Linus Pauling Institute
Micronutrient Research for Optimum Health

 
 
Search this site only.



 

Micronutrient Information Center

To receive more information about up-to-date research on micronutrients, sign up for the free, semi-annual LPI Research Newsletter here.

Fluoride

Fluorine occurs naturally as the negatively charged ion, fluoride (F^-). Fluoride is considered a trace element because only small amounts are present in the body (about 2.6 grams in adults), and because the daily requirement for maintaining dental health is only a few milligrams a day. About 95% of the total body fluoride is found in bones and teeth (1). Although its role in the prevention of dental caries (tooth decay) is well established, fluoride is not generally considered an essential mineral element because humans do not require it for growth or to sustain life (2). However, if one considers the prevention of chronic disease (dental caries) an important criterion in determining essentiality, then fluoride might well be considered an essential trace element (3). 

Function

Fluoride is absorbed in the stomach and small intestine. Once in the blood stream it rapidly enters mineralized tissue (bones and developing teeth). At usual intake levels, fluoride does not accumulate in soft tissue. The predominant mineral elements in bone are crystals of calcium and phosphate, known as hydroxyapatite crystals. Fluoride's high chemical reactivity and small radius allow it to either displace the larger hydroxyl (-OH) ion in the hydroxyapatite crystal, forming fluoroapatite, or to increase crystal density by entering spaces within the hydroxyapatite crystal. Fluoroapatite hardens tooth enamel and stabilizes bone mineral (4). 

Nutrient interactions

Both calcium and magnesium form insoluble complexes with fluoride and are capable of significantly decreasing fluoride absorption when present in the same meal. However, the absorption of fluoride in the form of monofluorophosphate (unlike sodium fluoride) is unaffected by calcium. Also, a diet low in chloride (salt) has been found to increase fluoride retention by reducing urinary excretion of fluoride (1).

Deficiency

In humans, the only clear effect of inadequate fluoride intake is an increased risk of dental caries (tooth decay) for individuals of all ages. Epidemiological investigations of patterns of water consumption and the prevalence of dental caries across various U.S. regions with different water fluoride concentrations led to the development of a recommended optimum range of fluoride concentration of 0.7-1.2 mg/liter or parts per million (ppm); the lower concentration was recommended for warmer climates where water consumption is higher, and the higher concentration was recommended for colder climates. A number of studies conducted prior to the introduction of fluoride-containing toothpastes demonstrated that the prevalence of dental caries was 40% to 60% lower in communities with optimal water fluoride concentrations than in communities with low water fluoride concentrations (5).

The Adequate Intake (AI)

The Food and Nutrition Board (FNB) of the Institute of Medicine updated its recommendations for fluoride intake in 1997. The FNB felt there were inadequate data to set a Recommended Dietary Allowance (RDA); instead, Adequate Intake (AI) levels were set based on estimated intakes (0.05 mg/kg of body weight) that have been shown to reduce the occurrence of dental caries most effectively without causing the unwanted side effect of tooth enamel mottling known as dental fluorosis (5). See the section below on Safety for a discussion of dental fluorosis.

 

Disease Prevention

Dental caries (cavities and tooth decay)

Specific cariogenic (cavity-causing) bacteria found in dental plaque are capable of metabolizing certain carbohydrates (sugars) and converting them to organic acids that can dissolve susceptible tooth enamel. If unchecked, the bacteria may penetrate deeper layers of the tooth and progress into the soft pulp tissue at the center. Untreated caries can lead to severe pain, local infection, tooth loss or extraction, nutritional problems, and serious systemic infections in susceptible individuals (6). Increased fluoride exposure, most commonly through water fluoridation, has been found to decrease dental caries in children and adults (7). Fluoride consumed in water appears to have a systemic effect in children before teeth erupt, as well as a topical (surface) effect in adults and children after teeth have erupted. Between 1950 and 1980 clinical studies in 20 different countries demonstrated that the addition of fluoride to community water supplies (0.7-1.2 ppm) reduced caries by 40%-50% in primary (baby) teeth and 50%-60% in permanent teeth (7).

Although the role of fluoride in preventing dental caries is well established, the mechanisms for its effects are not entirely understood. Originally, it was believed that fluoride incorporated into the enamel during tooth development resulted in a more acid-resistant enamel. More recent research indicates that the primary action of fluoride occurs topically (at the surface) after the teeth erupt into the mouth. When enamel is partially demineralized by organic acids, fluoride in the saliva can enhance the remineralization of enamel through its interactions with calcium and phosphate. In the presence of fluoride, remineralized enamel contains more fluoride and is more resistant to demineralization. In salivary concentrations associated with optimum fluoride intake, fluoride has been found to inhibit bacterial enzymes, resulting in reduced acid production by cariogenic bacteria (6, 7).

