The study, which was published in Environment International (November 16, 2019), is the third in the past two years to question the relationship between fluoride intake and IQ scores for children.
A study published earlier this year from many of the same authors found that children with greater prenatal fluoride exposure had lower IQ scores later on, and a 2017 study on children in Mexico previously linked prenatal fluoride exposure to lower IQ scores.
Lead study author Christine Till, PhD, and colleagues continued their research into early fluoride exposure and subsequent IQ scores by looking at a common source -- powdered infant formula that is reconstituted with water. The results were similar to their previous research.
"Exposure to increasing levels of fluoride in tap water was associated with diminished nonverbal intellectual abilities," wrote Till, an associate clinical psychology professor at York University in Toronto, and colleagues. "The effect was more pronounced among formula-fed children."
Powdered formula is one of the main nutrition sources for infants in both the U.S. and Canada, the authors noted. This type of formula is often reconstituted with optimally fluoridated tap water and may be a significant source of fluoride intake for infants. However, no prior studies had investigated the potential neurological effects of fluoride exposure from formula reconstituted with fluoridated water.
For their study, the researchers recruited 601 mothers and their children, the same cohort they used in their previous research. The children were divided into two groups: the breastfed group, which included children exclusively breastfed for at least six months, and the formula-fed group, which included children who were never breastfed or were exclusively breastfed for less than six months.
Then, when the children were between the ages of 3 years and 4 years, the researchers tested their intelligence. They used three different types of IQ tests: a general IQ test, a verbal-specific IQ test, and a nonverbal reasoning and visual-motor IQ test.
An estimated 0.5-mg daily increase in fluoride intake from formula made with fluoridated water was associated with an almost 9 point decrease in nonverbal IQ scores, the researchers found. After adjusting for prenatal fluoride exposure, an increase in daily fluoride exposure was still associated with a 7.62 point decrease in nonverbal reasoning scores; however, the results were not significant for the other two types of IQ scores.
In addition, a 0.5-mg/L increase in water fluoride concentration was associated with a 4.4 decrease in general IQ scores for formula-fed infants; however, the finding dropped out of significance when the researchers excluded two IQ outliers.
Water fluoride concentration was also significantly associated with lower nonverbal IQ in both groups of infants, although the interaction was not significant.
"Taken together, these findings suggest that using optimally fluoridated water (0.7 mg/L) to reconstitute infant formula may diminish the development of intellectual abilities in young children, particularly for nonverbal abilities," the authors wrote. "The findings also suggest that both prenatal and postnatal fluoride exposure affect the development of nonverbal intelligence to a greater extent than verbal intelligence."
It is important to note the study had a number of shortcomings, including that the researchers did not have data on whether each infant's formula was reconstituted with tap water or another type of water. The researchers tried to mitigate this shortcoming by only including children of women who reported drinking tap water.
In addition, the researchers could not estimate the amount of fluoride exposure from infant formula itself, only from the community water sources in the ZIP codes where children and their mothers lived.
Larger policy questions
The question of whether fluoride may hinder neurological development is making its way into U.S. public policy discourse. In September, the U.S. Department of Health and Human Services' National Toxicology Program issued a draft monograph on fluoride exposure and cognitive health effects, which concluded that fluoride is "presumed to be a cognitive neurodevelopmental hazard to humans."
The ADA opposed the conclusion based on the validity of the studies in the review, including the use of studies with fluoride exposure above the current recommended limit of 0.7 parts per million.
In addition, the ADA, U.S. Department of Health and Human Services, and U.S. Centers for Disease Control and Prevention websites currently note that fluoridated water is safe for reconstituted infant formula. Specifically, the ADA's Fluoride FAQs section on the website reads: "If your baby is primarily fed infant formula, using fluoridated water might increase the chance for mild enamel fluorosis, but enamel fluorosis does not affect the health of your child or the fitness of your child's teeth."
Nevertheless, the researchers called into question the safety of optimally fluoridated water for infants. In their conclusion, they wondered whether fluoridated water could exceed the tolerable limits of children younger than 6 months old.
"After adjusting for fetal exposure, we found that fluoride exposure during infancy predicts diminished nonverbal intelligence in children," the authors concluded. "In the absence of any benefit from fluoride consumption in the first six months, it is prudent to limit fluoride exposure by using nonfluoridated water or water with lower fluoride content as a formula diluent."
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