A study published Monday suggests that fluoride consumed by pregnant women can decrease the IQ of their children. No single study provides definitive answers, but the latest research on this controversial topic will no doubt stir debate.
Fluoride protects teeth from decay, so public health officials celebrate what has been accomplished by putting it in many water supplies. But Christine Till, an associate professor of psychology at York University in Toronto, also wondered about potential downsides.
"It's important that decisions about safety need to be based on evidence," she says, and she notes that evidence is thin when it comes to pregnant women and their babies. So Till and her colleagues tapped into a study of more than 500 Canadian women and their babies, focused on six cities.
"It turned out that about half of the sample were cities, like Toronto, where they add fluoride to drinking water," she says, "and the other half, like Montreal and Vancouver, were cities where they do not add fluoride to drinking water."
That mix gave her a sample of more than 500 women with a whole spectrum of fluoride exposure.
The scientists assessed fluoride exposure two ways. They measured fluoride in women's urine samples during pregnancy. They also calculated fluoride consumption based on how much is in a city's water supply and how much women recalled drinking.
As the team reports in the journal JAMA Pediatrics, it appears that higher levels of fluoride lead to lower IQs in the children.
"Only boys were affected when we looked at urinary fluoride," she says, "but both boys and girls were affected when we looked at maternal fluoride intake or water fluoride concentration."
The difference was typically a couple of IQ points, though the spread was wider when comparing those with highest exposure and those with the least. In general, there was a small difference for any individual child.
"We would feel an impact of this magnitude at a population level," Till says, "because you would have millions of more children falling in the range of intellectual disability, or an IQ of under 70, and that many fewer kids in the gifted range."
The study was funded by the Canadian government and the U.S. National Institute of Environmental Health Science.
"It's actually very similar to the effect size that's seen with childhood exposure to lead," says David Bellinger, a professor of neurology at Harvard Medical School and Boston Children's Hospital. He reviewed the paper before it was published and wrote a commentary about it.
He says it's important not to read too much into a single study, but this one certainly raises important issues.
Though it will no doubt play into the decades-long controversy over whether to add fluoride to public water supplies, he says that is misleading. The study found even in cities that had fluoridated water, women got most of their fluoride from other sources, such as food, tea and toothpaste, "so I think it's a mistake to focus too much on the water fluoridation piece here."
People have such strong feelings about fluoride one way or the other, he says, that "in truth, I don't think this paper is going to move the needle tremendously for a lot of people."
Scientists who advocate for fluoride will point to the weaknesses inherent in a population study like this, "and the people on the other side who think that fluoride is quite toxic are going to say 'see, we told you so.' "
The "decision to publish this article was not easy,"JAMA Pediatrics editor Dr. Dimitri Christakis wrote in an accompanying note. Given the sensitivity of the findings, he wrote, "we subjected it to additional scrutiny for its methods and the presentation of its findings."
"I think people are going to be shocked but I think people should realize that science is constantly evolving," senior study author Till says. A previous study in Mexico found a similar link, she says, so this is not the first to suggest a connection. This finding is likely to spur more research on the topic.
Fluoride consumption during pregnancy has no known health benefits to the baby, Till says, so she suggests a precautionary step as the science is explored further: "We recommend that women reduce their fluoride intake during pregnancy."
Bellinger agrees that makes sense, especially for women who find that the new results stir anxiety.
You can reach NPR science correspondent Richard Harris atrharris@npr.org.
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