“I still stand by the weight of the best available evidence, from 70 years of study, that community water fluoridation is safe and effective,” said Brittany Seymour, a dentist and spokeswoman for the American Dental Association. “If we’re able to replicate findings and continue to see outcomes, that would compel us to revisit our recommendation. We’re just not there yet.”
The American Academy of Pediatrics, likewise, recommends fluoride in toothpastes and tooth varnishes for children because the mineral prevents tooth decay. In drinking water, “fluoridation has been incredibly protective,” said Aparna Bole, a pediatrician who chairs the Council on Environmental Health at the American Academy of Pediatrics. Fluoridation reduces the prevalence of cavities by about one-fourth, according to the Centers for Disease Control and Prevention. The CDC considers water fluoridation one of the 10 top health achievements of the past century, on par with vaccines and antismoking campaigns.
Bole called the new study, published Monday in JAMA Pediatrics, “an important addition to our body of knowledge. It supports the public health community’s ongoing reevaluation of optimal fluoridation levels in drinking water.”
In January 1945, researchers added fluoride to municipal water in Grand Rapids, Mich., the first program to enlist fluoride to protect a city’s teeth. Opponents of fluoridation have since raised concerns both ludicrous — fluoridation is not a communist plot — and legitimate, such as fluorosis. In the mild form of fluorosis, faint white streaks appear on the teeth of young children. Severe fluorosis, which is much rarer, damages bones.
Dozens of cities in the United States and Canada, such as Portland, Ore., and Vancouver, do not add fluoride to city water. Elsewhere in the United States, fluoridation is the norm. As of 2014, per CDC data, two-thirds of people in the United States had fluoride in their drinking water. In 2015, to reduce the risk of mild fluorosis, the Department of Health and Human Services cut its fluoride recommendations almost in half, from 1.2 milligrams per liter to 0.7 milligrams per liter.
Few older studies addressed potential risks, or the lack thereof, associated with fluoride exposure during pregnancy, said study author Christine Till, a neuropsychologist at Toronto’s York University. She added that “whether we found an effect or not, the data would be really relevant because we would then address that gap in our knowledge.”
Till and her colleagues acquired data and frozen urine samples previously collected by Maternal-Infant Research on Environmental Chemicals, or MIREC. That project, run by Canada’s public health department, studied thousands of mothers who gave birth between 2008 and 2012. MIREC researchers measured the toddlers’ IQ after the children turned 3.
Pregnant women reported their consumption of tap water and black tea, which is high in fluoride, in questionnaires. The authors of the new study also calculated the amount of fluoride in municipal water, based on the levels at wastewater treatment plants linked to the women’s postal codes. The researchers estimated the women’s fluoride intake based on a combination of those measures.
The researchers compared the fluoride intake of 400 women, some who lived in fluoridated cities and some who did not. They controlled for factors such as household income and the women’s education. A 1 milligram daily increase in fluoride intake was associated with a 3.7-point drop in children’s IQ, they found.
As an additional step, Till and her colleagues measured fluoride biomarkers in urine from 500 pregnant women, collected during each trimester. Fluoride content in urine was only moderately related to the estimates of the mothers’ fluoride intake, suggesting that neither was a perfect measure of how much fluoride a pregnant woman drank.
The scientists observed that a 1 milligram-per-liter increase in urine fluoride predicted a drop in IQ of 4.5 points in young boys. When the researchers examined the urine of mothers who had daughters, however, fluoride had no association with IQ.
Previous observational studies claimed to find relationships between fluoride and IQ, but most were “of poorer quality due to various weaknesses in study design,” said David Bellinger, an expert in neuroepidemiology at Boston Children’s Hospital who was not associated with this study. The methods in this report, he said, are “very similar” to studies that showed low-dose lead and pesticide toxicities.
But Bellinger called for further research. “Generally, no single epidemiological study settles a question like this,” he said.
“The decision to publish this article was not an easy one,” said Dimitri A. Christakis, the editor of JAMA Pediatrics and a pediatrician at Seattle Children’s Hospital. Christakis appended a note to the study, a first in his career, explaining that the journal subjected the paper to “additional scrutiny.” This included multiple statistical reviews, he said.
“The findings are what they are,” Christakis said. “There is clearly an association. It by no means proves definitively that this is a risk.”
Several researchers unaffiliated with the report applauded this work’s publication in the face of intense review. “I believe that, in general, the dental community will discount these findings, minimize their importance and continue to recommend the use of fluoridated water during pregnancy,” said Pamela Den Besten, a pediatric dentist who studies tooth enamel at the University of California at San Francisco. She added: “This study has been carefully conducted and analyzed.”
“This is an excellent study,” said Philippe Grandjean, a physician who studies brain development and environmental pollutants at the Harvard School of Public Health. “CDC has to come out and look at the risk-benefit ratio again, because they can’t continue relying on studies that were carried out decades ago.”
The CDC declined to comment on the study itself because it was not a participant, said Amesheia Buckner, an agency spokeswoman. “Community water fluoridation is one of the most practical, cost-effective, equitable and safe measures communities can take to prevent tooth decay,” Buckner said.
“It has major drawbacks in terms of how the measurements have been made,” Ioannidis said. “The results are very borderline in terms of statistical significance.” It’s a weakness, he said, that the self-reported consumption was not linked directly to levels of fluoride measured in bodily fluids.
What’s more, the sex difference in IQ — the drop observed for boys but not girls — “makes no sense,” he said. “If you see a gender difference claim for this type of association, it’s far more likely to be a spurious finding rather than something true.”
Bole agreed. “I just don’t know how to interpret that,” she said.
Some physicians offered advice based on this study. “The answer for me, I can say, is I would not have my wife drink fluoridated water” if she were pregnant, Christakis said. Grandjean, likewise, suggested pregnant women drink bottled water and limit black tea to a single cup per day.
Others did not. “I’m hoping people don’t conclude on the basis of this one study, ‘Oh, boy, we should all be drinking bottled water.’ No,” Bole said. “Tap water in most communities is the healthiest and most environmentally responsible choice.” And adding more restrictions to what pregnant women can consume, Ioannidis said, creates a “burden of feasibility."
Seymour said the damage from dental disease goes beyond teeth. “Kids in the U.S. who don’t have access to community water fluoridation have significantly higher dental disease, and we know that impacts their ability to learn and grow,” she said, including harm to sleep, self-esteem and school performance. Seymour, whose young daughter drinks city water, would “honestly be more concerned if there was a decision to stop fluoridating our water,” she said.
Ioannidis’s stance "on whether fluoride exposure during pregnancy is a bad thing for the IQ of the child goes up about threefold,” he said. “If you thought that it was maybe 1 percent likely to be true, that 1 percent now would become 3 percent. It would still not be true.”