However,work by Featherstone and others(1990) has shown that fluoride's greatest anti-cares benefit is gained from topical application. Research has shown that the solubility of the tooth mineral with fluoride incorporated at the time of development is not much different than that formed without ststemic fluoride and is insuffcient to have a measurable effect on its acid solubility. However, fluoride in the saliva surrounding the tooth is incor- porated into the surface of enamel crystals during remineralization(replacing minerals lost from the tooth surface) to form a surface veneer containing fluorapatite with a much lower solubility than the original carbonated tooth mineral. The pH (a measure of acidity) at which tooth mineral dissolves is 5.5(7.0 is neutral pH-neither acid nor base). However, when the mineral is converted to fluorap-atite, the pH at which it dissolver is lowered to 4.5(a lower number indicates that it is ore acidic;e.g.,stomach acid has a pH of less than 1.0) Therefore fluoride makes it more difficult for the acids pro-duced by cariogen (decay-causing) bacteria in plaque to demineralize tooth structure and cause dental cares. There is evidence that fluoride from drinking water, toothpastes, mouth rinses, and some foods remains in the saliva for several hours and has a prolonged topical effect.
PROTECTION AGAINST EROSION
Highly acidic foods and beverages such as citrus fruits, sodas, and wine can contribute to loss of tooth mineral that is called erosion. Erosion differs from caries in that bacteria are not involved and most of the mineral loss is at the surface. It is important to maintain a well-balanced diet to mini-mize excess acidic foods. Some midical conditions also cause erosion of the teeth by causing stomach acid to enter the mouth. Examples are acid reflux(burping up stomach acid) and bulimia( chronic forced vomiting to control weight gain after binge eating). By making the tooth structure less soluble in acid, fluoride provides some degree of protection against erosion.
BACTERIA INHIBITION
Fluoride interferes with the essential enzyme activity of the bacteria. Although the fluoride ion has been shown not to cross the bacterial cell wall, it can travel through it in the form of hydrofluoric acid (HF). As the decay-causing bacteria produce acids during the metabolism of sugars and cllked starch, some of the fluoride present in the plaque fluid com-bines with the hydrogen ion of the acid to become HF and rapidly diffuse into the cell. Once in the alkaline cytoplasm of the cell, the HF separates into the fluoride ion and the hydrogen ion again. These ions disrupt the enzyme activities essential to the functioning of the bacteria and cause their death.