CAMILA PERES BUZALAFa, ALINE DE LIMA LEITEb, AND
MARÍLIA AFONSO RABELO BUZALAF*b
aPró-Reitoria de Pesquisa e Pós-Graduação, Universidade do Sagrado
Coração, Bauru, São Paulo 17011-160, Brazil; bDepartment of Biological
Sciences, Bauru Dental School, University of São Paulo, Bauru, São Paulo
4.1 Introduction to Fluoride Metabolism
Fluoride is an inorganic anion of ﬂuorine, which is naturally found in the
biosphere with ubiquitous presence in the environment and is the most
electronegative and reactive among all elements. This latter characteristic
explains why ﬂuorine is mainly found as inorganics instead of the elemen-
tal form. Its well-known cariostatic eﬀect led to the implementation of ﬂuo-
ride in systemic (such as water, salt, sugar, milk and supplements) and topic
sources (such as toothpastes, gels, mouth rinses and varnishes). The ben-
eﬁts of ﬂuoride include both caries control and bone formation (Bratthall
et al. 1996; Kobayashi et al., 2014). However, it can negatively aﬀect the qual-
ity of the developing mineralized tissues, causing dental and skeletal ﬂuoro-
sis. The diﬀerential responses seem to be determined by the dose and time
of exposure (Everett, 2011). Thus, the knowledge of ﬂuoride metabolism
is crucial not only to understand its biological eﬀects but also to optimize
ﬂuoride-driven preventive and therapeutics strategies.
Food and Nutritional Components in Focus No. 6
Fluorine: Chemistry, Analysis, Function and Eﬀects
Edited by Victor R Preedy
© The Royal Society of Chemistry 2015
Published by the Royal Society of Chemistry, www.rsc.org
4.2 General Aspects of Fluoride Metabolism
Fluoride metabolism includes absorption, distribution and excretion,
where each step depends on the pH. Hydrogen ﬂuoride (HF) is a weak acid
with a pKa of 3.4. By deﬁnition, at pH 3.4, 50% of ﬂuoride is in the undis-
sociated form (HF), while the remaining 50% is in the ionic form (F–).
As pH decreases from 3.4, the concentration of HF increases, and as pH
increases, the concentration of F– increases (Whitford, 1996) (Figure 4.1).
This means that ﬂuoride permeates cell membranes as HF, in response to
a pH gradient between adjacent body-ﬂuid compartments, going from the
more acidic to the more alkaline compartment (Figure 4.1). Aer inges-
tion, plasma ﬂuoride reaches a peak within 20–60 min, due to its pH-de-
pendent gastric absorption, followed by a rapid decline as a result of both
uptake in calciﬁed tissues and urine excretion (Figures 4.2 and 4.3). The
small intestine also contributes to ﬂuoride absorption in a pH-indepen-
dent mechanism. Nonabsorbed ﬂuoride is excreted in feces (Figure 4.3).
From plasma, ﬂuoride is distributed to both hard and so tissues followed
Figure 4.1 Metabolism of ﬂuoride is dependent on pH. HF is a weak acid with pKa
of 3.4, meaning that at pH 3.4, half of ﬂuoride is in the nonionic form
(HF) and half is in the ionic form (F−). As pH decreases from 3.4, the con-
centration of HF enhances, while as pH increases, the concentration of
F− increases. Besides, the coeﬃcient of permeability of cell membranes
to HF is much higher than that of F−. Thus, ﬂuoride drives through cell
membranes as HF, in response to a pH gradient between adjacent com-
partments, going from more acidic to more alkaline compartments.
by its renal excretion (Figure 4.3). A minor portion of absorbed ﬂuoride
is found in so tissues through a steady-state distribution between extra-
cellular and intracellular ﬂuids. However, about 50% of the absorbed ﬂu-
oride is incorporated in calciﬁed tissues, mainly in bone, where 99% of
the ﬂuoride content in the body is found (Whitford, 1994). Its incorpora-
tion in calciﬁed tissue is not irreversible, from where it can be released
back to plasma compartment. The ﬂuoride excreted in urine is mostly a
result of the total absorbed amount subtracted by the incorporated quan-
tity in hard tissues. Each step related to the ﬂuoride metabolism will be
described in sequence.
