Introduction

Fluorine occurs naturally in the Earth's crust, water, and food as the negatively charged ion, fluoride (F^-). Fluoride is considered to be 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. 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. 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.

Food Sources

The fluoride content of most foods is low (less than 0.05 mg/100g) and may vary from place to place because soil mineral content varies geographically. 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, canned meats, hot dogs, and infant foods also add fluoride to the diet. Foods generally contribute 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.

Some important food sources of fluoride:

Tea	Chicken	Sardines	Salmon	Cod
Corned beef	Shrimp	Mackerel	Seaweed	Sausages

Recommended Dietary Allowance (RDA)

The European Union has not set an RDA for the general population. In the US the Food and Nutrition Board set an Adequate Intake (AI) level of 4 mg/day for adults and lower levels for children.

Inhibitors/stimulators:

The following food components have been found to stimulate the absorption of fluoride:

Chloride (salt) – a diet low in chloride (salt) has been found to increase fluoride retention by reducing urinary excretion of fluoride.

The following food components have been found to inhibit the absorption of fluoride :

Calcium – forms insoluble complexes with fluoride and is 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.

Magnesium – forms insoluble complexes with fluoride and is capable of significantly decreasing fluoride absorption when present in the same meal.

Functions in the Body

Fluoride is absorbed in the stomach and small intestine. Once in the bloodstream, it rapidly enters mineralized tissue (bone 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.

Deficiency

The only clear effect of inadequate fluoride intake is an increased risk of dental caries (tooth decay) for individuals of all ages. 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.

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 life-threatening symptoms is considered to be 5 mg/kg of body weight. Nausea, abdominal pain, and vomiting almost always accompany acute fluoride toxicity. Other symptoms like diarrhoea, excessive salivation and tearing, sweating, and generalized weakness may also occur.

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 only at fluoride intakes about 5 times the recommended level. 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.

Regulation

Generally, most of the fluoride in food or water that is swallowed enters the bloodstream quickly through the digestive tract. However, the amount that enters the bloodstream also depends on factors such as how much of the fluoride was swallowed, and how well the fluoride dissolves in water. Factors such as age and health status affect what happens to the fluoride ion once it is in your body. After entering the body, about half of the fluoride leaves the body quickly in urine, usually within 24 hours unless large amounts (20 mg or more, which is the amount in 20 or more liters of optimally fluoridated water) are ingested. Most of the fluoride ion that stays in the body is stored in your bones and teeth.