The Relationship Between Fluoride Exposure and Child IQ
Post by Lila Metko
The takeaway
Researchers have yet to determine to what extent fluoride exposure could cause neurotoxic effects. The authors examined multiple studies that measured the relationship between prenatal and child fluoride exposure and child IQ scores. They reported an inverse association between fluoride exposure and child IQ, meaning that IQ went down as fluoride exposure levels went up.
What's the science?
It is estimated that on average the largest percentage of an American’s fluoride consumption comes from fluoridated drinking water. In 2006, the National Research Council issued a report outlining the possible neurotoxic effects of high fluoride exposure from drinking water. Multiple meta-analyses in the past decade have suggested an inverse relationship between fluoride exposure and child IQ. This week in JAMA Pediatrics, Taylor and colleagues conducted a meta-analysis of 74 studies on this topic, including a study quality assessment (also called risk of bias).
How did they do it?
The authors systematically searched eight large network databases including PubMed, Scopus, and PsycINFO. The criterion for inclusion in the meta-analysis necessitated that the study “estimated the association between exposure to fluoride…and a quantitative measure of children’s intelligence.” Each study in the meta-analysis was evaluated with the OHAT risk of bias tool, a method developed by the National Toxicology Program. The OHAT risk of bias tool is an 11-question assessment including key questions that evaluate how well individual studies address potential confounding, exposure characterization, and outcome assessment. The majority of studies included reported group averages but 19 reported individual-level exposure, typically determined through fluoride content in drinking water or fluoride concentration in urine. The authors did a mean effects meta-analysis and a regression slopes meta-analysis that evaluated group-level and individual-level fluoride exposures respectively. A mean effects meta-analysis estimates standardized mean differences, a summary statistic that calculates the difference in IQ between children living in areas with high fluoride exposure and children living in areas with low fluoride exposure. A regression slopes meta-analysis uses regression coefficients from individual studies to estimate the change in IQ per a 1 mg/L increase in fluoride exposure. Some studies were excluded from these primary analyses because of factors such as a lack of reported mean IQ scores for outcome measures and overlapping populations.
What did they find?
The authors found an inverse relationship between fluoride exposure and IQ in both the mean effects and regression slopes meta-analyses. Findings were consistent across high-risk of bias and low-risk of bias studies. Associations remained inverse when the exposure groups were exposed to less than 4 mg/L and less than 2 mg/L in drinking water. In the regression slopes meta-analysis the authors found that for every 1 mg/L increase in urinary fluoride concentration, there is a decrease of 1.63 points in a child’s IQ. While this study only assesses associations, it is significant to note that the inverse relationship between fluoride level exposure and child IQ was intact across different study designs, methods of assessing fluoride exposure, and IQ assessments.
What's the impact?
This study is one of several in the past decade to find an inverse relationship between fluoride exposure and child IQ. This meta-analysis is notable because it used a rigorous and transparent process to identify all studies relevant to the specific research question, extract data from each study, and assess each of the 74 studies for risk of bias based on pre-specified criteria. Interestingly, associations remained inverse even when the exposure groups were exposed to less than 4 mg/L and less than 2 mg/L in drinking water. The EPA enforces that drinking water cannot have more than 4 mg/L fluoride and recommends that drinking water should have less than 2 mg/L fluoride.
