Fluoride: Toxicity, Genetic diseases, Requirements, dietary sources, Micronutrient interactions

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.

Toxicity

Fluorine, like other trace elements, is toxic when consumed in excessive amounts. The primary adverse effects associated with chronic, excessive fluoride intake are enamel and skeletal fluorosis. Enamel fluorosis is a dose-related effect caused by fluoride ingestion during the pre-eruptive development of the teeth. After the enamel has completed its pre-eruptive maturation, it is no longer susceptible. Inasmuch as enamel fluorosis is regarded as a cosmetic effect, it is the anterior teeth that are of most concern. The pre-eruptive maturation of the crowns of the anterior permanent teeth is finished and the risk of fluorosis is over by 8 years of age. Therefore, fluoride intake up to the age of 8 years is of most interest. Mild fluorosis (which is not readily apparent) has no effect on tooth function and may render the enamel more resistant to caries. In contrast, the moderate and severe forms of enamel fluorosis are generally characterized by esthetically objectionable changes in tooth color and surface irregularities.

Skeletal fluorosis has been regarded as having three stages. Stage 1 is characterized by occasional stiffness or pain in joints and some osteosclerosis of the pelvis and vertebrae, whereas the clinical signs in stages 2 and 3, which may be crippling, include dose-related calcification of ligaments, osteosclerosis, exostoses, and possibly osteoporosis of long bones, muscle wasting, and neurological defects owing to hypercal-cification of vertebrae. The development of skeletal fluorosis and its severity are directly related to the level and duration of exposure. Most epidemiological research has indicated that an intake of at least 10 mg/ day for 10 or more years is needed to produce the clinical signs of the milder form of the condition. Crippling skeletal fluorosis is extremely rare. For example, only five cases have been confirmed in the USA since the mid-1960s.

Based largely on the data on the association of high fluoride intakes with risk of skeletal fluorosis in chil-dren (>8 years) and adults, the US Food and Nutri-tion Board has established a tolerable UL of fluoride of 10 mg/day for children (>8 years), adolescents, and adults, as well as pregnant and lactating women.

Assessing status

A high proportion of the dietary intake of fluoride appears in urine. Urinary output in general reflects the dietary intake.

Requirements and dietary sources

Most foods have fluoride concentrations well below 0.05 mg/100 g. Exceptions to this observation include fluoridated water, beverages, and some infant formu-lae that are made or reconstituted with fluoridated water, teas, and some marine fish. Because of the ability of tea leaves to accumulate fluoride to concen-trations exceeding 10 mg/100 g dry weight, brewed tea contains fluoride concentrations ranging from 1 to 6 mg/l depending on the amount of dry tea used, the water fluoride concentration and brewing time.

Intake from fluoridated dental products adds considerable fluoride, often approaching or exceeding intake from the diet, particularly in young children who have poor control of the swallowing reflex. The major contributors to nondietary fluoride intake are toothpastes, mouth rinses, and dietary fluoride supplements.

In 1997 the US Food and Nutrition Board estab-lished AI values for fluoride: infants 0.01 mg (first 6 months), 0.5 mg (7–12 months), children and adoles-cents 0.7, 1.0, and 2.0 mg (1–3, 4–8, and 9–13 years, respectively), male adolescents and adults 3 and 4 mg (14–18 and 19 years and older, respectively), female adolescents and adults 3 mg (over 14 years, including pregnancy and lactation). The AI is the intake value of fluoride (from all sources) that reduces the occur-rence of dental caries maximally in a group of indi-viduals without causing unwanted side-effects. With fluoride, the data are strong on caries risk reduction but the evidence upon which to base an actual require-ment is scant, thus driving the decision to adopt an AI as the reference value.

Micronutrient interactions

The rate and extent of fluoride absorption from the gastrointestinal tract are reduced by the ingestion of foods particularly rich in calcium (such as milk or infant formulae).