Figure 4.2 Plasma ﬂuoride levels aer a small amount of ﬂuoride ingested. (A)
Aer ingestion, plasma ﬂuoride reaches a peak within 20–60 min, due
to its pH-dependent gastric absorption, followed by a rapid decline as
a result of both uptake in calciﬁed tissues and urine excretion. The ﬂu-
oride plasma level returns to baseline levels within 3–4 h. The small
intestine also contributes to ﬂuoride absorption in a pH-independent
mechanism. (B) From plasma, ﬂuoride is distributed to both hard and
so tissues followed by its renal excretion. Plasma ﬂuoride levels are
rapidly decreased mainly due to ﬂuoride uptake in hard tissues and
renal excretion, while the nonabsorbed ﬂuoride is excreted in feces (A
4.3 Fluoride Absorption
Approximately 80–90% of ﬂuoride ingested is absorbed from the gastro-
intestinal tract by passive diﬀusion and does not seem to require speciﬁc
transporters (Whitford, 1996). However, “Fluc” proteins-based channel was
recently puriﬁed in phospholipid bilayers with extreme selectivity for F–
over Cl–, but its expression and role in ﬂuoride absorption in mammalian
is still unknown (Stockbridge et al., 2013). Recently, selective bacterial CLC
anion-transporting proteins that catalyze transport of ﬂuoride in liposomes
were discovered (Stockbridge et al., 2012). In addition, a novel ﬂuoride
Figure 4.3 General aspects of ﬂuoride metabolism. Aer infestion, ﬂuoride is
absorbed in the stomach and small intestine by a pH-dependent and
-independent mechanisms, respectively. Nonabsorbed ﬂuoride is excreted
in feces. A minor portion of absorbed ﬂuoride is found in so tissues
through a steady-state distribution between extracellular and intracellu-
lar ﬂuids. However, about 50% of the absorbed ﬂuoride is incorporated
in calciﬁed tissues, mainly in bone, where 99% of the ﬂuoride content in
the body is found. Its incorporation in calciﬁed tissue is not irreversible,
from where it can be released back to the plasma compartment. From
systemic circulation, ﬂuoride content is followed by renal excretion. The
ﬂuoride excreted in urine is mostly a result of the total absorbed amount
subtracted by the quantity incorporated in hard tissues.
exporter named FEX functioning as ﬂuoride eﬄux was recently described
in eukaryotic cells (Li et al., 2013). This exporter allows organisms such as
fungi to evoke from ﬂuoride-mediated toxicity but has not been identiﬁed
in mammalian cells as yet. Together, these novel ﬁndings open new per-
spectives regarding ﬂuoride-mediated inter- and intracellular transport.
While about 20–25% of the total ingested ﬂuoride is absorbed in the stom-
ach, the remainder is absorbed in the proximal small intestine (Nopakun et
al., 1989; Whitford, 1996). Its absorption occurs in a half-life of 30 min, reach-
ing a peak within 20–60 min (Whitford, 1996), independently of the amount
ingested. Aer that, the ﬂuoride concentrations are rapidly decreased and
correlate to both uptake in calciﬁed tissues and renal excretion (Whitford,
1996). The gastric absorption is very quick and is determined by acidity and
content gastric and fashion of gastric emptying (Whitford and Pashley, 1984).
This is explained by the fact that the gastric absorption is inversely related
to the pH. i.e., the higher the gastric acidity, the higher the ﬂuoride absorp-
tion (Whitford and Pashley, 1984). When ionic ﬂuoride enters the acid gastric
lumen environment, it is converted into HF, which is an uncharged molecule
that readily crosses cell membranes, including gastric mucosa (Gutknecht
and Walter, 1981). Thus, peak plasma ﬂuoride concentration is higher in
acidic compared to neutral environments. Thus, the pH dependency of ﬂuo-
ride absorption has a direct implication not only for the treatment in cases of
acute ﬂuoride toxicity but also in therapeutic applications. Moreover, delayed
gastric emptying may promote both slower and smaller ﬂuoride absorption
leading to a decreased plasma concentration (Nopakun et al., 1989).
About 75% of the fluoride not absorbed in the stomach is absorbed
predominantly as ionic fluoride by the proximal small intestine in a
pH-independent mechanism. In this case, F– crosses the tight junctions
of intestine epithelia arranged between cells and paracellular channels
(Nopakun et al., 1989). The greatest absorption in the small intestine may
be a consequence of about 100-thousand times more [F–] compared to
[HF] in neutral pH (Messer et al., 1989).
