Fluoride: Absorption, transport, tissue distribution, Metabolic function, essentiality, Deficiency symptoms

Fluoride

Fluorine occurs chiefly in fluorspar and cryolite, but is widely distributed in other minerals. Fluoride is the ionic form of fluorine, a halogen, and the most elec-tronegative of the elements in the periodic table; the two terms are often used interchangeably. Fluorine and its compounds are used in producing uranium and more than 100 commercial fluorochemicals, including many well-known high-temperature plas-tics. Hydrofluoric acid is extensively used for etching the glass of light bulbs, etc. Fluorochlorohydrocarbons are extensively used in air conditioning and refrigera-tion. Fluorine is present in small but widely varying concentrations in practically all soils, water supplies, plants and animals, and is a constituent of all diets.

Absorption, transport and tissue distribution

Fluoride appears to be soluble and rapidly absorbed, and is distributed throughout the ECF in a manner similar to chloride. The concentrations of fluorine in blood, where it is bound to albumin, and tissues are small. The elimination of absorbed fluoride occurs almost exclusively via the kidneys. Fluoride is freely filtered through the glomerular capillaries and under-goes tubular reabsorption in varying degrees.

Fifty percent of orally ingested fluoride is absorbed from the gastrointestinal tract after approximately 30 minutes. In the absence of high dietary concentra-tions of calcium and certain other cations with which fluoride may form insoluble and poorly absorbed compounds, 80% or more is typically absorbed. Body fluid and tissue fluoride concentrations are propor-tional to the long-term level of intake; they are not homeostatically regulated. About 99% of the body’s fluoride is found in calcified tissues (bone and teeth), to which it is strongly but not irreversibly bound.

In general, the bioavailability of fluoride is high, but it can be influenced to some extent by the vehicle with which it is ingested. When a soluble compound such as sodium fluoride is ingested with water, absorp-tion is nearly complete. If it is ingested with milk, baby formula, or foods, especially those with high concentrations of calcium and certain other divalent or trivalent ions that form insoluble compounds, absorption may be reduced by 10–25%. Fluoride is absorbed passively from the stomach, but protein-bound organic fluoride is less readily absorbed.

The fractional retention (or balance) of fluoride at any age depends on the amount absorbed and the amount excreted. In healthy, young, or middle-aged adults, approximately 50% of absorbed fluoride is retained by uptake in calcified tissues and 50% is excreted in urine. In young children, as much as 80% can be retained owing to the increased uptake by the develop-ing skeleton and teeth. In later life, it is likely that the fraction excreted is greater than the fraction retained. However, this possibility needs to be confirmed.

Metabolic function and essentiality

Although there is no known metabolic role in the body for fluorine, it is known to activate certain enzymes and to inhibit others. While the status of fluorine (fluoride) as an essential nutrient has been debated, the US Food and Nutrition Board in 1997 established a dietary reference intake for the ion that might suggest their willingness to consider fluorine to be a beneficial element for humans, if not an “essen-tial nutrient.”

The function of fluoride appears to be in the crys-talline structure of bones; fluoride forms calcium fluorapatite in teeth and bone. The incorporation of fluoride in these tissues is proportional to its total intake. There is an overall acceptance of a role for fluoride in the care of teeth. The cariostatic action (reduction in the risk of dental caries) of fluoride on erupted teeth of children and adults is owing to its effect in the metabolism of bacteria in dental plaque (i.e., reduced acid production) and on the dynamics of enamel demineralization and remineralization during an acidogenic challenge. The ingestion of fluo-ride during the pre-eruptive development of the teeth also has a cariostatic effect because of the uptake of fluoride by enamel crystallite and formation of fluorhydroxyapatite, which is less acid soluble than hydroxyapatite. When drinking water contains 1 mg/l there is a coincidental 50% reduction in tooth decay in children. Fluoride (at relatively high intakes) also has the unique ability to stimulate new bone formation and, as such, it has been used as an experimental drug for the treatment of osteoporosis. Recent evi-dence has shown an especially positive clinical effect on bone when fluoride (23 mg/day) is administered in a sustained-release form rather than in forms that are quickly absorbed from the gastrointestinal tract.

Deficiency symptoms

The lack of exposure to fluoride, or the ingestion of inadequate amounts of fluoride at any age, places the individual at increased risk for dental caries. Many studies conducted before the availability of fluoride-containing dental products demonstrated that dietary fluoride exposure is beneficial, owing to its ability to inhibit the development of dental caries in both children and adults. This was particularly evident in the past when the prevalence of dental caries in communities without water fluoridation was shown to be much higher than that in communities who had their water fluoridated. Both the intercommunity trans-port of foods and beverages and the use of fluoridated dental products have blurred the historical difference in the prevalence of dental caries between communities with and without water fluoridation. This is referred to as a halo or diffusion effect. The overall difference in caries prevalence between fluoridated and nonfluoridated area regions in the USA was reported to be 18% (data from a 1986–1987 national survey), whereas the majority of earlier studies reported differences of approximately 50%. Therefore, ingestion of adequate amounts of fluoride is of importance in the control of dental caries.