Q: How does fluoride affect teeth? Does ingesting fluoride in drinking water affect teeth more than fluoride toothpaste? A: Tooth decay is caused by the acids produced from the metabolism of certain carbohydrates by bacteria, notably Streptococcus mutans , in dental plaque. Dental plaque is the soft, cheeselike material that forms on teeth that are not adequately cleaned. Its formation is enhanced by diets high in sugars. Good oral hygiene practices (brushing, flossing) can reduce its accumulation on most tooth surfaces. Plaque has a complex composition. In addition to water, it contains many species of bacteria and many kinds of inorganic compounds and organic molecules. Tooth enamel is chiefly composed of crystals of a calcium phosphate mineral called hydroxyapatite [Ca10 (PO4 )6 OH2 ].This mineral, like other minerals, begins to dissolve at the microscopic level when exposed to a solution with a sufficiently low pH (approximately 5.0 or less). If the exposures to acid are repeated daily for several weeks or months, an early, clinically observable carious lesion may occur.
The mechanism by which fluoride slows or prevents tooth decay involves at least three actions. First, it inhibits demineralization of enamel. This effect is due in part to the incorporation of fluoride into the apatite crystal, either before or after the tooth erupts into the mouth. Fluoride substitutes for the hydroxyl ions to form hydroxyfluorapatite [Ca10 (PO4 )6 (OH)F] or fluorapatite [Ca10 (PO4 )6 F2 ], both of which are less soluble in acid than hydroxyapatite.
Second, fluoride promotes the remineralization of enamel. It is not entirely clear how this phenomenon occurs. It has been shown, however, that fluoride increases the concentration of calcium in plaque fluid. This retards enamel demineralization through the common ion effect but also promotes the formation of calcium-phosphate-fluoride salts which precipitate onto the enamel surface.
Provided that the fluoride concentration in dental plaque is sufficiently high, as may result from a topical fluoride treatment in the dental office, the third mechanism is a reduction in the activity of bacterial enzymes responsible for production of acid from fermentable sugars. The most studied enzyme is enolase, the inhibition of which blocks the production of lactic acid. Relatively high concentrations of fluoride also inhibit the phosphotransferase system, which facilitates the membrane transport of sugars into the cells. Thus, the availability of the substrate required for the production of acid is reduced.
It is not known which of these three mechanisms is the most important. Currently, it is believed that fluoride in plaque fluid at the plaque-enamel interface plays the dominant role in the cariostatic (decay-retarding) effect of fluoride.
The ability of fluoride to prevent tooth decay (dental caries) was discovered in the United States during the 1930s, about 15 years before the first controlled water fluoridation program in Grand Rapids, Michigan, and 30 years before fluoridated dental products were available to the public. The discovery was made by epidemiologists who were searching for the cause of Colorado brown stain, later known as dental fluorosis, a condition in which the enamel contains more protein than is normal. This increases the porosity of the enamel which, in advanced cases, may result in discoloration. The researchers discovered, to their surprise, that persons with dental fluorosis had fewer dental caries. Compared with communities with water containing negligible amounts of fluoride, the prevalence of caries was approximately 500wer in communities where the water contained 1.0/part per million (1.0mg/L). Five to six years after Grand Rapids started water fluoridation, there were 50 0.000000ewer caries compared with the control city of Muskegon. In addition, this average reduction in caries also occurred in other cities that subsequently implemented water fluoridation programs. It compares favorably with the average 20% reduction associated with the regular use of fluoridated toothpastes, despite the fact that the fluoride concentration in toothpastes is 1000 parts per million. This paradox may be due to the fact that most studies of the effectiveness of toothpastes have been of relatively short duration (2-3 years).
Gary M. Whitford, PhD, DMD