The ﬂuoride absorption is also aﬀected by the diet and other foods with
which ﬂuoride interacts. Sodium ﬂuoride (NaF) in water solutions is 100%
absorbed (Whitford, 1996). However, when in combination with foods or
drinks containing calcium, aluminum and magnesium (bi- and -trivalent
cations), ﬂuoride forms insoluble complexes that decrease ﬂuoride absorp-
tion (Trautner and Siebert, 1986). This explains the use of calcium-containing
solutions in acute ﬂuoride toxicity (Li, 2003).
The absorption is also altered by the type of administered ﬂuoride. So far,
there are divergences in the literature about similarities in ﬂuoride absorp-
tion when it comes from NaF or disodium monoﬂuorophosphate (Na2PO3F,
SMFP) (Whitford et al., 1990). The fact is that the absorption of ﬂuoride
from SMFP requires phosphatases-mediated enzymatic hydrolysis, leading
to lower and delayed peak plasma ﬂuoride levels compared to those seen
from NaF. However, the bioavailability of ﬂuoride from both compounds is
roughly the same (Buzalaf et al., 2008).
4.4 Fluoride Distribution
Aer ingestion, plasma ﬂuoride levels rapidly increase until a peak is reached
(around 20–60 min). From then on, the levels start declining to baseline levels
within 3 to 11 h, depending on the dose that is ingested (Whitford, 1996). Blood
plasma is considered the central compartment from which ﬂuoride is distrib-
uted to so and mineralized tissues and specialized body ﬂuids, followed by its
elimination or clearance (Whitford, 1996). In so tissues, ﬂuoride establishes
a steady-state distribution between extracellular and intracellular ﬂuids. Vari-
ations in the plasma ﬂuoride concentrations lead to proportional alterations
in intracellular compartments (Ekstrand, 1996). In mineralized tissues, ﬂuo-
ride is reversibly incorporated mainly in bone, being released back to plasma
during bone remodeling. Hereaer, detailed aspects related to each compart-
ment involved in the ﬂuoride distribution will be described.
4.4.1 Plasma Fluoride
Plasma ﬂuoride is found both as inorganic (ionic or free ﬂuoride) and
organic (nonionic) forms and together they represent the total of plasma ﬂu-
oride (Ekstrand, 1996; Whitford, 1996). Inorganic ﬂuoride is not bound to
plasmatic macromolecules and can be quantiﬁed by a speciﬁc electrode. Its
biological importance in dentistry, medicine and public health is well estab-
lished. Instead, organic ﬂuoride is composed chieﬂy of diﬀerent types of lip-
id-like molecules that bind to plasmatic proteins. Although present in greater
amounts, the biological role of organic ﬂuoride is still unknown. In contrast
to other ions, such as calcium, sodium, phosphorus and chlorine, plasma
ﬂuoride is not homeostatically regulated but reﬂects the balance between
the amount and time of intake, absorption, distribution and uptake, and
renal ﬁltration and excretion (Ekstrand, 1996; Whitford, 1996). In humans
living in areas with ﬂuoridated water (containing 1 ppm F or ∼52.6 µmol L−1),
plasma ﬂuoride concentrations ranged between 0.5 and 1.5 µmol L−1 (Whit-
ford, 1996). Thus, although plasma ﬂuoride levels are modulated by several
physiological factors regardless of ﬂuoride intake, they constitute an import-
ant biomarker of past and present exposures (Ohmi et al., 2005).
4.4.2 Fluoride in So Tissues
The rate of ﬂuoride distribution from plasma to so tissues is determined
by the velocity of blood ﬂow to diﬀerent tissues and organs. It is inﬂuenced
by both the magnitude and direction of transmembrane pH gradient. This
means that the proportion of ﬂuoride levels in plasma or tissue is dependent
on the ratio between the extracellular pH (epH) and intracellular pH (ipH).
It is important to consider that ﬂuoride accumulates in the more alkaline
compartment in response to a pH gradient (Figure 4.1) and that the extra-
cellular ﬂuid is usually more alkaline than the cytosol of mammalian cells.
