adult. It seems that fluoride exposure at birth affects tooth loss at age 40 and older, which is evidence for a systemic mechanism.
Fluoridation of municipal drinking water at safe levels (~1 part per million) provides dental protection through both topical and systemic mechanisms. This has helped reduce rates of tooth decay in the U.S. to the frequency archeologists observe in skeletons from 1000 years ago.