Canadian researchers have rekindled a long-standing divisive debate around the link between fluoride and IQ. The observational research suggests exposure to fluoride in pregnancy can result in lower IQ scores in children, however, many experts have quickly responded, calling the findings "weak" and "non-significant."
The new research investigated around 500 mother-child pairs, with fluoride exposure measured by tracking levels in a mother's urine during pregnancy, and self-reported consumption of tap water. IQ was assessed in children when they reached between three and four years of age.
The general conclusion in the study was that a 1-mg/L increase in urinary fluoride during pregnancy could be associated with a 4.5 point reduction in childhood IQ score. This association was only seen in boys and not girls. Tracking self-reported fluoride intake revealed a 1-mg higher intake of fluoride correlated with a 3.7 point drop in IQ scores for both boys and girls.
As with all science, the devil is in the details, and many experts are urging caution when interpreting these results. A number of limitations quickly appear when the study is closely examined. Stuart Ritchie, from King's College London, calls the findings "weak and borderline," pointing out the inconsistent effects found in boys versus girls.
"In the first [urinary fluoride] analysis, there's only a statistically significant result if they split the sample up into boys and girls: the effect only exists in boys," says Ritchie. "In the second [self-reported fluoride analysis], there's an overall effect, but it's no stronger in boys than girls. So those two results are inconsistent."
Thom Baguley, from Nottingham Trent University, frankly calls the study's conclusions "non-significant." He suggests the data in the study is so noisy there can be no statistically significant conclusion to be made.
"They did observe a decrease for male children and a slight increase in IQ (but non-significant) for girls," says Baguley. "This is an example of subgroup analysis – which is frowned upon in these kinds of studies because it is nearly always possible to identify some subgroup which shows an effect if the data are noisy. Here the data are very noisy."
Another concern raised surrounding the paper's conclusions is the tenuous jump between maternal fluoride levels and IQ scores in children recorded several years later. The research cannot account for exposure to a multitude of substances across the first few years of a child's life, with Alastair Hay, an environmental toxicologist from the University of Leeds, pointing to lead exposure as one example of something that can have a major influence on childhood IQ.
"Although the authors adjust for a range of substances (including lead) in maternal blood and say this has no effect on findings, for me, the major serious gap is the range of exposure to multifarious substances, including lead, that the children would have had between birth and IQ assessment at ages three or four," says Hay. "We know that lead exposure has devastating effects on IQ in children and this study takes no account of postnatal lead exposure."
Both the researchers and the journal publishing the research seem well aware of the divisive nature of the topic. Dimitri Christakis, editor of the journal JAMA Pediatrics, penned a short editor's note accompanying the new article, suggesting extra care was taken in making the decision to publish this particular piece.
"This decision to publish this article was not easy," Christakis writes. "Given the nature of the findings and their potential implications, we subjected it to additional scrutiny for its methods and the presentation of its findings."
David Bellinger, a Harvard Medical School neurologist, also published an editorial accompanying the new research. While Bellinger did not work on this research, he did review the paper ahead of its publication. Bellinger agrees the research is by no means definitive, and there are acknowledged limitations, but he suggests the hypothetical neurotoxicity of fluoride is worthy of serious consideration.
"These considerations notwithstanding, the hypothesis that fluoride is a neurodevelopmental toxicant must now be given serious consideration," Bellinger writes.
The new study was published in the journal JAMA Pediatrics.