Thus, it is reasonable to expect higher ﬂuoride concentration in plasma and
extracellular compared than those found in intracellular ﬂuids (ranging
between 10 and 50%) (Whitford et al., 1979). Moreover, as the pH gradient
between membranes can be changed by altering extracellular pH, a ﬂux of
ﬂuoride into and out of cells may be achieved. When epH increases, ﬂuoride
leaves the cell. Inversely, when epH decreases, ﬂuoride goes into the cell. This
is the basis for the alkalinization in cases of acute ﬂuoride toxicity, in order to
promote releasing of ﬂuoride out of cells (Whitford et al., 1979). Besides pH
gradient control, a complex molecular mechanism involved in ﬂuoride toxic-
ity was recently described. It was shown that many species present ﬂuoride
RNA-based riboswitches to control the expression of proteins that soen the
deleterious eﬀects of this anion (Baker et al., 2012). Thus, diﬀerential degrees
of ﬂuoride toxicity may be due to diﬀerences in expression and/or usage of
those ﬂuoride-sensing RNAs, rather than only the pH gradient.
Fluoride concentrations were screened and compared among tissues and
organs using 18F, revealing that the kidney presents the greatest ratio in ﬂuo-
ride concentration, followed by liver, lung, spleen and others (Whitford et al.,
1979). This may explain why these organs are targets for the ﬂuoride-mediated
toxicity (Guo et al., 2003; Pereira et al., 2013). In this context, some compounds
have been used to ameliorate the ﬂuoride-driven toxic response by decreasing
its accumulation in so tissues and organs (Blaszczyk et al., 2012). Reduced
distribution of ﬂuoride in so tissue is accompanied of a greater ﬂuoride
uptake in bone by uncertain mechanisms (Blaszczyk et al., 2012).
4.4.3 Fluoride in Specialized Body Fluids
Specialized body ﬂuids do not present the same ﬂuoride concentration
observed in plasma but changes in ﬂuoride levels are directly modulated
according to plasma concentration. From those, cerebrospinal ﬂuid presents
50% less ﬂuoride from those seen in plasma (Whitford, 1996). Moreover,
gingival crevicular present ﬂuoride levels in a slightly greater fashion com-
pared to plasma (Whitford, 1994). Instead, concentrations in parotid and
submandibular ductal saliva are slightly decreased, showing ductal saliva
and plasma ﬂuoride concentrations ratios around 0.8–0.9 (Whitford, 1994).
Similarities with plasma allowed them to be used as a biomarker of ﬂuoride
exposure aiming to analyze ﬂuoride bioavailability from ﬂuoridated prod-
ucts (Olympio et al., 2007). Fluoride levels in whole saliva are predominantly
more variable and higher compared to ductal saliva because they are oen
contaminated by exogenous ﬂuoride sources such as food, water and ﬂuori-
dated dental products (Whitford et al., 1999).
4.4.4 Fluoride in Mineralized Tissues
About 99% of total ﬂuoride retained in the organism is found in calciﬁed tis-
sues, mainly in bone. Enamel and dentin also contribute but in a lesser extent
(Whitford, 1994). Age seems to be a factor that alters ﬂuoride concentration
in bone. About 36% and 55% of the ﬂuoride daily absorbed in incorporated in
healthy adults (18–75 years) and children (<7 years), respectively (Villa et al.,
2010). This is due to the continuous ﬂuoride intake throughout life (Richards
et al., 1994). Chronic intake of ﬂuoride negatively impacts mineralized tissues,
promoting dental and skeletal ﬂuorosis dependent on dose, time and duration
of exposure. Controversy persists concerning the impact of community water
ﬂuoridation on bone health in adults, and few studies have assessed relation-
ships with bone at younger ages. Recently, bone in adolescents measurements
were performed aer life-long ﬂuoride intake through a prospective cohort
study (Levy et al., 2014). Daily ﬂuoride intake was 0.66 mg for females and 0.78
mg for males and no signiﬁcant relationships between daily ﬂuoride intake
and adolescents’ bone measures we found. The ﬁndings suggest that ﬂuoride
exposures at the typical levels for most US adolescents in ﬂuoridated areas do
not have signiﬁcant eﬀects on bone mineral measures (Levy et al., 2014).
