Scientists Urge Caution Around Study on Fluoridated Water and Children's IQs

"No single observational study provides a definitive test of a hypothesis."

Unsplash / Jacek Dylag

For decades, fluoridated water has been a favorite villain of conspiracy theorists claiming that it’s a mind-control tool. There’s no evidence to support that idea, but there is plenty showing it’s good for your teeth. Research published Monday in JAMA Pediatrics, however, raises questions about fluoride’s safety.

In a paper published on Monday, researchers show evidence that the children of mothers who drink fluoridated water during pregnancy have lower IQs.

But before you go out and start ordering untreated “raw water” to replace your tap water, though, you should know that there are some important caveats to this new research.

The study used data from 512 mother-child pairs in six Canadian cities to look for an effect of maternal exposure to fluoridated water on her children. When measured in a mother’s urine, each 1-milligram increase in fluoride per liter was associated with a 4.49-point decrease in IQ score in boys. There was no statistically significant association for girls.

With mothers’ self-reported water consumption, however, 1 milligram more of fluoride per day was associated with a 3.66-point decrease in IQ for both boys and girls.

It’s a startling conclusion, given that nearly 75 percent of people in the United States have access to fluoridated water. It is likely to worry many people who are pregnant or who plan on getting pregnant, so these results need to be placed in their proper context.

This study shows a connection between maternal fluoride consumption and children's IQs, but experts warn against drawing strong conclusions from just one study.

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First of all, the observational design of this experiment makes it hard to conclusively say that maternal exposure to fluoride lowers children’s IQs. The data, already collected through the Maternal-Infant Research on Environmental Chemicals program, showed that about 40 percent of mothers studied in the program had access to fluoridated water, while the rest didn’t. This natural environment, in which some people were exposed to fluoride while others weren’t, created good conditions for an experiment. In an observational study like this, however, there are many potential complicating factors that can’t easily be accounted for.

Second, the effect size the researchers observed from the data was relatively small — just a few IQ points on average. Given that IQ tests only measure specific dimensions of intelligence, it’s hard to draw a definitive conclusion about the real-world effects of a test score that’s a few points lower. That being said, Christine Till, Ph.D., an associate professor of psychology at York University in Toronto and the study’s senior author, notes that the differences become more pronounced when comparing children with the lowest levels of exposure to those with the highest.

“We would feel an impact of this magnitude at a population level,” Till told NPR, “because you would have millions of more children falling in the range of intellectual disability, or an IQ of under 70, and that many fewer kids in the gifted range.”

In an accompanying editorial published with the study, David Bellinger, Ph.D., a professor of neurology and psychology at Harvard Medical School, wrote that despite the caveats, this research should be taken seriously by the scientific community.

“No single observational study provides a definitive test of a hypothesis, and early studies of an association that is subsequently confirmed tend to report larger effect sizes than do later studies,” he writes. “These considerations notwithstanding, the hypothesis that fluoride is a neurodevelopmental toxicant must now be given serious consideration.”

Fluoride limits in bottled water are also a point of contention

Unsplash / Jonathan Chng

Given the controversial nature of the topic, JAMA Pediatrics editor Dimitri Christakis, M.D., M.P.H., a professor of pediatric medicine at Seattle Children’s Hospital, issued a statement along with the article about the journal’s decision to publish it:

This decision to publish this article was not easy. 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. The mission of the journal is to ensure that child health is optimized by bringing the best available evidence to the fore. 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. That said, scientific inquiry is an iterative process. It is rare that a single study provides definitive evidence. This study is neither the first, nor will it be the last, to test the association between prenatal fluoride exposure and cognitive development. We hope that purveyors and consumers of these findings are mindful of that as the implications of this study are debated in the public arena.

Echoing this note of caution, authorities have noted that they are not about to hastily adopt any new fluoride restrictions.

The US Centers for Disease Control and Prevention told The Daily Beast that it doesn’t comment on outside research, and the American Academy of Pediatrics said it would await further research before issuing new guidelines.

Nevertheless, plenty of US cities have taken it upon themselves to act on early results of previous fluoride research. As recently as April, the US Food and Drug Administration lowered the limit of fluoride in bottled water from 0.8 milligrams per liter to 0.7.

So while further research may be necessary for the scientific community to draw a strong conclusion, plenty of people in the US have been there for awhile already.

Importance: The potential neurotoxicity associated with exposure to fluoride, which has generated controversy about community water fluoridation, remains unclear.
Objective: To examine the association between fluoride exposure during pregnancy and IQ scores in a prospective birth cohort.
Design, Setting, and Participants: This prospective, multicenter birth cohort study used information from the Maternal-Infant Research on Environmental Chemicals cohort. Children were born between 2008 and 2012; 41% lived in communities supplied with fluoridated municipal water. The study sample included 601 mother-child pairs recruited from 6 major cities in Canada; children were between ages 3 and 4 years at testing. Data were analyzed between March 2017 and January 2019.
Exposures: Maternal urinary fluoride (MUFSG), adjusted for specific gravity and averaged across 3 trimesters available for 512 pregnant women, as well as self-reported maternal daily fluoride intake from water and beverage consumption available for 400 pregnant women.
Main Outcomes and Measures: Children’s IQ was assessed at ages 3 to 4 years using the Wechsler Primary and Preschool Scale of Intelligence-III. Multiple linear regression analyses were used to examine covariate-adjusted associations between each fluoride exposure measure and IQ score.
Results: Of 512 mother-child pairs, the mean (SD) age for enrollment for mothers was 32.3 (5.1) years, 463 (90%) were white, and 264 children (52%) were female. Data on MUF^^SG concentrations, IQ scores, and complete covariates were available for 512 mother-child pairs; data on maternal fluoride intake and children’s IQ were available for 400 of 601 mother-child pairs. Women living in areas with fluoridated tap water (n = 141) compared with nonfluoridated water (n = 228) had significantly higher mean (SD) MUF^^SG concentrations (0.69 [0.42] mg/L vs 0.40 [0.27] mg/L; P = .001; to convert to millimoles per liter, multiply by 0.05263) and fluoride intake levels (0.93 [0.43] vs 0.30 [0.26] mg of fluoride per day; P = .001). Children had mean (SD) Full Scale IQ scores of 107.16 (13.26), range 52-143, with girls showing significantly higher mean (SD) scores than boys: 109.56 (11.96) vs 104.61 (14.09); P = .001. There was a significant interaction (P = .02) between child sex and MUF^^SG (6.89; 95% CI, 0.96-12.82) indicating a differential association between boys and girls. A 1-mg/L increase in MUF^^SG was associated with a 4.49-point lower IQ score (95% CI, −8.38 to −0.60) in boys, but there was no statistically significant association with IQ scores in girls (B = 2.40; 95% CI, −2.53 to 7.33). A 1-mg higher daily intake of fluoride among pregnant women was associated with a 3.66 lower IQ score (95% CI, −7.16 to −0.14) in boys and girls.
Conclusions and Relevance: 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. These findings indicate the possible need to reduce fluoride intake during pregnancy.
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