Wednesday, February 6, 2013

Water fluoridation may have no effect on children's IQ

Online new sites erupted in criticism and some fanfare to the City of Windsor's decision to stop fluoridating their drinking water, with Jonathan Kay at the National Post staunchly against the move.  There are ongoing arguments against fluoridation of water, like those summarized by Dr. Gifford-Jones at the Toronto Sun last fall, but since the water fluoridation debate has evolved over at least 35 years, I can't see it going away any time soon.

However, one study that's being cited thrown around by critics of fluoridation shows that "children in high-fluoride areas had significantly lower IQ scores than those who lived in low-fluoride areas".  Clearly, therefore, fluoridation of drinking water has done irreparable harm to the children of Windsor, and will continue to do so unless something is done about it.  Now!  

At least, that's how fear-mongers see it.  Has anyone actually read this report? Books like The Case Against Fluoride love these kind of studies.

To be more precise, the authors showed that "the standardized weighted mean difference in IQ score between exposed and reference populations was –0.45 (95% confidence interval: –0.56, –0.35) using a random-effects model".  According to this news release on the paper, this -0.45 mean difference is "approximately equivalent to seven IQ points for commonly used IQ scores with a standard deviation of 15", which means that there's huge variability in the estimate of "IQ loss", even though it's statistically significant.

Another important thing to bear in mind about this study is that it's a meta-analysis of 27 other studies reporting IQ differences among populations in China (and two in Iran).  Most of these examined the effects of levels of fluoride in natural drinking water that's well in excess of levels used in North America:

The exposed groups had access to drinking water with fluoride concentrations up to 11.5 mg/L; thus, in many cases concentrations were above the levels recommended (0.7–1.2 mg/L; DHHS) or allowed in public drinking water (4.0 mg/L; U.S. EPA) in the United States (U.S. EPA 2011).

And yes, the authors do conclude that some research into fluoride effects should be further studied:
Although the studies were generally of insufficient quality, the consistency of their findings adds support to existing evidence of fluoride-associated cognitive deficits, and suggests that potential developmental neurotoxicity of fluoride should be a high research priority. 
But that's no different from the vast majority of research papers that end up presenting some valid work, solve part of a problem, reveal additional studies that ought to be pursued (based on the evidence just shared), and keep researchers researching.

But here's the part that no one bothers to point out:
Drinking water may contain other neurotoxicants, such as arsenic, but exclusion of studies including arsenic and iodine as coexposures in a sensitivity analysis resulted in a lower estimate, although the difference was not significant.
The emphasis is mine.  Here, the authors point out, indirectly, that water fluoridation studies that do not exclude arsenic and iodine, etc., as confounding effects will overestimate the harmful effects of fluoride. 

In the end, the best answer to the question "Does fluoride in drinking water lower IQ scores?" must be "Yes, slightly, but not to statistical significance.  We need to examine this further".  It may, it may not, but we'll know a follow up study, hence the title of this post "water fluoridation may have no effect on children's IQ". 

Many urban regions fluoridate their water.  Toronto, for example, has a 0.6 parts per million target for fluoride in drinking water, partly justified by the following:
Fluoride has been added to the Toronto drinking water supply since 1963.  Studies of Toronto children 12 years after the introduction of water fluoridation and again in 2000 show that by 2000, there was a 77.4% mean reduction in decayed, missing, and filled baby teeth for five year-old children.
Granted, a lot has changed in the 37 years needed to yield this result, which Toronto curiously acknowledges in the next paragraph of their information, such as the use of toothpaste, better nutrition, and better dental care in general.  So like the meta study of fluoridation in Chinese drinking water, there are confounding factors that make it difficult to firmly say that fluoridation of Toronto water is the biggest contributor to the reduction of tooth decay.

Speaking as a scientist, I think Windsor has presented an opportunity.  With its decision to do away with drinking water fluoridation, Windsor has set itself up to be a great region for a natural experiment: if fluoridation really does have an effect on tooth decay on young children, we should start seeing the results in about 5-10 years.  Windsor has great dental care and nutrition levels like any other city in North America, so the problems with Toronto's statement above are nonexistent.

What Windsor's city council can still do is defer the removal of fluoride from this spring to 2014, and let some motivated people design a good study to track the effects.  The city doesn't even have to pay - encourage people to sign up for the Ontario Health Study.  When 2019 or 2024 come around and the results roll in, Windsor's city council will be seen as either brilliant or backwards, but it'll be hard to say anything without some data on the matter.