Retention of ﬂuoride in bone is not uniform and is more pronounced in
the periosteal and endosteal regions and in cancelous rather than contact
bone (Richards et al., 1994). Bone ﬂuoride is divided into surface and inner
compartments. The ﬁrst is rapidly exchangeable while the latter is not. In
surface and inner bone, ﬂuoride uptake occurs in diﬀerent steps. First, iso-
and heteroionic exchanges on the hydration shells of bone crystallites take
place in contact with extracellular ﬂuids. This proximity allows a steady-state
relationship between the extracellular ﬂuid (that represents physiological
plasma ﬂuoride concentration) and hydration shells of bone regulating ﬂuo-
ride concentration. This characteristic corroborates the use of bone surface
as a terminal biomarker of acute ﬂuoride exposure (Buzalaf et al., 2005). Sec-
ondly, ﬂuoride incorporates into precursors of hydroxyﬂuorapatite followed
by the apatitic lattice itself (Neuman and Neuman, 1958).
Fluoride associated with the nonexchangeable inner compartment, is irre-
versibly bound. Instead, it is continuous mobilized during bone remodeling
and bone absorption (Rao et al., 1995).
Levels of ﬂuoride in dentin are quite similar to bone and increase aer
long-term intake. Higher concentration is found in pulp from which it starts
to reduce as it approaches the dentin-enamel junction (Hallsworth and
Weatherell, 1969). Enamel ﬂuoride content also enhances with age but is
comparatively lower and unrelated to dentin (Vieira et al., 2006). Usually, ﬂu-
oride concentration in tooth enamel reﬂects ﬂuoride exposure during amelo-
genesis (Hallsworth and Weatherell, 1969). However, although enamel is the
target tissue for dental ﬂuorosis, correlations between ﬂuoride content in
enamel and ﬂuorosis occurrence were not found (Vieira et al., 2006).
4.5 Kidney Excretion
Fluoride renal excretion is one of the most important mechanisms for the
regulation of ﬂuoride levels in the body (Buzalaf and Whitford, 2011). About
60% and 45% of the daily ingested ﬂuoride is excreted in urine of healthy
adults and children, respectively (Villa et al., 2010). Increases in urinary ﬂu-
oride were observed in children aged 5–8 years living in a nonﬂuoridated
community aer using ﬂuoride containing-dental varnishes (Garcia-Hoyos
et al., 2012). Thus, plasma and the kidney excretion rate constitutes the
physiologic balance determined by ﬂuoride intake, uptake to and removal
from bone and the capacity of ﬂuoride clearance by the kidney. Fluoride
is freely ﬁltered through the glomerulus and undergoes a variable degree
of proximal tubular reabsorption, directly related to glomerular ultraﬁl-
trate pH (Whitford, 1994). Aer ﬂuoride enters the renal tubules, a variable
amount (from 10 to 90%) is reabsorbed to systemic circulation, depending
on the urinary pH because transmembrane migration occurs by diﬀusion of
HF (Whitford et al., 1976). When the pH of tubular ﬂuid is relatively high,
the amount of ﬂuoride as HF is lower, while it is higher as F–. Thus, low ﬂu-
oride concentration is available to be reabsorbed crossing the epithelium of
the epithelium renal tubule into the interstitial ﬂuid. Instead, the majority
of nonreabsorbed is excreted in urine as F– (Ekstrand, 1996) (Figure 4.4).
In addition, any factor that aﬀects urinary pH will have an impact on the
amount of ﬂuoride that is excreted in urine (Buzalaf and Whitford, 2011).
Urinary F excretion is also inﬂuenced by the glomerular ﬁltration rate since
its reduction, as occurs in chronic renal dysfunction as well as in the last
decades of life, results in lower excretion and increased plasma F levels
Fluoride causes kidney injury seen by tubular dysfunction at plasma con-
centrations of ﬂuoride 0.625 µg ml−1 and renal cell apoptosis (Bai et al.,
Figure 4.4 Fluoride reabsorption from renal tubule to systemic circulation. The
extent of ﬂuoride reabsoption is depedent on urine pH. When the
urine is alkaline, the concentration of HF, the most permeable form
of ﬂuoride, is lower and most of the ﬂuoride remains in the tubule to
be excreted (on the le). Conversely, when the urine is acidic, the con-
centration of HF is higher that crosses to the renal tubule membrane
towards interstitium. Herein, it dissociates forming F– that diﬀuses into
the peritubular capillaries returning to systemic circulation (on the
2010). Several urine markers such as urinary kidney injury molecule (Kim-1),
clusterin (Clu), osteopontin, β-2-microglobulin, cystatin-C and heat shock
protein (HSP)-72 were shown to be upmodulated in 50 ppm ﬂuoride-treated
rats compared to untreated rats (Cardenas-Gonzalez et al., 2013), while a tox-
icity marker (aﬂatoxin B1 aldehyde reductase) was up-modulated (Kobayashi
et al., 2011).
