Minerals and Trace Elements 229minutes. In the absence of high dietary concentra-
tions of calcium and certain other cations with which
fl uoride may form insoluble and poorly absorbed
compounds, 80% or more is typically absorbed. Body
fl uid and tissue fl uoride concentrations are propor-
tional to the long-term level of intake; they are not
homeostatically regulated. About 99% of the body’s
fl uoride is found in calcifi ed tissues (bone and teeth),
to which it is strongly but not irreversibly bound.
In general, the bioavailability of fl uoride is high,
but it can be infl uenced to some extent by the vehicle
with which it is ingested. When a soluble compound
such as sodium fl uoride 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 fl uoride is less readily absorbed.
The fractional retention (or balance) of fl uoride at
any age depends on the amount absorbed and the
amount excreted. In healthy, young, or middle-aged
adults, approximately 50% of absorbed fl uoride is
retained by uptake in calcifi ed 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 confi rmed.
Metabolic function and essentiality
Although there is no known metabolic role in the
body for fl uorine, it is known to activate certain
enzymes and to inhibit others. While the status of
fl uorine (fl uoride) 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 fl uorine to
be a benefi cial element for humans, if not an “essen-
tial nutrient.”
The function of fl uoride appears to be in the crys-
talline structure of bones; fl uoride forms calcium
fl uorapatite in teeth and bone. The incorporation of
fl uoride in these tissues is proportional to its total
intake. There is an overall acceptance of a role for
fl uoride in the care of teeth. The cariostatic action
(reduction in the risk of dental caries) of fl uoride 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 fl uo-
ride during the pre-eruptive development of the teeth
also has a cariostatic effect because of the uptake of
fl uoride by enamel crystallite and formation of
fl uorhydroxyapatite, 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 forma-
tion 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 fl uoride (23 mg/day) is administered
in a sustained-release form rather than in forms that
are quickly absorbed from the gastrointestinal tract.Defi ciency symptoms
The lack of exposure to fl uoride, or the ingestion of
inadequate amounts of fl uoride at any age, places the
individual at increased risk for dental caries. Many
studies conducted before the availability of fl uoride-
containing dental products demonstrated that dietary
fl uoride exposure is benefi cial, owing to its ability to
inhibit the development of dental caries in both chil-
dren and adults. This was particularly evident in the
past when the prevalence of dental caries in commu-
nities without water fl uoridation was shown to be
much higher than that in communities who had their
water fl uoridated. Both the intercommunity trans-
port of foods and beverages and the use of fl uoridated
dental products have blurred the historical difference
in the prevalence of dental caries between communi-
ties with and without water fl uoridation. This is
referred to as a halo or diffusion effect. The overall
difference in caries prevalence between fl uoridated
and nonfl uoridated 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 fl uoride is of
importance in the control of dental caries.Toxicity
Fluorine, like other trace elements, is toxic when
consumed in excessive amounts. The primary adverse
effects associated with chronic, excessive fl uoride