Osteoporosis

Although fluoride in pharmacologic doses has been shown to be a potent therapeutic agent for increasing spinal bone mass (see Disease treatment), there is little evidence that water fluoridation at optimum levels for the prevention of dental caries is helpful in the prevention of osteoporosis. The majority of studies conducted to date have failed to find clinically significant differences in bone mineral density or fracture incidence when comparing residents of areas with fluoridated water supplies to residents in areas without fluoridated water supplies (8). However, two studies found that drinking water fluoridation was associated with decreased incidence of hip fracture in the elderly. In addition, one study in Italy found a significantly greater risk of femoral (hip) fractures in men and women residing in an area with low water fluoridation (0.05 ppm) compared to the risk in a similar population whose water supply was naturally fluoridated (1.45 ppm) at higher than optimum levels for prevention of dental caries (9). Another study in Germany found no significant difference in bone mineral density between residents of a community whose water supply had been optimally fluoridated for 30 years (1 ppm) compared with those who resided in a community without fluoridated water. However, this study reported that the incidence of hip fracture in men and women, age 85 years or older, was significantly lower in the community with fluoridated water compared to the community with nonfluoridated water, despite higher calcium levels in the nonfluoridated water supply (10). More recently, a community-based study in 1,300 women found that elevated serum fluoride concentrations were not related to bone mineral density or to osteoporotic fracture incidence (11).

Disease Treatment

Osteoporosis

Osteoporosis is characterized by decreased bone mineral density (BMD) and increased bone fragility and susceptibility to fracture. In general, decreased BMD is associated with increased risk of fracture. However, the usual relationship between BMD and fracture risk does not always hold true when very high (pharmacologic) doses of fluoride are used to treat osteoporosis. Most available therapies for osteoporosis (e.g., estrogen, calcitonin, and bisphosphonates) decrease bone loss (resorption), resulting in very small increases in BMD. Pharmacologic doses of fluoride are capable of producing large increases in the BMD of the lumbar spine. Overall, therapeutic trials of fluoride in patients with osteoporosis have not consistently demonstrated significant decreases in the occurrence of vertebral fracture despite dramatic increases in lumbar spine BMD (12). A meta-analysis of 11 controlled studies, including 1,429 participants, found that fluoride treatment resulted in increased BMD at the lumbar spine but was not associated with a lower risk of vertebral fractures (13). This meta-analysis also found that higher concentrations of fluoride were associated with increased risk of non-vertebral fractures. Early studies using high doses of sodium fluoride (75 mg/day) may have induced rapid bone mineralization in the absence of adequate calcium and vitamin D, resulting in denser bones that were not mechanically stronger (14). Some controlled studies using lower doses, intermittent dosage schedules, or slow release formulations (enteric coated sodium fluoride) have demonstrated a decreased incidence of vertebral fracture along with increased bone density of the lumbar spine (15-17). Analysis of bone architecture has also shed some light on the inconsistent effect of fluoride therapy in reducing vertebral fractures. Recent research indicates that osteoporosis may be associated with an irreversible change in the architecture of bone known as decreased trabecular connectivity. Normal bone consists of a series of plates interconnected by thick rods. Severely osteoporotic bone has fewer plates, and the rods may be fractured or disconnected (decreased trabecular connectivity). Despite fluroide therapy increasing bone density, it probably cannot restore connectivity in patients with severe bone loss. Thus, fluoride therapy may be less effective in osteoporotic individuals who have already lost substantial trabecular connectivity (12, 18).

Safety of fluoride therapy for osteoporosis

Serious side effects have been associated with the high doses of fluoride used to treat osteoporosis. They include gastrointestinal irritation, joint pain in the lower extremities, and the development of calcium deficiency as well as stress fractures. The reasons for the occurrence of lower extremity joint pain and stress fractures in patients taking fluoride for osteoporosis remain unclear, but they may be related to rapid increases in bone formation without sufficient calcium to support such an increase (12). Presently, enteric coated sodium fluoride or monofluorophosphate preparations offer a lower side effect profile than the high-dose sodium fluoride used in earlier trials. Additionally, sufficient calcium and vitamin D must be provided to support fluoride-induced bone formation. Although fluoride therapy may be beneficial for the treatment of osteoporosis in appropriately selected and closely monitored individuals, uncertainty about its safety and benefit in reducing fractures has kept the Food and Drug Administration (FDA) from approving fluoride therapy for osteoporosis (19). Combinations of lower doses of fluoride with antiresorptive agents, such as estrogen or bisphosphonates, may improve therapeutic results while minimizing side effects, and thus these therapies are considered worthy of further study (20, 21).

