Fluoride Linked With Lower Intelligence in Children

Photo by Dubravko Sorić.

Dentistry Today
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Photo by Dubravko Sorić.

Fluoride in the urine of pregnant women shows a correlation with lower measures of intelligence in their children, according to a team of international researchers working in Mexico. However, the ADA and the American Fluoridation Society (AFS) caution consumers against reading the results as a reason to avoid community fluoridation efforts. 

“Our study shows that the growing fetal nervous system may be adversely affected by higher levels of fluoride exposure,” said Howard Hu, MD, MS, ScD, principal investigator and professor of environmental health, epidemiology, and global health at the Dalla Lana School of Public Health at the University of Toronto. “It also suggests that the prenatal nervous system may be more sensitive to fluoride compared to that of school-aged children.” 

Tap water and dental products have been fluoridated in Canada and the United States for more than 60 years to prevent cavities and improve bone health, though the practice has drawn criticism in recent years. In the United States, fluoridation is set at 0.7 ppm in public water supplies. Dental fluorosis is possible with excess fluoride ingestion during tooth formation. Skeletal fluorosis, though rare, may occur at levels 5 to 10 times higher than those recommended for public water.    

“Relatively little is known, with confidence, about fluoride’s impact on neurodevelopment,” said Hu, whose team included experts from the National Institute of Public Health of Mexico, the University of Toronto, the University of Michigan, McGill University, Indiana University, Mount Sinai School of Medicine, and the Harvard School of Public Health.

The study analyzed data from 287 mother-child pairs in Mexico City who were part of the Early Life Exposures in Mexico to ENvironmental Toxicants (ELEMENT) project, which recruited pregnant women from 1994 to 2005 and has followed them and their children ever since. The ELEMENT team analyzed urine samples taken from mothers during pregnancy and from their children at the age of 4 years and between the ages of 6 and 12 years to reconstruct personal measures of fluoride exposure.

“This is significant because previous studies estimated exposures based on neighborhood measurements of drinking water fluoride levels, which are indirect and much less precise measures of exposure,” said Hu. “They also looked at children’s exposures instead of prenatal exposures or had much smaller sample sizes of subjects to study.”

The researchers then analyzed how levels of fluoride in urine related to the children’s verbal, perceptual-performance, quantitative, memory, and motor abilities at the age of 4 years and once more between the ages of 6 and 12 years. Analyses were adjusted for other factors that influence neurodevelopment, such as gestational age at birth, birthweight, birth order, sex, maternal marital status, smoking history, age at delivery, IQ, education, socioeconomic status, and lead exposure.

Estimates from the researchers’ adjusted linear regression models suggest that mean General Cognitive Index and IQ scores were about 3 and 2.5 points lower in association with a 0.5 mg/L increase in prenatal exposure, respectively. However, there was no clear, statistically significant association between fluoride exposure and cognitive development after birth.

The researchers also found that urinary fluoride levels in pregnant women were somewhat higher than, but within the general range of, urinary fluoride levels seen in non-pregnant general populations in Canada and the United States. Yet Hu said that the findings do not provide enough information to suggest there is no safe level of fluoride exposure. 

“The potential risks associated with fluoride should be further studied, particularly among vulnerable populations such as pregnant women and children, and more research on fluoride’s impact on the developing brain is clearly needed,” said Hu.

The ADA, meanwhile, notes that the study’s findings aren’t applicable to the United States. In Mexico, fluoride is added to salt and is naturally found in community water to varying degrees. In the United States, fluoride isn’t added to salt, and it is only added to water where natural fluoride does not meet 0.7 ppm, which is necessary to prevent tooth decay. Furthermore, the ADA reports, the study did not examine how its participants ingested fluoride, so no conclusions about community water fluoridation should be drawn.

Additionally, the AFS points out that every public water system in the United States is required to report its fluoride levels to ensure compliance with Environmental Protection Agency standards, while the level of fluoride in water supplies is not regularly monitored or reported in Mexico. Also, the natural fluoride levels of the water where the study subjects lived varied from 0.15 to 1.38 ppm, potentially affecting the results.

“There are other reasons why we cannot draw firm conclusions from the Mexico study,” said Johnny Johnson, DMD, MS, president of the AFS. “For example, some urine samples for this study were collected at a time when the fluoride concentration in salt was higher than it is today. In addition, some questions have been raised about the method in which the fluoride urine samples were collected. To put it simply, this study raises more questions than it answers.”  

The ADA further notes that the best available scientific evidence shows no association between the recommended fluoride totals in the United States and intelligence, adding that more than 70 years of research and practical experience have found fluoridation safe and effective. The AFS adds that, based on this research, there is no compelling reason to believe that women in the United States should fear or avoid fluoridated water or toothpaste.

The study, “Prenatal Fluoride Exposure and Cognitive Outcomes in Children at 4 and 6-12 Years of Age in Mexico,” was published by Environmental Health Perspectives.

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