Encyclopedia of Food & Culture | Fluoride

FLUORIDE. Fluoride is an important trace element in human nutrition. Daily exposure to small quantities is widely considered to be vital for maintenance of sound tooth structure. Ingested or systemic fluoride has long been known to offer significant benefit when supplied during tooth formation in childhood. More recently, topical exposure (that is, making fluoride available at the tooth surface) has been shown to provide benefits throughout life, even for older adults.

Sources of Fluoride

Water, rocks, soil, and living tissue all have naturally occurring fluoride as a constituent. Crystalline and carbonate minerals containing fluoride are common throughout the earth's near-surface crust. As water flows through the environment, fluoride and many other ions dissolve from sedimentary rock layers and soil into aquifers, streams, rivers, and oceans. Dissolved ions are essential for humans and all living things. Fluoride ions are absorbed directly from the water we drink.

Fluoride in Bone and Tooth Tissue

Fluoride ions taken systemically can become incorporated within bone and tooth tissue. Although bones and teeth have an organic matrix, it is their inorganic or crystalline hydroxyapatite composition that gives them their strength and hardness. Living human cells use available calcium and other minerals to form strong hydroxyapatite matrices. When fluoride ions are also available to the cells, an additional material called fluorapatite is formed. Integration of a small amount of fluorapatite within a hydroxyapatite matrix may produce a more durable substance than is found with hydroxyapatite alone.

Topical Mechanism

Fluoride ions can also provide a very strong surface or topical effect for teeth when available on a regular basis. One such effect is that topical fluoride inhibits the ability of some bacteria to produce dental plaque by blocking the function of important intracellular bacterial enzymes. Much more significantly, topical fluoride also leads to reduced demineralization and increased remineralization of enamel surfaces.

Bacterial Acid and Chemical Balance

Demineralization of a tooth occurs when bacteria create an acidic or low pH environment at the tooth surface. The acidity dissolves hydroxyapatite, releasing positively charged calcium ions and negatively charged carbonate and phosphate ions into saliva. When normal saliva flow dilutes the acidity, the positive and negative ions recombine and remineralize the surface.

This cycle represents a balance. Diets rich in fermentable carbohydrates such as mono-and disaccharides, which are relatively simply sugars, disrupt the balance. They stimulate some oral bacteria to produce dental plaque and acid. Dental plaque is a substance that attaches to tooth enamel and is colonized by the bacteria that form it. Once such a colony is established, each ingestion of fermentable carbohydrate causes approximately one half-hour of intense acid production by the bacteria. This burst of acid production lowers the pH near the tooth surface, demineralizing large amounts of hydroxyapatite. The balance is disrupted and, as the cycle is repeated, it damages the tooth's surface.

Topical Fluoride and Stronger Enamel

When sufficient amounts of negatively charged fluoride ions are routinely present topically at the tooth surface, a different pattern emerges for this cycle. The balance of demineralization and remineralization actually builds fluoride into the tooth's surface structure. Over long exposure to fluoride in saliva, more and more fluoride is incorporated, and the enamel surface becomes stronger. A much greater increase in acidity is then necessary before a destructive imbalance in the cycle will be initiated. This surface or topical effect is thought to be the primary means by which fluoride prevents dental caries.

Benefits of Community Water Fluoridation

In studies of many communities over several decades, it has become clear that there is great benefit to maintaining proper fluoride levels in the public water supply. A concerted public health effort throughout the decades since the 1950s has led to the maintenance of fluoride at these levels in many public water supplies.

Community water fluoridation is intended to provide fluoride at concentrations ranging from 0.7 to 1.2 ppm. Coincidentally, this is about the same concentration of fluoride that is found in ocean water. Levels are adjusted within this range regionally and throughout the year. This provides lower concentrations of fluoride when people are likely to drink more water and higher concentrations when less water consumption is expected.

Without other significant sources of fluoride during the 1950s and 1960s, community water fluoridation produced reductions of 40 to 50 percent in the number of cavities or dental caries among children. Their teeth had enamel that was more resistant to caries both when it was formed and throughout life.

Other countries have assessed a variety of alternative means for delivering protective levels of fluoride. These have included supplementation with tablets or drops, salt fluoridation, and milk fluoridation. However, in the United States, fluoridation of public water as part of purification treatment remains the most effective and economical means for providing this benefit to communities. Currently about 60 percent of the U.S. population has fluoride maintained at these levels in their drinking water.

In the 1980s, it became clear that the positive effects of water fluoridation were not limited to developing teeth. Studies of people age sixty-five and older showed that it was beneficial even when all of the fluoride exposure took place after tooth eruption. Those who lived in communities with fluoridated water as adults had significantly lower rates of dental caries on exposed tooth root surfaces than comparable older adults without fluoridated water.

Fluoride and Osteoporosis

There has been interest in potential positive effects of fluoride supplementation on increased bone density. When ingested, fluoride is absorbed primarily from the upper gastrointestinal tract and is excreted in urine. Fluoride that is not excreted is deposited in calcified tissues—bones and teeth.

