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A new Canadian study suggests children exposed to higher levels of fluoride in utero may have slightly lower intelligence scores than those not exposed to fluoride, controversial findings that are already being challenged by a number of experts.

The findings, published in JAMA Pediatrics on Monday, are so contentious the journal editors wrote a note alongside it explaining their decision to publish the research.

“Given the nature of the findings and their potential implications, we subjected it to additional scrutiny for its methods and the presentation of its findings,” the note says. The editors wrote that they are disseminating the best science, “regardless of how contentious the results may be.”

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The researchers looked at more than 500 pregnant women in six Canadian cities and compared those living in areas with fluoridated water to those in areas with no public fluoridation program. The cities involved in the study are Vancouver, Montreal, Kingston, Toronto, Hamilton and Halifax. The researchers then looked at IQ scores of the women’s children when they were three and four years old.

The researchers measured fluoride exposure in two ways. They looked at fluoride levels in urine samples taken from the women during pregnancy. In a second method, they used a questionnaire the women had answered during pregnancy and combined that with other calculations to estimate how much fluoride they had been exposed to through drinking water during pregnancy.

When they looked at the urine samples, the researchers found that boys born to women with higher concentrations of fluoride had slightly lower IQ scores at ages three and four. According to the findings, an increase of one milligram per litre of urinary fluoride levels was linked to a 4.5 point IQ score decrease in boys. There was no impact on the IQ scores of girls.

But when the researchers used the estimate of a woman’s fluoride intake from the questionnaire and other calculations, they found that an increased daily intake of one milligram was linked to an IQ reduction of 3.66 points in boys and girls.

Small amounts of fluoride have been added to drinking water in Canada since the 1940s as a way to prevent tooth decay. Many health agencies, including the World Health Organization, recommend adding fluoride to water to help improve oral health. But adding fluoride to water has provoked controversy for decades, with some anti-fluoride advocates pointing to flawed research suggesting it poses a health risk.

Christine Till, one of the study’s authors and an associate professor of psychology at York University in Toronto, said she hopes the findings will start conversations about policy changes, such as decreasing or eliminating fluoride from the water supply.

“I think the public should be aware of both the benefits and potential risks of fluoride,” she said.

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She said the findings surprised her and that it's important to challenge assumptions about fluoride.

“I didn’t think we were going to find an effect because we were told it’s safe and effective, and that’s the dogma that we hear,” she said.

But several experts not involved with the study are questioning the findings, which they say need to be interpreted with a high degree of caution.

Stuart Ritchie, a lecturer at the Institute of Psychiatry, Psychology and Neuroscience at King’s College in London, said the study’s findings are inconsistent, given that in one scenario, there was a difference between boys and girls and in the other, there was no difference. He said this raises questions about the methods used and reliability of the data.

“So overall, I think the findings here are pretty weak and borderline," he said.

Grainne McAlonan, professor of translational neuroscience at the Sackler Centre for Translational Neurodevelopment at King’s College, said the overall IQ between children living in fluoridated and non-fluoridated areas is very similar and that any small differences found in the study likely don’t mean much. IQ is also subject to error and can fluctuate, so differences in a few points probably aren’t significant, she said.

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Ray Copes, chief of environmental and occupational health at Public Health Ontario, said he found it puzzling that researchers found a different effect on IQ based on which measure of fluoride exposure they used. He said that the urine results are likely the most scientifically accurate way to assess fluoride exposure.

He added that it’s debatable whether a three- or four-point difference in IQ scores is meaningful.

“I would suggest that both for dental health and for IQ, there are much greater determinants or influences than fluoride," Dr. Copes said.

In 2017, a study in Mexico found children exposed to fluoride in utero had slightly lower IQ scores. But many experts pointed out that the results aren’t applicable in Canada or the United States because the sources of fluoride are different between countries.

Dr. Copes said more work is necessary to determine what, if any, impact fluoride exposure has on intelligence.

“I think it would be a mistake to focus on a single study and think that provides the last word on any particular science question,” he said.

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