What are the proven effects of fluoride?

Strengthen enamel: Every meal, most drinks, and snacks create damage via acids or bacterial acids produced by the sugar intake. This causes demineralizatoin and loss of hydroxide from hydroxyapetite weakening the tooth. Fluoride replaces the lost mineral and forms a stronger fluorapetite. Remineralizes. However too much Fluoride during tooth developement can actually cause damage to the teeth.

Hardens tooth enamel: Enamel has a crystalline structure. Fluoride gets incorporated in and on the enamel to make it harder and less likely to soften when exposed to acid producing bacteria.