I have never seen any study showing that the fluoride from the blood can have any influence on the teeth.
The enamel of the teeth does not have blood vessels, so it cannot "grab fluoride" from blood. As far as it is known, the biomineralized calcium phosphate is always hydroxyapatite. When the hydroxyapatite of the enamel of the teeth is in contact with a solution containing fluoride ions, there is a passive ion exchange process (i.e. not mediated by biological enzymes) which results in partial replacement of the more soluble hydroxyapatite with the less soluble fluorapatite.
There is no doubt that using a tooth paste with fluoride or washing the mouth with a fluoride solution can prevent caries.
However there has never been any evidence whatsoever that drinking fluoride has any beneficial effect, beyond that of the teeth being washed by the fluoride solution before ingesting it. On the contrary, it is well known that too much fluoride in the drinking water interferes with bone growth, as it happens in some countries where the drinking water is naturally rich in fluoride.
The enamel of the teeth does not have blood vessels, so it cannot "grab fluoride" from blood. As far as it is known, the biomineralized calcium phosphate is always hydroxyapatite. When the hydroxyapatite of the enamel of the teeth is in contact with a solution containing fluoride ions, there is a passive ion exchange process (i.e. not mediated by biological enzymes) which results in partial replacement of the more soluble hydroxyapatite with the less soluble fluorapatite.
There is no doubt that using a tooth paste with fluoride or washing the mouth with a fluoride solution can prevent caries.
However there has never been any evidence whatsoever that drinking fluoride has any beneficial effect, beyond that of the teeth being washed by the fluoride solution before ingesting it. On the contrary, it is well known that too much fluoride in the drinking water interferes with bone growth, as it happens in some countries where the drinking water is naturally rich in fluoride.