Sources

Water fluoridation

The major source of dietary fluoride in the U.S. diet is drinking water. When water is fluoridated, it is adjusted to between 0.7 and 1.2 milligrams (mg) of fluoride per liter, which is 0.7-1.2 ppm. This concentration has been found to decrease the incidence of dental caries while minimizing the risk of dental fluorosis and other adverse effects (see Safety). Approximately 62% of the U.S. population consumes water with sufficient fluoride for the prevention of dental caries. The average fluoride intake for adults living in fluoridated communities ranges from 1.4 to 3.4 mg/day. Because well water can vary greatly in its fluoride content, people who consume water from wells should have the fluoride content of their water tested by their local water district or health department. Water fluoride testing may also be warranted in households that use home water treatment systems. While water softeners are not thought to change water fluoride levels, reverse osmosis systems, distillation units, and some water filters have been found to remove significant amounts of fluoride from water. However, Brita-type filters do not remove fluoride (5, 19).

Bottled water sales have grown exponentially in the U.S. in recent years, and studies have found that most bottled waters contain sub-optimal levels of fluoride, although there is considerable variation. For example, a study of 78 different bottled water products in Iowa found that over 80% had fluoride concentrations of less than 0.3 ppm; however, 10% of the tested products had fluoride concentrations of 0.7 ppm or greater (22). Several other studies have reported similar findings, with most bottled waters relatively low in fluoride, but a few in the optimal range or higher (23, 24).

While consumption of fluoride from water presents very little risk of adverse effects in adults except in extreme circumstances (see Safety), consumption of relatively large amounts of water mixed with formula concentrates appears to increase the risk for the development of dental fluorosis in infants (25, 26). One study found that, on average, at least half of all fluoride ingested by infants 6 months and younger was from water mixed with formula concentrates (27).

Food and beverage sources

The fluoride content of most foods is low (less than 0.05 mg/100 grams). Rich sources of fluoride include tea, which concentrates fluoride in its leaves, and marine fish that are consumed with their bones (e.g., sardines). Foods made with mechanically separated (boned) chicken, such as canned meats, hot dogs, and infant foods, also add fluoride to the diet (28). In addition, certain fruit juices, particularly grape juices, often have relatively high fluoride concentrations (29). Foods generally contribute only 0.3-0.6 mg of the daily intake of fluoride. An adult male residing in a community with fluoridated water has an intake range from 1-3 mg/day. Intake is less than 1 mg/day in non-fluoridated areas (2). The table below provides a range of fluoride content for a few fluoride-rich foods (5). 

 

Fluoride supplements

Fluoride supplements, available only by prescription, are intended for children living in areas with low water fluoride concentrations for the purpose of bringing their intake to approximately 1 mg/day (5). The American Dental Association recommends fluoride supplements for those children living in areas with suboptimal water fluoridation (30). The supplemental fluoride dosage schedule in the table below was recommended by the American Dental Association, the American Academy of Pediatric Dentistry, and the American Academy of Pediatrics (31). It requires knowledge of the fluoride concentration of local drinking water as well as other possible sources of fluoride intake. For more detailed information regarding fluoride and the prevention of dental caries, visit the American Dental Association Web site. 

American Dental Association Fluoride Supplement Schedule

Toothpaste

Fluoridated toothpastes are very effective in preventing dental caries but also add considerably to fluoride intake of children, especially young children who are more likely to swallow toothpaste. Researchers estimate that children under 6 years of age ingest an average of 0.3 mg of fluoride from toothpaste with each brushing. Children under the age of 6 years who ingest more than 2 or 3 times the recommended fluoride intake are at increased risk of a white speckling or mottling of the permanent teeth, known as dental fluorosis. A major source of excess fluoride intake in this age group comes from swallowing fluoride-containing toothpaste. To prevent dental fluorosis while providing optimum protection from tooth decay, it is recommended that parents supervise children under 6 years of age while brushing with fluoridated toothpaste. In addition to discouraging the swallowing of toothpaste, children should be encouraged to use no more than a pea-size application of toothpaste and to rinse their mouths with water after brushing (1, 5).

Safety

Adverse effects

Fluoridation of public drinking water in the U.S. was initiated over 50 years ago. Since then, a number of adverse effects have been attributed to water fluoridation. However, extensive scientific research has uncovered no evidence of increased risks of cancer, heart disease, kidney disease, liver disease, Alzheimer's disease, birth defects, or Down's syndrome (32, 33). The use of high doses of fluoride to treat osteoporosis has been associated with some adverse effects, which are discussed in the Disease Treatment section above.

