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How is the fluoride in toothpaste absorbed by our body? Does the tooth absorb the molecule directly, or is it absorbed by the mouth?

The answers to this question suggest that some materials can be absorbed by the mouth, but fluoride is absent. Additionally, many municipalities add fluoride to tap water which I should imagine is actually absorbed in the stomach by a different mechanism than that of toothpaste.

Note that toothpaste generally comes with stern warnings not to swallow. Looking at the ingredients it seems the only thing that may be harmful is the fluoride itself. Therefore I suspect that toothpaste has a much higher fluoride concentration than tap water, which supports the notion that its fluoride is absorbed by a different mechanism than tap water.

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The chemical mechanism by which fluoride protects teeth is the remineralization of hydroxyapatite $\ce{Ca5(PO4)3(OH)}$ in the tooth enamel into fluorapatite $\ce{Ca5(PO4)3F}$:

$$\ce{Ca5(PO4)3(OH) \ (s) + F- \ (aq) -> Ca5(PO4)3F \ (s) + OH- \ (aq)}$$

This reaction occurs at the surface of the enamel, i.e. directly within the mouth. The fluoride ions participating in the reaction are dissolved in the saliva and other fluids present in the mouth.

Thus, the fluoride does not need to be absorbed into any part of the body (although, as part of the remineralization process, it can be said to be absorbed directly into the tooth enamel), and, in particular, there is no need for it to be swallowed. Indeed, because of potential adverse effects of high levels of fluoride when absorbed into the body through the digestive tract, swallowing strongly fluoridated dental care products like toothpaste or mouthwash should be avoided.

That said, any fluoride that is swallowed and absorbed into the body, e.g. through fluoridated (or naturally fluoride-rich) drinking water, can still contribute to the remineralization of teeth, simply because some of it will be secreted back into the mouth as part of saliva.

While the fluoride levels in saliva (assuming a safe level of fluoride intake) are much lower than those in, say, toothpaste, your teeth will be constantly exposed to the fluoride in saliva, whereas they will only be exposed to toothpaste for a few minutes each day. Thus, while the rate of remineralization due to systemic fluoride may be much slower than that due to e.g. fluoridated toothpaste, the total amount of remineralization per day may well be of a comparable magnitude.

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  • $\begingroup$ Thank you! This is exactly the information that I was looking for, including both the mechanisms by which the fluoride gets to the tooth. $\endgroup$
    – dotancohen
    Jun 11, 2014 at 13:50
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From different sources, I think that it can be safely assumed that fluoride stays in the mouth and is absorbed by the teeth while brushing unless ingested (reference). Most dental doctors agree that the fluoride is not absorbed in the mouth and that it may be retained on the surface of your oral mucosa, where it is then available to help re-mineralize your teeth after acid exposure (reference).

Fluoride levels in toothpaste differs according to the company that is producing it.

It may appear after the label "Active ingredient" or as a component under "Ingredients" on the toothpaste tube. Whereas previously fluoride content was given as a percent of volume (% w/v) or weight (% w/w), it is now accepted that the most efficient method of informing people of the amount of fluoride in a toothpaste is to give the "parts per million" fluoride (ppm F). Most manufacturers now give fluoride content in ppm F. The EU has prohibited over the counter sales of toothpastes with a ppm F value of 1500 (reference).

Of course ingestion of the toothpaste will lead to the fluoride being absorbed by the stomach.This could increase the dietary recommended intake of fluoride leading to fluorosis and many other issues (reference). There is also a study estimating the fluoride absorbed from toothpastes through ingestion. In the study, it states that

Measuring changing fluoride levels in saliva appears to be an acceptable non-invasive technique for following systemic fluoride absorption(reference).

The 1984 issue of Clinical Toxicology of Commercial Products lists Fluoride as "more poisonous than lead and slightly less poisonous than arsenic". Lead has a toxicity rating of 3, Fluoride has a toxicity rating of 4 (reference).

Hence it is recommended that the brushing habits of your young ones be supervised to check ingestion of toothpaste and use the appropriate amounts of toothpaste for brushing.

Health effects of fluoride can be read about in this article.