4.6 Modulators of Fluoride Metabolism and Their
Any systemic, metabolic and genetic alteration can modify ﬂuoride metab-
olism. They interfere with absorption, excretion and so, with ﬂuoride fate
in the body. Imbalances may lead to pathological conditions such as acute
and chronic toxicities, dental and skeletal ﬂuorosis. They include acid–base
disturbances, physical activity, circadian rhythm and hormones (Whitford,
1996). Other factors such as nutritional status, diet composition, renal
impairment and genetic predisposition modify ﬂuoride metabolism. They
are described in detail below.
4.6.1 Acid–Base Disturbances
Considering that urinary pH is a limitant factor in determining ﬂuoride reab-
sorption and/or renal excretion (Figure 4.4), it seems obvious that acid–base
disturbances promote variations in ﬂuoride tissue concentration. The acid–
base disturbances are usually induced by diet composition. In this sense,
vegetarian-based diet turns towards alkaline pH, while a meat-rich diet turns
towards acidic pH. Acute respiratory disorders similarly impact renal excre-
tion as well as metabolic disorders (Whitford, 1996).
4.6.2 Physical Activity, Circadian Rhythm and Hormones
Physical activity is associated with either decreased or increased circulating
ﬂuoride levels (Whitford, 1996). Prolonged exercise causes enhancement of
diﬀusion of HF from extracellular to intracellular ﬂuid in skeletal muscle
cells, a phenomenon caused by reduction in the pH gradient between cell
A biological rhythmicity of ﬂuoride was found to be opposite to that of
calcium and phosphate, discouraging the role of bone as a regulator. The
mean ﬂuoride peak is reached around 9 a.m., and decreased around 9 p.m
in animals (Whitford, 1996). In humans, plasma ﬂuoride concentrations
were positively correlated with urinary ﬂuoride excretion rates and serum
parathormone levels, suggesting that both the renal system and hormones
regulate ﬂuoride rhythmicity (Cardoso et al., 2008). In agreement with this,
Villa et al. (2008) demonstrated that ﬂuoride urinary excretion is lower in the
diurnal compared to nocturnal period.
4.6.3 Nutritional Status
There is a consensus that malnutrition and dental ﬂuorosis prevalence are
related. However, this association still needs to be scientiﬁcally proven. The lack
of evidence so far is due to diﬃculties in data interpretations. A relationship
between water ﬂuoride concentration, socioeconomic status, nutritional status
and prevalence of enamel lesions in child from Saudi Arabia was demonstrated
(Rugg-Gunn et al., 1997). In Brazil, using assessment of malnutrition by height-
for-age and weight-for-age indexes, dental ﬂuorosis was shown to be dependent
on water ﬂuoride concentration but independent of malnutritional status (Cor-
reia Sampaio et al., 1999). To clarify this controversy, longitudinal studies should
be conducted where variables such as nutritional status, diet composition and
ﬂuoride intake are all implicated and controlled during tooth formation.
4.6.4 Diet Composition
Diet that interferes in the acidiﬁcation and alkalinization of urine promotes
signiﬁcant diﬀerences in urinary ﬂuoride clearance and plasma half-lives. As
described above, this principle applies for protein-rich and vegetarian diets.
Long-term diet-induced changes in urinary pH could decrease (vegetari-
an-induced alkaline urine) or increase (protein-induced acidic urine) the risk
of dental ﬂuorosis (Whitford, 1997). Thus, vegetarianism is inversely related
to dental ﬂuorosis prevalence (Awadia et al., 1999).