Acute toxicity

Fluoride is toxic when consumed in excessive amounts, so concentrated fluoride products should be used and stored with caution to prevent the possibility of acute fluoride poisoning, especially in children and other vulnerable individuals. The lowest dose that could trigger adverse symptoms is considered to be 5 mg/kg of body weight, with the lowest potentially fatal dose considered 15 mg/kg of body weight. Nausea, abdominal pain, and vomiting almost always accompany acute fluoride toxicity. Other symptoms like diarrhea, excessive salivation and tearing, sweating, and generalized weakness may also occur (32). In order to prevent acute fluoride poisoning, the American Dental Association has recommended that no more than 120 mg of fluoride (224 mg of sodium fluoride) be dispensed at one time (19).

Dental fluorosis

The mildest form of dental fluorosis is detectable only to the trained observer and is characterized by small opaque white flecks or spots on the enamel of the teeth. Moderate dental fluorosis is characterized by mottling and mild staining of the teeth, and severe dental fluorosis results in marked staining and pitting of the teeth. In its moderate to severe forms, dental fluorosis becomes a cosmetic concern when it affects the incisors and canines (front teeth). Dental fluorosis is a result of excess fluoride intake prior to the eruption of the first permanent teeth (generally before 8 years of age). It is also a dose dependent condition, with higher fluoride intakes being associated with more pronounced effects on the teeth. The risk of mild to moderate dental fluorosis appears to increase significantly at an intake 2-3 times that recommended for children of a susceptible age, while severe dental fluorosis has been seen in the U.S. only at fluoride intakes about 5 times the recommended level (33). The incidence of mild and moderate dental fluorosis has increased over the past 50 years, mainly due to increasing fluoride intake from toothpaste, although inappropriate use of fluoride supplements may also contribute. In 1997, the Food and Nutrition Board (FNB) of the Institute of Medicine based its recommendation for upper levels of fluoride intake (UL) on the prevention of moderate enamel fluorosis (5).

 

Skeletal fluorosis

Intake of fluoride at excessive levels for long periods of time may lead to changes in bone structure known as skeletal fluorosis. The early stages of skeletal fluorosis are characterized by increased bone mass, detectable by x-ray. If very high fluoride intake persists over many years, joint pain and stiffness may result from the skeletal changes. The most severe form of skeletal fluorosis is known as "crippling skeletal fluorosis," which may result in calcification of ligaments, immobility, muscle wasting, and neurological problems related to spinal cord compression. Most estimates indicate that crippling skeletal fluorosis occurs only when fluoride intakes exceed 10-25 mg/day for at least ten years. Crippling skeletal fluorosis is extremely rare in the U.S.; in fact, only five cases have been confirmed in the last 35 years. Interestingly, studies of communities in the U.S. where water fluoride concentrations were as high as 20 mg/L (ppm), allowing for fluoride intakes as high as 20 mg/day, did not find evidence of crippling skeletal fluorosis. Such water fluoride concentrations are higher than those known to have resulted in crippling skeletal fluorosis in other countries, suggesting that metabolic or dietary factors might render some populations more susceptible (5, 32).

Drug Interactions

Calcium supplements, as well as calcium and aluminum containing antacids, can decrease the absorption of fluoride. It is best to take these products two hours before or after fluoride supplements (34).

Linus Pauling Institute Recommendation

The safety and public health benefits of optimally fluoridated water for prevention of tooth decay in people of all ages have been well-established. The Linus Pauling Institute supports the recommendations of the American Dental Association and the Centers for Disease Control and Prevention, which include optimally fluoridated water as well as the use of fluoride toothpaste, fluoride mouthrinse, fluoride varnish, and when necessary, fluoride supplementation. Due to the risk of fluorosis, any fluoride supplementation should be prescribed and closely monitored by a dentist or physician.

References

---

Written in February 2001 by:
Jane Higdon, Ph.D.
Linus Pauling Institute
Oregon State University

Updated in September 2007 by:
Victoria J. Drake, Ph.D.
Linus Pauling Institute
Oregon State University

Reviewed in September 2007 by:
John J. Warren, D.D.S., M.S.
Professor
Preventive & Community Dentistry
College of Dentistry
The University of Iowa

Copyright 2001-2013  Linus Pauling Institute

---

Disclaimer

The Linus Pauling Institute Micronutrient Information Center provides scientific information on the health aspects of dietary factors and supplements, foods, and beverages for the general public. The information is made available with the understanding that the author and publisher are not providing medical, psychological, or nutritional counseling services on this site. The information should not be used in place of a consultation with a competent health care or nutrition professional.

The information on dietary factors and supplements, foods, and beverages contained on this Web site does not cover all possible uses, actions, precautions, side effects, and interactions. It is not intended as nutritional or medical advice for individual problems. Liability for individual actions or omissions based upon the contents of this site is expressly disclaimed.