Osteoporosis, loss of bone density, is an increasingly prevalent problem in the U.S. population among both men and women. Unfortunately, research to date does not suggest a useful effect of fluoride on bone strength, even when it is supplemented at concentrations twenty times greater than that found in fluoridated water.

Early Research on Fluoride

It was research on the effects of prolonged intake of excessive amounts of naturally occurring fluoride that led scientists to understand the protection afforded by healthy fluoride levels. In the 1930s, a dentist in Colorado, Dr. Frederick McKay, became curious about a brown surface stain seen on some of his patients' teeth. These teeth often had a rough and porous surface texture, yet they were also far less prone to develop dental caries.

McKay's early observations led to a long series of investigations. It became clear that this problem, a severe form of fluorosis, resulted from very high levels of naturally occurring fluoride in drinking water. McKay's water samples had fluoride concentrations as much as fourteen times greater than that recommended today for community water systems. These investigations led to the discovery that when fluoride was present at the low levels that are now widely used, it offered powerful protection from dental caries without any adverse effects.

Reevaluation of Fluoride Use

By the 1990s, the wide availability of fluoridated water led scientists to reevaluate fluoride use practices. Particular attention was paid to the potential for a diffuse exposure to fluoride throughout the population. Many packaged foods are processed in communities with fluoridated water, becoming sources of small amounts of fluoride to those who consume them. Far more important, however, is the use of toothpaste and other products containing fluoride. It was concluded that community water fluoridation levels remain appropriate, but that greater care must be taken in the use of fluoride toothpaste.

Levels of fluoride in treated drinking water are extremely low when compared to concentrations in common therapeutic products. For example, fluoride concentration in over-the-counter fluoride mouth rinses is generally about 230 parts per million (ppm); toothpastes contain about 1,000 ppm; prescription home-use mouth rinses and home-use gels range from 1,000 to 5,000 ppm; professionally applied fluoride gels contain 10,000 to 12,300 ppm; and professionally applied fluoride varnishes contain about 22,000 ppm.

The additional sources of fluoride, primarily toothpaste, have led to lower rates of dental caries in U.S. communities not provided with fluoridated water. However, even with these lower background rates of dental caries in the population, it is estimated that community water fluoridation alone still provides an additional reduction of 20 to 40 percent in dental caries when comparison is made to caries rates for Americans who do not have fluoridated water but who use fluoride toothpaste.

Fluoride Issues for the Future

During the reevaluation of fluoride in the 1990s, concerns were raised regarding the potential for fluorosis. In contemporary studies of fluorosis in the U.S. population, nearly all observed cases have been classified as "very mild" or "mild." These are categories of "white-spot" discoloration that are usually only apparent to a dentist conducting an intraoral examination. Ingestion of fluoride toothpaste is considered the primary explanation for these white-spot discolorations.

Children are likely to swallow toothpaste while brushing, ingesting an unintended and excessive amount of fluoride. The most effective strategy for avoiding mild fluorosis is to limit children to a pea-sized quantity of toothpaste at each brushing. This quantity is adequate for caries prevention and oral hygiene, but it should not lead to development of fluorosis.

Use of infant formula and some baby foods has also raised a degree of concern. Because of infants' very small body mass, the proper intake of systemic fluoride is lower than that for slightly older children. Some studies have identified varying levels of fluoride in these products, some approaching levels that are associated with increased risk for very mild or mild fluorosis in infants. Physicians and dentists are urged to use caution in prescribing fluoride supplements for infants and very young children living in communities without fluoridated water because they might be consuming these fluoride-containing products.

The U.S. Environmental Protection Agency has set a standard of 4.0 ppm as the maximum allowable fluoride level in drinking water. Within the United States, fluoride levels in drinking water are actually maintained at about one-fourth of this level. However, in some developing countries, particularly in southern Asia and northern Africa, natural fluoride is present at extremely high levels. In India, for example, a study sponsored by the World Health Organization found natural fluoride levels exceeding 1.5 ppm in about 8 percent of samples, with some concentrations as high as 22.0 ppm. In such areas, public health workers actively engage in efforts to reduce fluoride exposure and eliminate fluorosis.

Conclusion

Nearly one hundred organizations with related expertise, including the World Health Organization, the U.S. Public Health Service, the American Medical Association, the American Public Health Association, the American Society for Clinical Nutrition, the American Society for Nutritional Sciences, the International Association for Dental Research, the FDI World Dental Federation, and the American Cancer Society have recognized the importance of daily fluoride intake for dental health. Particularly when supplied through community water fluoridation, ensuring adequate dietary fluoride exposure has been an extremely safe and cost-effective public health measure. Fluoride is a trace element that has extremely important personal and public health benefits for promotion and maintenance of optimal oral health.

See also Dentistry; Digestion.

Rob Berg