High dietary concentrations of calcium also reduce ﬂuoride absorption
(Whitford, 1994). Its impact over dental ﬂuorosis prevalence was also evi-
denced (Chen et al., 1997). The rate of dental ﬂuorosis of a milk-drinking
group was 7.2%, whereas that of the nonmilk-drinking group was 37.5%
(Chen et al., 1997). This and other data support the use of calcium supple-
mentation in endemic ﬂuorosis areas.
Regular diets also seem to regulate ﬂuoride retention in the body. Intake of
sorghum was associated with higher ﬂuoride-induced toxicity compared to
rice- and wheat-based diets. These data were accompanied by less and more
ﬂuoride urinary excretion, respectively (Lakshmaiah and Srikantia, 1977).
Also, a multigrain diet enriched with protein was showed to attenuate ﬂuoro-
sis toxicity (Vasant and Amaravadi, 2013).
4.6.5 Renal Impairment
Greater plasma ﬂuoride was observed in patients with renal failure (Schmidt
and Leuschke, 1986). However, no diﬀerences were observed in plasma insu-
lin level, as a result of ﬂuoride intake, when control and renal-deﬁcient rats
were compared (Lupo et al., 2011). Moreover, renal impairment in children
has been associated with tooth defects that include enamel pitting and hypo-
plasia. Intake of ﬂuoride by nephrectomized rats impaired ﬂuoride clearance
from plasma and worsened tooth development (Lyaruu et al., 2008). This
relationship was also demonstrated in humans (Farge et al., 2006).
4.6.6 Genetic Predisposition
Increasing evidence has led to the aﬃrmative that dental ﬂuorosis is geneti-
cally inﬂuenced (Polzik et al., 1994). This is clariﬁed through observation that
not all children living in the same ﬂuoridated area develop dental ﬂuorosis
(Yoder et al., 1998). The genetic character is reinforced when diﬀerences in
urinary ﬂuoride levels and meal ﬂuoride values are not found among these
children. The genetic predisposition to dental ﬂuorosis is also well established
in mice (Everett et al., 2002; Everett, 2011). Examination of 12 inbred strains of
mice showed diﬀerences in susceptibility to dental ﬂuorosis. The A/J strain is
“susceptible”, with a rapid onset and severe development of dental ﬂuorosis,
while the 129P3/J is “resistant”, with minimum development of dental ﬂuoro-
sis (Everett et al., 2002). These strains also diﬀer regarding their susceptibili-
ties to the eﬀects of ﬂuoride in bone (Mousny et al., 2006; Everett, 2011). To
determine whether such diﬀerences were due to diﬀerences in F metabolism,
a metabolic study was conducted in which total F intake and excretion were
measured. The data showed that, compared to A/J mice, 129P3/J mice ingested
less water, excreted less urine, had lower urinary ﬂuoride excretion and conse-
quently had higher F retention and plasma and femur ﬂuoride levels (Carvalho
et al., 2009). Despite this, the amelogenesis in 129P3/J mice was remarkably
unaﬀected by ﬂuoride (Carvalho et al., 2009). Recently, the molecular mecha-
nisms underlying the renal ﬂuoride metabolism in A/J and 129P3/J mice that
may account for their diﬀerential metabolic handling of ﬂuoride were deter-
mined using a proteomic approach (Carvalho et al., 2013). In F-treated groups,
Na(+)/H(+) exchange regulatory cofactor NHE-RF3, a protein involved in the
regulation of renal tubular reabsorption capacity was downmodulated in the
kidney of 129P3/J mice (Carvalho et al., 2013). Exclusive proteins expressed in
A/J or 129P3/J mice exhibited the same proﬁle, regardless of exposure to ﬂuo-
ride, suggesting that genetic background per se accounts for such diﬀerences
between these two strains of mice. In addition, some proteins that are codiﬁed
by chromosomes 2 and 11, loci previously characterized to determine suscep-
tibility and resistance to dental ﬂuorosis in A/J and 129P3/J mice, respectively
(Everett et al., 2009; Everett et al., 2011), were found and may disclose the
molecular mechanism of dental ﬂuorosis (Carvalho et al., 2013).
Besides, several gene polymorphisms have been addressed to explore pos-
sible correlation with dental ﬂuorosis in humans. They include parathyroid
hormone (Wen et al., 2012), myeloperoxidase (Zhang et al., 2013), collagen1A2
(Huang et al., 2008) and estrogen receptor (Ba et al., 2011) gene polymor-
phisms. Except for parathyroid hormone gene, the others showed an associa-
tion between polymorphisms and risk and/or development of dental ﬂuorosis.
