DAT Practice Test 5

Fluoride and the Chemistry of Enamel

¹Tooth enamel is the hardest substance in the human body. By weight it is about 96 percent mineral, most of it in the form of hydroxyapatite, a calcium phosphate whose tiny, rod-shaped crystals are packed together in bundles. Unlike bone, which is constantly broken down and rebuilt by living cells, mature enamel contains no cells at all. The cells that build it are lost once a tooth erupts into the mouth, so enamel that is destroyed is never regrown. What enamel can do, however, is gain and lose mineral at its surface, and that slow chemical give-and-take turns out to be the key to understanding both tooth decay and the most widely used means of preventing it.

²At the neutral pH of resting saliva, the fluid around the teeth is supersaturated with calcium and phosphate ions, so there is no tendency for enamel to dissolve; if anything, minerals tend to be deposited. The situation changes after a person eats or drinks something containing sugar. Bacteria in dental plaque, the sticky film that coats the teeth, ferment the sugar and release acids. Within a few minutes the pH of the plaque can fall from about 7 to below 5. Once it drops below roughly 5.5, a value often called the critical pH, the plaque fluid becomes undersaturated with respect to hydroxyapatite, and mineral begins to leave the enamel. Saliva gradually neutralizes the acid, largely through its bicarbonate buffer, and over the next 30 to 60 minutes the pH climbs back toward neutral, at which point lost mineral can begin to be replaced.

³Seen this way, tooth decay, or dental caries, is not a single event but the outcome of a long contest between periods of mineral loss and periods of repair. When repair keeps pace, the enamel stays sound. When acid attacks come too often for saliva to restore the balance, mineral is lost faster than it returns. For this reason, how often a person consumes sugar matters more than the total amount: a single sweet dessert produces one acid period, whereas the same quantity of sugar sipped throughout an afternoon may produce many. The earliest visible sign of net loss is a chalky white spot, where mineral has been removed from beneath a still-intact surface layer. At that stage the damage can be halted and even partly reversed. Only when the weakened surface finally collapses does a true cavity form.

⁴The story of fluoride began not in a laboratory but in a dental office. In the early twentieth century, a dentist practicing in Colorado noticed that many of his patients had brown stains on their teeth, a condition then known locally as “Colorado brown stain.” Curiously, the stained teeth were also unusually resistant to decay. It took decades to trace both effects to high levels of fluoride occurring naturally in the local drinking water. In the 1930s, a dental officer of the U.S. Public Health Service surveyed communities with different natural fluoride levels and found that water containing about one part per million was associated with far less decay and with only very mild discoloration.

⁵These findings led to a public health experiment. In 1945, Grand Rapids, Michigan, became the first city to add fluoride deliberately to its water supply, while a nearby city served as a comparison. Over the following years, decay among Grand Rapids schoolchildren fell substantially, and community water fluoridation spread across the United States and to many other countries.

⁶The early explanation for these results was that fluoride works from the inside. Children drank fluoridated water while their teeth were still forming, the reasoning went, and fluoride ions took the place of some hydroxide ions in the growing crystals, producing fluorapatite, a mineral less soluble in acid than hydroxyapatite. On this view, fluoride’s benefit was built into the enamel before a tooth ever erupted, and its value lay mainly in childhood.

⁷Later research steadily undermined this picture. Measurements showed that even in fluoridated communities only a small fraction of the hydroxyapatite in enamel is converted to fluorapatite, and the fluoride content of a person’s enamel turned out to be a poor predictor of how much decay he or she would develop. At the same time, studies of the tooth surface showed that very low concentrations of fluoride dissolved in saliva and plaque fluid have a strong effect on the balance between mineral loss and repair. By the end of the twentieth century, public health authorities had concluded that fluoride’s protective effect is mainly topical: what matters most is fluoride present in the mouth at the moment an acid attack occurs, not fluoride locked inside the enamel.

⁸The topical effect works in several ways. During an acid attack, fluoride ions adsorb to the surfaces of enamel crystals and slow their dissolution. As the pH recovers, fluoride attracts calcium and phosphate ions back to the damaged surface, and the new mineral that forms incorporates fluoride and resembles fluorapatite, which does not begin to dissolve until the pH falls to about 4.5. The rebuilt surface is thus more resistant than the original. Fluoride also acts on the bacteria themselves: in acidic plaque, some of it combines with hydrogen ions and diffuses into bacterial cells, where it inhibits enzymes the bacteria need to break down sugar, reducing the amount of acid they produce.

⁹This revised understanding has practical consequences. If fluoride works mainly at the surface, then small, frequent exposures, such as twice-daily brushing with fluoride toothpaste or drinking fluoridated water throughout the day, are more useful than occasional large doses. It also means that adults, whose teeth are fully formed, benefit as well as children. The new picture has also tempered enthusiasm for high intakes. Swallowing too much fluoride while teeth are still developing, generally before about age eight, can cause dental fluorosis, a change in enamel that usually appears as faint white flecks and, in severe cases, as brown staining like that once seen in Colorado. Partly for this reason, in 2015 the U.S. Public Health Service lowered its recommended level for community water to 0.7 milligrams per liter, and young children are advised to use only a small amount of toothpaste.

¹⁰Fluoride is not a cure-all. It cannot rebuild enamel once a cavity has formed; such damage requires a filling or other restoration. Nor does it remove the underlying cause of decay, which is the repeated production of acid by plaque bacteria fed on sugar. Some researchers have also asked how much additional benefit community water fluoridation provides now that fluoride toothpaste is used almost everywhere, and the size of that benefit remains a subject of active study. What is no longer in doubt is the basic mechanism: fluoride helps the tooth win the daily chemical contest at its surface, one acid attack at a time.