2 June 2014. Research in the mid-1900s revealed that children had fewer cavities in their teeth if they lived in communities with fluoride in the drinking water, though not too much, as fluoride may cause damage to tooth development. Due to these observations, dentists began to promote the addition of fluoride to drinking water when “optimal” amounts were not available. The US Centers for Disease Control has hailed community water fluoridation as a main victory in disease prevention.
Whether the data from 50 years ago or more are still relevant is controversial. Caries incidence has decreased in many countries, also in the absence of fluoridation. Fluoride-containing toothpastes are probably part of the reason, and they have the advantage of supplying the protective agent right at the target. Nonetheless, water fluoridation is still hailed by many dentists as a public health measure that is the most effective in preventing caries.
The evidence of adverse effects is also unclear, especially in regard to the fluoride doses that may be toxic. A recent review and meta-analysis of studies, mainly from China, on decreased IQ at elevated water-fluoride concentrations has caused much consternation. Newspapers misquoted the report’s conclusions, and a major magazine decided to withdraw a report when it became clear that it had propagated some of the mistaken allegations. Erroneous statements were made both by apparent proponents and opponents of water fluoridation.
Fortunately, the controversy has spurred renewed interest in research on the possible implications of elevated fluoride exposure on IQ. Results have just been published from a study in Dunedin, New Zealand. IQ data from almost 1,000 subjects born in 1972-1973 were available, and questionnaire data included past use of fluoride-containing toothpaste or fluoride tablets. Residence information showed that only 99 of the subjects had never lived in an area with fluoridated water, while 22 had never used fluoride toothpaste. The researchers did not find any differences in IQ associated with fluoride exposure indicators, also in regard to the IQ at age 38 years. The report does not consider the size of a fluoride effect that would have been detectable in a study of this size, within a narrow range of exposures, and considering the imprecise classification of fluoride exposure. The statistical confidence limits suggest that a loss of 2-3 IQ points could not be excluded by their findings. Nonetheless, the authors conclude that the absence of significant effects can be generalized to similar western populations.
In support of their results, the authors write: “No dramatic historical decreases in IQ have been seen following widespread implementation of [community water fluoridation] or worldwide introduction of fluoride toothpastes; instead, historical comparisons have documented substantial IQ gains across countries since the mid-1900s.” What to make of this confused linkage of independent facts is difficult to see, though. Still, the New Zealand dentists offer the following advice: “Scientists and policy makers should be reminded of the necessity of caution in attributing causality when evidence for it does not exist.”
Indeed, but we should also be reminded that an adverse effect may well be present, even when it could not be demonstrated in a particular study, especially one that can be characterized as weak or non-informative.