The benefits to teeth of fluoridating community water supplies are widely acknowledged.1 A comprehensive summary of the most recent evidence is included in the systematic review by McDonagh et al in this issue of the journal (p855)2 but some concerns persist about possible adverse effects on bone.3 The paper by Phipps et al in this issue of the BMJ (p860) introduces new evidence on the safety of fluoridating community water supplies.4 In their multicentre prospective study, Phipps et al found that ambulatory women aged 65 years or older who had been continuously exposed to fluoridated water for the past 20 years had higher bone mineral density at the lumbar spine and hip and a slightly lower risk of hip and vertebral fractures than women who had not been exposed to fluoridated water. The potential confounding effect of other factors known to be associated with fractures, such as oestrogen use, smoking, and body weight, was controlled for at the level of the individual woman.
This was not the case in earlier ecological studies (cited by Phipps et al ) in which higher rates of fractures were found among communities that had fluoridated drinking water when compared with communities without fluoridation. However, a study among residents of the English county of Cleveland who were aged 50 or older and who had had lifelong exposure to naturally high concentrations of fluoride in their drinking water showed no increase in their risk of hip fracture when compared with community controls who used water with naturally low concentrations of fluoride.5 In this study potential confounders were also controlled for at the level of the individual woman. Thus there seems to be reasonably strong evidence that an optimal amount of fluoride in drinking water—either added or occurring naturally—does not increase the risk of osteoporotic fractures in elderly people. No evidence of an elevated risk of fractures attributable to using the optimal level of fluoride in drinking water was found in the systematic review by McDonagh et al.2
The association between the fluoridation of community water supplies and the rate of fractures is part of a wider question about the potential of using fluoride for controlling osteoporosis, which is a major public health problem that causes much pain and disability and has considerable costs for society.6 The prevalence of osteoporosis is increasing as the population ages. Bone mineral density and the frequency of fractures are used to assess the exposure of a population to the risk of osteoporosis. Fluoride seems to be the only drug capable of increasing osteoblastic activity and thus bone mineral density. The true value of the gain in bone mineral density remains questionable, however, since its increase after the use of fluoride has been accompanied by both a higher rate of fractures and a lower rate.7,8 One theory about the conflicting results is that high doses of fluoride may be harmful and low doses beneficial.9 It has also been proposed that adjuvant calcium is necessary for fluoride to be effective.9 Finally, the conflicting results may be caused by biases in the studies. Currently, these issues are under thorough scrutiny, and a systematic review is to be published in the near future.6
The finding that long term exposure to fluoridated water does not increase the risk of osteoporotic fractures among elderly people should alleviate remaining concerns about the safety of fluoridation.
In terms of benefits, the only aim of community water fluoridation is to prevent dental caries. A recent review of the effectiveness of water fluoridation in the United States shows that previous reductions in mean caries scores of one half to two thirds are no longer attainable because other methods of providing fluoride and the availability of products containing fluoride have reduced the prevalence of caries, thus diluting this measurement of effectiveness.10 Similar findings have been reported from the United Kingdom.11 The reduction in the relative effect of fluoridation, which is also seen in the systematic review by McDonagh et al has generally not been big enough to call into question the justification for fluoridating water.2
However, a much larger reduction occurred between 1973 and 1982 among 13-15 year olds in Finland: in 1973 the score of decayed and filled teeth was 43% lower in a fluoridated area than in a low fluoride area, whereas in 1982 there was no difference.12 In Finland preventive dental care is provided free to all children, and this reduction shows that the relative effect of fluoridation can vary strongly depending on different circumstances. Over 360 million people in about 60 countries worldwide are exposed to fluoridated water: more than 10000 communities and 145 million people are exposed in the United States alone.13 Given the huge numbers and the fact that the idea of fluoridating community water divides public opinion, the benefits and potential risks of fluoridation require careful and continuous monitoring.