The study included data on fluoride exposure and IQ scores from hundreds of mothers and their children in six major Canadian cities. About 40% of the participants lived in communities with a fluoridated water supply, all of which were fluoridated at or below the recommended level.
"In this study, maternal exposure to higher levels of fluoride during pregnancy was associated with lower IQ scores in children aged 3 to 4 years," wrote the authors, led by Rivka Green, a clinical developmental neuropsychology doctoral candidate from York University in Ontario (JAMA Pediatr, August 19, 2019). "These findings indicate the possible need to reduce fluoride intake during pregnancy."
A potential fluoride-IQ link
A 2017 study on Mexican children previously linked prenatal fluoride exposure to lower IQ scores. However, fluoride exposure in the previous study came from fluoridated salt, as opposed to the regulated water systems of the U.S. and Canada.
“These findings indicate the possible need to reduce fluoride intake during pregnancy.”
— Rivka Green and colleagues
In the current study, the researchers wondered whether the association between fluoride exposure and IQ scores would hold true for communities with optimal water fluoridation. To find out, they parsed fluoride exposure and IQ data from 512 mothers and children in Toronto, Vancouver, and four other major Canadian cities.
The researchers measured prenatal fluoride exposure in two ways:
- Estimated daily water fluoride intake based on postal code and reported water consumption
- Fluoride concentration in maternal urine samples from each trimester of pregnancy
Children with higher levels of prenatal fluoride exposure had significantly lower IQ scores, the researchers found. A 1-mg increase in estimated daily water fluoride intake during pregnancy was associated with a 3.66 lower IQ score for children. This was true even after adjusting for lead, mercury, arsenic, and other known neurotoxins.
The link between prenatal exposure and lower IQ scores was especially prominent for boys. Boys whose mothers had the highest concentration of urinary fluoride scored about 3 IQ points lower than those whose mothers had lower fluoride concentrations. This finding was not significant for girls.
|Change in IQ score by prenatal fluoride exposure
||Difference between the 10th and 90th percentiles
|Estimated water fluoride concentration
|Maternal urinary fluoride concentration, boys
|Maternal urinary fluoride concentration, girls
"Testing whether boys are potentially more vulnerable to neurocognitive effects associated with fluoride exposure requires further investigation, especially considering that boys have a higher prevalence of neurodevelopmental disorders, such as ADHD, learning disabilities, and intellectual disabilities," the authors wrote. "Adverse effects of early exposure to fluoride may manifest differently for girls and boys, as shown with other neurotoxicants."
Deciding whether to publish
This was one of the first studies to estimate fluoride exposure using a large number of children living in communities with optimally fluoridated water, the authors noted. Of the women who lived in communities with water fluoridation, 94% had fluoride lower than the 0.7-mg/L level considered optimal by the U.S. and Canadian governments.
"The evidence showing an association between fluoride exposure and lower IQ scores raises a possible new concern about cumulative exposures to fluoride during pregnancy, even among pregnant women exposed to optimally fluoridated water," the authors wrote.
The study does have shortcomings, the researchers noted. They did not have maternal IQ data, nor did they assess postnatal fluoride exposure or consumption. One of the researchers is also serving as an expert witness in an upcoming water fluoridation case involving the U.S. Environmental Protection Agency.
Nevertheless, the researchers tried to reduce the effects of outside influences on the results, such as by testing maternal fluoride concentrations at multiple points during pregnancy and using two different fluoride exposure metrics.
In an editorial note, JAMA Pediatrics Editor Dimitri Christakis, MD, MPH, noted that the decision to publish the study was not easy and that the journal scrutinized the findings before publication. However, he doesn't think this will be the last study on the subject.
"This decision to publish this article was not easy," he wrote. "Publishing it serves as testament to the fact that JAMA Pediatrics is committed to disseminating the best science based entirely on the rigor of the methods and the soundness of the hypotheses tested, regardless of how contentious the results may be."
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