Can fluoride affect IQ? Study points to risks but raises questions

By Priyanjana Pramanik, MSc.Reviewed by Susha Cheriyedath, M.Sc.Jan 7 2025

Researchers reveal a potential link between high fluoride exposure and lower IQ in children, but there are significant study limitations, including geographic bias and high risk of bias in many studies.

Fluoride Exposure and Children’s IQ Scores -  A Systematic Review and Meta-Analysis. Image Credit: Lopolo / Shutterstock

In a recent review article in JAMA Pediatrics, researchers systematically analyzed epidemiological studies that investigated the relationship between exposure to fluoride in the prenatal or postnatal period and children’s intelligence quotient (IQ) scores.

Existing research appears to indicate that fluoride exposure in urine and drinking water is inversely associated with children’s IQ scores and suggests a significant dose-response relationship.

Background

Fluoride occurs naturally in certain community water sources. In other drinking water supplies in the U.S., it is added to prevent tooth decay. The U.S. Public Health Service guidelines recommend that fluoride concentration should be set at 0.7 mg/L, while the Environmental Protection Agency (EPA) sets non-enforceable and enforceable limits at 2 mg/L and 4 mg/L, respectively.

The World Health Organization suggests a guideline of 1.5 mg/L. In the U.S., water and beverages comprise about 75% of fluoride intake, with fluoridated water contributing between 40% and 70%. Fluoride also comes from other sources, such as dental products, food, and industrial emissions.

Emerging evidence suggests that fluoride exposure may influence brain development. A report published in 2006 by the National Research Council highlighted concerns about high fluoride levels causing neurotoxic effects. These concerns were informed by studies from China with some design limitations.

Since then, additional research from 10 countries, including Canada, Mexico, and India, has raised similar concerns, with recent research linking fluoride exposure to lower children's IQ. Four new studies have further examined this connection, focusing on urinary fluoride levels in mothers and children.

About the Study

To update evidence and improve fluoride research analysis, researchers used a systematic literature review and meta-analysis of individual-level and group-level exposure to fluoride and its relationship to children's IQ scores.

The research process, including study selection and evaluation of risk-of-bias, adhered to a standardized protocol. Notably, 52 of the 74 studies included were rated as high risk of bias. Data were gathered from multiple databases until October 2023 and analyzed using the Health Assessment Workspace Collaborative (HAWC) system.

Studies were independently reviewed and assessed for bias by applying the National Toxicology Program's Office of Health Assessment and Translation (OHAT) approach. Subgroup analyses were conducted to understand the impact of studies with a high risk of bias and differing geographic locations on results.

Statistical analysis included three separate meta-analyses. The first compared average IQ scores between exposed and reference groups using standardized mean differences (SMDs). A second focused on dose-response associations, examining the relationship between fluoride exposure and IQ utilizing restricted maximum likelihood estimation approaches to account for possible non-linear relationships. The third analysis pooled data from studies reporting regression slopes and estimated the effects of fluoride exposure on IQ at the individual level.

Subgroup analyses further explored variations by the risk of bias, country of study (notably, most were conducted in China), exposure type, and timing (postnatal or prenatal). Researchers evaluated publication bias using statistical tests and funnel plots.

Findings

Overall, the study highlights a consistent link between fluoride exposure and reduced IQ in children, with variations depending on study design and risk of bias.

The study reviewed 74 publications, including 64 that were cross-sectional and 10 that utilized a prospective cohort design, primarily from China. No studies included U.S. populations, limiting direct applicability to U.S. public health recommendations.

Of the 65 studies used in the primary analysis, most found an inverse relationship between fluoride exposure and children's IQ. Specifically, 59 studies reported average IQ scores, and 19 focused on individual-level fluoride exposure.

The meta-analysis of 59 studies involving 20,932 children revealed that higher fluoride exposure was linked to significantly lower IQ scores with an SMD value of -0.45. High heterogeneity and publication bias were noted. Adjusted analyses confirmed the inverse association. Among the 59 studies, 52 showed negative associations, with SMDs ranging from -5.34 to -0.04, while seven studies reported no inverse association.

Further analysis of 12 low-risk studies showed a smaller but still significant negative impact on IQ (SMD of -0.19), while 47 high-risk studies indicated a more substantial effect (SMD of -0.52). Dose-response analyses involving 38 studies confirmed that increased fluoride levels in water and urine were associated with lower IQ scores, even at fluoride concentrations below 4 mg/L.

However, the association was null at concentrations below 1.5 mg/L when using water fluoride as the exposure measure. Regression analysis of 13 studies found that a 1 mg/L increase in urinary fluoride led to an average IQ decrease of 1.63 points.

Conclusions

This systematic review and meta-analysis found that higher fluoride exposure is linked to lower IQ in children. The review included studies with varying risk levels and examined group and individual fluoride exposure.

Consistent inverse relationships were found across different study designs, exposure levels, and geographic locations. Although group-level fluoride measurements, like those from drinking water, may underestimate total exposure, individual urinary fluoride levels provided more accurate assessments.

Despite limitations such as the predominance of high-bias studies and limited data at lower fluoride levels (<1.5 mg/L), the findings align with previous research, suggesting a need for further studies to better understand the impact of fluoride on cognitive development.

The authors recommend future research focus on prospective cohort studies with robust individual exposure measurements, particularly in populations outside China, to improve the generalizability of findings.

A takedown of the fluoride paper from yesterday, front page @latimes https://t.co/O5m0l80QGm @LATkarenkaplan https://t.co/DOn3Txx67D pic.twitter.com/4sFwK6xW5Z

— Eric Topol (@EricTopol) January 7, 2025