The action of fluoride on teeth and treatments with fluoride is very comprehensively given in this article.

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  • $\begingroup$ The tooth bone directly absorbs the fluoride, then? It passes through the enamel? $\endgroup$
    – dotancohen
    Jun 11, 2014 at 10:14
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    $\begingroup$ @dotancohen Most articles call it absorb but I would like to say deposited. and no it doesn't pass through the enamel but becomes part of it which is why the term absorbed is used. $\endgroup$
    – The Last Word
    Jun 11, 2014 at 10:25
  • $\begingroup$ Thank you. If the enamel is the final destination of the fluoride, then how does digested fluoride get there? By the saliva only? $\endgroup$
    – dotancohen
    Jun 11, 2014 at 10:39
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    $\begingroup$ @dotancohen For fluoride in the mouth, I would have to say that saliva would be the way to get to the teeth. $\endgroup$
    – The Last Word
    Jun 11, 2014 at 10:42
  • $\begingroup$ I'm a bit dubious about that "more poisonous than lead" quote: it's repeated (in various mutated forms) on many anti-fluoridation websites, but I wasn't able to find anything like it in the claimed source (which, apparently, is actually a book, not a journal issue). That said, Google provides only a rather inconvenient form of snippet search access to the book, so, without an actual copy, I can't be quite 100% sure that it's bogus. $\endgroup$
    – Ilmari Karonen
    Jun 11, 2014 at 12:23
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Fluoride is absorbed mostly in the stomach and small intestine.

Several aspects of fluoride metabolism - including gastric absorption, distribution and renal excretion - are pH-dependent because the coefficient of permeability of lipid bilayer membranes to hydrogen fluoride (HF) is 1 million times higher than that of F(-). This means that fluoride readily crosses cell membranes as HF, in response to a pH gradient between adjacent body fluid compartments. After ingestion, plasma fluoride levels increase rapidly due to the rapid absorption from the stomach, an event that is pH-dependent and distinguishes fluoride from other halogens and most other substances. The majority of fluoride not absorbed from the stomach will be absorbed from the small intestine. In this case, absorption is not pH-dependent. Fluoride not absorbed will be excreted in feces. Peak plasma fluoride concentrations are reached within 20-60 min following ingestion. ... Many factors can modify the metabolism and effects of fluoride in the organism, such as chronic and acute acid-base disturbances, hematocrit, altitude, physical activity, circadian rhythm and hormones, nutritional status, diet, and genetic predisposition.

Source: Buzalaf MA, Whitford GM. Fluoride metabolism. Monogr Oral Sci. 2011;22:20-36. doi: 10.1159/000325107. Epub 2011 Jun 23.

Fluoride concentration:

  • tap water: maximum contaminant level goal is 4.0 mg/L or 4.0 ppm [1]
  • optimally fluoridated municipal water: 0.7 parts per million (ppm) of fluoride ion (F) [2]
  • toothpaste for adults: 1000 ppm or 1100 ppm F from three fluoride compounds. These are sodium monofluorophosphate (MFP), sodium fluoride (NaF) and stannous fluoride (SnF2) [2].

When ingesting toothpaste, fluoride is less absorbed from dicalcium phosphate dihydrate toothpaste [3].

References:

  1. http://water.epa.gov/drink/contaminants/basicinformation/fluoride.cfm
  2. American Dental Asociation http://www.ada.org/EPUBS/science/2012/may/page.shtml
  3. Drummond BK, Curzon ME, Strong M. Estimation of fluoride absorption from swallowed fluoride toothpastes. Caries Res. 1990;24(3):211-5.
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    $\begingroup$ If the toothpaste is not ingested, then how does it get to the stomach and small intestine? Just whatever seeps down the throat? Why not just take a fluoride pill then, which would give a more controlled dose, instead of relying on however uncontrolled amount seeps down the throat. $\endgroup$
    – dotancohen
    Jun 11, 2014 at 10:16
  • $\begingroup$ Dotancohen, if fluoride is in the digestive tract, isn't the most likely source fluoridated water? $\endgroup$
    – electronpusher
    Feb 8, 2017 at 2:35

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