Fluoride amounts in the body are tightly regulated and have a direct impact
on cellular and systemic levels. It is clear that the beneﬁcial as well as the
adverse impact can be attributed to the magnitude and duration of ﬂuoride
exposure. In addition to the individual physiological diﬀerences in han-
dling ﬂuoride, ﬂuoride levels are also modulated by environmental, bio-
chemical, physiological, pathological and molecular factors. In the past
few years, this area of research was aimed to understand the molecular
mechanisms involved in ﬂuoride metabolism and its biological eﬀects.
Although considerable progress has been achieved, there is still much to
● Fluoride is a cariostatic agent and is implemented in several systemic
and local sources.
● Fluoride presents both positive and negative impacts on hard tissues
depend on the dose and time of exposure.
● Fluoride metabolism includes its absorption in the gastrointestinal
tract, distribution through so and hard tissues followed by renal
● The coeﬃcient of permeability to HF is much higher compared to ionic
● Absorption of ﬂuoride in the stomach occurs in a pH-dependent man-
ner and the tubular reabsorption depends on the urinary pH.
● Plasma is an important biomarker of ﬂuoride exposure.
● Most ﬂuoride absorbed is incorporated in bone.
● Several conditions aﬀect ﬂuoride metabolism including acid–base
disturbances, physical activity, circadian rhythm and hormones,
nutritional status, diet composition, renal impairment and genetic
Deﬁnitions and Explanation of Key Terms
Systemic toxicity: Requires absorption and distribution of the toxicant to a
site distant from its entry point, at which point eﬀects are produced. Most
chemicals that produce systemic toxicity do not cause a similar degree of
toxicity in all organs, but usually demonstrate major toxicity to one or two
Coeﬃcient of permeability: A rate of particle diﬀusion through cell
membranes that depends on the particle and total area through which it
pH gradient: A gradient of hydrogen ion concentration between extracel-
lular and intracellular compartments.
Weak acid: An acid that is partially dissociated in an aqueous solution. It
is measured by the acid dissociation constant (Ka).
Steady-state distribution: A condition of a physical system or device
that does not change over time, or in which any one change is contin-
ually balanced by another, such as the stable condition of a system in
Fluc proteins-based channel: Fluoride ion exporters formed by a group of
small membrane proteins known as the “crcB” family or Fluc, a protein
that is widespread among unicellular organisms.
CLC anion-transporting proteins: A subclass of the widespread CLC super-
family of anion-transport proteins.
Fluoride exporter named FEX: A widespread family of ﬂuoride export pro-
teins that mediated eukaryotic resistance to ﬂuoride toxicity.
Tight junctions: Form the closest contact between adjacent cells known
Bone remodeling: A lifelong process where mature bone tissue is
removed from the skeleton, a process called bone resorption, and
new bone tissue is formed, a process called ossification or new bone
RNA-based riboswitches: Riboswitches are specific components of an
mRNA molecule that regulates gene expression. The riboswitch is a part
of an mRNA molecule that can bind and target small target molecules.
Glomerular ﬁltration rate: The ﬂow rate of ﬁltered ﬂuid through the
Amelogenesis: The formation ofenamel onteeth and begins when the
crown is forming during the bell stage oooth development aer dentino-
genesis, which is the formation of dentin. It has three stages: the induc-
tive, the secretory, and the maturation stages.
Dental ﬂuorosis: A developmental disturbance of dental enamel caused
by excessive exposure to high concentrations of ﬂuoride during tooth
development. It presents diﬀerent appearances, varying from tiny white
streaks or specks in the enamel to discoloration or brown markings in the
tooth. The enamel may be pitted and rough.
Skeletal ﬂuorosis: A bone disease caused by excessive consumption of ﬂu-
oride that causes pain and damage to bones and joints.
List of Abbreviations
HF hydrogen ﬂuoride
pKa the logarithmic form of acid dissociation constant
F– ﬂuoride ion
NaF sodium ﬂuoride
SMFP disodium monoﬂuorophosphate
epH extracellular pH
ipH intracellular pH
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