A landmark U.S. study tracking students from high school to age 60 reveals that recommended fluoride levels support stronger academic performance in adolescence but offer no measurable cognitive edge in later adulthood.
Study: Childhood fluoride exposure and cognition across the life course. Image credit: Lopolo/Shutterstock.com
A study in Science Advances examines the impact of fluoride exposure on cognitive performance during adolescence and midlife. Children exposed to recommended fluoride levels in drinking water exhibit better cognitive performance in secondary school; however, no significant benefits were noted by age 60.
Effect of fluorides in drinking water
Multiple studies have demonstrated that fluoride in drinking water improves oral health in children and adults, particularly by reducing the risk of tooth decay. The US Centers for Disease Control recognized the policy of fluoridating drinking water as one of the “10 great public health interventions of the 20th century”.
A previous study found that fluoride concentrations in drinking water exceeding 1.5 mg/L adversely affected children’s IQ. However, the finding cannot be generalized due to the limited sample size and because most included studies examined non-U.S. populations exposed to much higher fluoride levels than those typically found in the United States. Scientists have identified that no research has been conducted in the US or has used nationally representative data to assess the effect of fluoride exposure on children’s IQ.
A nationally representative study revealed that only 4.3 % of Americans between 6 and 19 years of age were exposed to fluoride in drinking water at levels greater than 1.2 mg/L. A 2023 survey of community water systems across the US indicated that 4.5 % of US citizens are exposed to 1.5 mg/L or more of fluoride. According to current US recommendations, 0.7 mg/L of fluoride is allowed in drinking water.
Extremely high levels of fluoride exposure in children could cause fluoride toxicity, which is also known as fluorosis. Recent meta-analyses have shown a statistically significant inverse relationship between fluoride exposure through drinking water and childhood IQ at exposure levels uncommon in the United States. Although many studies have shown the beneficial oral effects of exposing children and adults to fluorides, more research is required to explore any long-term adverse effects.
Assessing cognitive effects of childhood fluoride exposure in adolescence and adulthood
The current study used a nationally representative dataset of 58,270 sophomores and seniors from 1,020 American high schools in 1980. A total of 26,820 participants were randomly selected for follow-up in 2021.
Relevant data on fluoride levels in drinking water were obtained from archival sources. For example, the US Geological Survey provided information on untreated groundwater from 38,105 US wells between 1988 and 2017, and additional data were extracted from the 1967-1993 editions of the US Department of Health and Human Services’ Fluoridation Census.
Community water fluoridation guidelines have been amended multiple times over the years. Between 1962 and 2015, the allowed levels ranged from 0.7 to 1.2 mg/L; however, after 2015, the recommended level was lowered to 0.7 mg/L.
Fluoride exposure was categorized into three groups: Firstly, consistently insufficient if students lived in areas without municipal fluoridation and with natural fluoride below 0.7 mg/L at conception and during academic testing; secondly, consistently sufficient if students lived in areas with municipal fluoridation or natural fluoride at or above 0.7 mg/L at both times; and thirdly, partially sufficient if fluoridation was implemented in their area between conception and academic testing.
Cognitive performance of all students in 1980 was assessed using test scores in reading comprehension, vocabulary, and mathematics in the 12th grade. Cognitive functioning was also assessed at age 60 using a hierarchical item-response theory model.
Fluoride exposure improves cognitive performance
Students were consistently exposed to sufficient fluoride concentrations through 12th grade. However, a disproportionate exposure was noted, particularly between students from urban areas and those from the southern and midwestern United States. These geographic patterns contributed to the fact that students in consistently fluoridated areas were somewhat more likely to be Black and to have modestly lower average baseline cognitive scores, reflecting underlying geographic and demographic patterns rather than fluoridation itself.
A multinomial logistic regression model predicted fluoride exposure category as a function of spatial, demographic, and socioeconomic factors. Spatial factors were found to drive the fluoride exposure category. Supplementary analyses suggest that area-level social, economic, and political factors may not fully explain the observed associations, although unmeasured confounding cannot be ruled out.
Young people exposed to recommended levels of fluoride in drinking water were found to perform better on mathematics, reading, and vocabulary achievement tests in secondary school than those who were never exposed to sufficient fluoride. Fluoride exposure was not statistically significantly associated with cognitive functioning at age 60.
Fluoride helps, temporarily.
The current study highlights that exposure to fluoride at levels commonly found in U.S. drinking water is associated with cognitive benefits during adolescence, without evidence of harm to cognitive functioning in later adulthood.
These findings support the continued use of municipal water fluoridation as a public health measure. However, the authors recognized certain limitations in the exposure assessment, particularly the assumption that high school location reflects residence across childhood, and the need for further research to understand long-term effects fully.
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