What is the primary mechanism by which fluoride prevents tooth decay?

Fluoride primarily prevents tooth decay through the mechanism of re-mineralizing enamel. When fluoride is present in the oral cavity, it can be incorporated into the crystalline structure of tooth enamel, forming fluorapatite. This compound is more resistant to acid dissolution than the original hydroxyapatite of enamel, thereby enhancing the enamel's strength and making it less susceptible to decay.

Additionally, fluoride facilitates the re-mineralization process in areas of demineralization that have occurred due to acid attacks from bacteria or dietary acids. This helps restore and maintain the mineral content of enamel, effectively reversing early stages of decay.

While other mechanisms such as inhibiting bacteria growth, neutralizing acids, and increasing saliva production contribute to overall oral health and decay prevention, re-mineralization is specifically the primary and most direct action through which fluoride strengthens and protects tooth enamel against decay.