• Epidemiological methods
• EPIDEMIOLOGY
• Clinical epidemiology
• EDUCATION

The overwhelming health benefits of water fluoridation has been clearly demonstrated for decades; it’s sheer effectiveness and relatively low cost prompted the Centre for Disease Control to declare it as one of the top 10 public health achievements of the 20th century.1 Despite this, as long as there has been water fluoridation there has been an opposition to it on health grounds, although many of the arguments have been debunked.2 ,3 In this respect, water fluoridation remains controversial from a political standpoint.

The recent work published in this journal by Peckham et al4 has re-ignited the debate. However, there are numerous reasons to be sceptical of the work. First, the mantra that correlation is not causation cannot be repeated often enough. Second, Peckham et al state in the analysis that studies have suggested a link between fluoride and hypothyroidism; however, the review paper they referenced5 actually lists a number of research articles of various quality, most of which find no effect in pathological or functional thyroid differences. That review article makes clear that the evidence for direct fluoride involvement in the causation of hypothyroidism is poor; despite the fact that it has been examined on numerous occasions. A similar position is articulated in the European Union scientific committee on health and environmental risks 2011 review on Fluoride, which concluded that “a systematic evaluation of the human studies does not suggest a potential thyroid effect at realistic exposures to fluoride.”6 Interestingly, Peckham et al cite this work but ignore this conclusion. Finally, the authors warn that certain areas of the UK may be above recommended limits; however, these are the recommended limits for dental fluorosis, and have unknown relationship with thyroid function.

By contrast, the causative role of iodine deficiency on hypothyroidism is well-established.7 Previous studies8 suggested that the UK is widely iodine deficient, which seems a much more likely cause of hypothyroidism, with a well understood and clear mechanism of action. Peckham et al concede that iodine matters but argue that it is unlikely there are significant differences between the UK populations studied. Without any socioeconomic data, or dedicated analysis, this claim cannot be tested. An alternative test of the hypothesis would be comparison of thyroid function in nations that fluoridate versus those that do not; such comparison has not yet been done.

A major weakness of this study is the fact that other potential confounding factors have not been taken into account; this makes the conclusions regarding the community health utility of water fluoridation problematic. The strong conclusion of the paper by Peckham et al is not supported by the published literature.

What is without question is that fluoride has had a remarkable and positive effect on our dental health, and the evidence base for this is overwhelming. The WHO recommends up to 1.5 mg/L for optimum dental health9 on the basis of decades of epidemiological study, which has consistently shown water fluoridation to be safe and cost-effective. The conclusions of the study by Peckham et al are simply not convincing, and should perhaps be taken with a large pinch of (fluoridated) salt.