Flouride

BACKGROUND

Fluoride is a normal constituent of the human body, involved in the mineralisation of both teeth and bones (Fairley et al 1983, Varughese & Moreno 1981). The fluoride concentration in bones and teeth is about 10,000 times that in body fluids and soft tissues (Bergmann & Bergmann 1991, 1995). Nearly 99% of the body's fluoride is bound strongly to calcified tissues. Fluoride in bone appears to exist in both rapidly- and slowly-exchangeable pools. Because of its role in the prevention of dental caries, fluoride has been classified as essential to human health (Bergmann & Bergmann 1991, FNB:IOM 1997).

Ingestion of fluoride in the pre-eruptive development of teeth has the effect of reducing caries due to uptake of fluoride by enamel crystallites and formation of fluorohydroxyapatite which is less soluble than hydroxyapatite (Brown et al 1977, Chow 1990). The post-eruptive effect on reducing caries is due to reduced acid production by bacteria and increased enamel remineralisation in acidogenic challenge (Bowden 1990, Hamilton 1990, Marquis 1995). Fluoride also has a unique ability to stimulate new bone formation and as such has been used as an experimental drug for the treatment of osteoporosis (Kleerekoper & Mendlovic 1993) although results have been variable depending on site assessed and the outcome measured (Kroger et al 1994, Riggs et al 1990, Sowers et al 1986, 1991).

Because of the low natural levels of fluoride in some water supplies and high levels of dental caries, many authorities worldwide, including Australia and New Zealand, have permitted, or instigated, fluoridation of water supplies. Although this has met some opposition, partly because of the potential health or dental effects that include fluorosis, the NHMRC concluded that a concentration of 1 mg/L secures most of the caries preventive effect available from fluoridated water, while maintaining minimal contribution of water fluoride to dental fluorosis in children and that there was no evidence of adverse health effects attributable to fluoride in communities exposed to a combination of fluoridated water (1 mg/L) and contemporary discretionary sources of fluoride (NHMRC 1991).

Not all Australian water supplies are fluoridated, notably those in parts of Queensland such as Brisbane. Concentrations in fluoridated areas are within the range identified by the NHMRC as safe and effective, varying from 0.6 mg/L in Darwin to 1.1 mg/L in Hobart. In New Zealand, the Ministry of Health (MOH) has recommended fluoridation of water supplies since the 1950s as the most effective and efficient way of preventing dental caries in communities receiving a reticulated water supply. In the Drinking Water Standards 2000, fluoridation is recommended at a level of 0.7–1.0 mg/L in drinking water. Around 85% of the New Zealand population is on what the government considers to be satisfactorily safe community water supplies in terms of fluoride content. Another 5% of the population are on community water supplies. Some of the larger centres without fluoridated water supplies currently are Whangarei, Tauranga, Wanganui, Napier, Nelson, Blenheim, Christchurch, Timaru and Oamaru.

The World Health Organization states in a review of chronic disease and diet that evidence that both locally applied and systemic fluoride are preventive for dental caries is convincing (WHO 2003). One of the concerns expressed about fluoridation of the water supply relates to increasing rates of fluorosis in children seen in some communities over the same period as fluoridation has been practised. Dental fluorosis is a biomarker of over-exposure to fluoride among young children and results in a mottling of teeth. Recent research in Australia among children not exposed and exposed to water fluoridation indicated prevalences of 19% and 34%, respectively (Puzio et al 1993). However, Kumar et al (1989) have shown that the increases in fluorosis in other communities have been greater in areas with non-fluoridated water supplies and are likely to be due to increased intake of fluoride from supplements and ingestion from toothpaste and reconstituted infant formula (Osuji et al 1988, Pendrys & Stamm 1990).

Fluoride intake from most foods is low. Foods generally have concentrations well below 0.05 mg/ 100 g (Taves 1983). However, water in fluoridated areas, as well as beverages, teas, some marine fish and some infant formulas, especially those that are made or reconstituted with fluoridated water, generally have higher concentrations. Other sources of fluoride include supplements and dental products. Water-soluble fluoride eg sodium fluoride, is nearly completely absorbed. The bioavailability may be reduced by the presence of calcium, magnesium, aluminium, iron or other cations. Absorbed fluoride is rapidly bound to the minerals in bones and teeth. Most of the non-retained or metabolic fluoride is excreted through the kidneys and the remainder via the intestines. In healthy young or middle-aged adults, about 50% of absorbed fluoride is retained and 50% excreted, but young children may retain as much as 80% (Eksterand et al 1994a,b).

Indicators used to assess the requirements for fluoride include prevalence of dental caries, measures of bone mineral content and fluoride balance studies.