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By Jamie Ducharme
Updated: August 21, 2019 8:25 AM ET | Originally published: August 20, 2019

A study published in JAMA Pediatrics has given new life to a long-running debate: whether adding fluoride to drinking water is a prudent way to prevent tooth decay, or a potentially toxic mistake.

The research, which focused on mother-child pairs from six Canadian cities, found that high fluoride exposure during pregnancy was correlated with lower IQ scores among young children, especially boys.

“Based on the current evidence, it is a reasonable recommendation to tell women to reduce their fluoride intake during pregnancy,” says study co-author Christine Till, an associate professor of psychology at York University in Toronto. “It’s a low-hanging-fruit recommendation to [protect] the safety of the fetus. That’s a no-brainer.”

But even JAMA Pediatrics’ editor-in-chief, Dr. Dimitri Christakis, cautions that the fluoridation debate is far from settled, as he wrote in an accompanying editor’s note.

“We were well aware that this would probably be overplayed, and misplayed, in the public arena. But it needs to be out there,” says Christakis, who is also a professor of pediatrics at the University of Washington. “It’s an important study. It’s not the definitive study. Science is an incremental process.”

The U.S. began adding fluoride to some public water supplies in the 1940s, in an effort to strengthen teeth and prevent decay, and research on it has been accumulating in the subsequent decades. At high doses, fluoride can actually damage people’s teeth, according to the World Health Organization—and some research, much of it in animals, suggests it’s also tied to more serious side effects, including bone cancer and cognitive impairments. In part due to that controversy, more than 300 communities in North America have voted to end fluoridation programs over the past 20 years, according to the anti-fluoride activist group Fluoride Action Network. Today, about 66% of Americans and 39% of Canadians had access to fluoridated water.

The U.S. in 2015 lowered the recommended amount of fluoride in drinking water supplies, from up to 1.2 milligrams per liter to 0.7 milligrams per liter, mainly to cut down on potential damage to teeth. (Canada also defines 0.7 milligrams per liter as the optimal level of fluoridation.) The U.S. Centers for Disease Control and Prevention maintains that community water fluoridation is safe, and in 1999 even crowned it one of the 10 greatest public-health achievements of the 20th century.

The new study is the latest to call that into question. Between 2008 and 2011, researchers recruited expectant mothers during the first 14 weeks of a healthy pregnancy. About 500 women provided urine samples during each trimester of pregnancy, which the researchers used to measure fluoride levels. Four-hundred women also answered questions about whether they lived in a community that added fluoride to its water, and how often they drank tap water. After the women gave birth, their children took IQ tests at age 3 or 4.

The researchers found that self-reported high-fluoride intake was associated with lower IQ scores in both boys and girls. But when looking at actual fluoride measurements from maternal urine samples, and after adjusting for factors such as maternal education and household income, higher intake was correlated with lower IQ scores in boys, but not in girls. That’s likely because self-reported fluoride intake dealt specifically with fluoride consumed through beverages, meaning kids were likely exposed to the same water as they grew up. Maternal urine samples, however, also captured fluoride from dietary sources and dental products, providing a fuller picture of prenatal risk factors, Till says. Prior research in lab animals also suggests males are more susceptible to damage from fluoride, and other research suggests they are at higher risk of neurodevelopmental disorders such as ADHD, so it’s not entirely surprising that their IQ scores would be more closely linked to fluoride exposure, she adds.

Drinking an extra milligram of fluoride per day—about a liter of properly fluoridated water, plus a mug of tea (which is itself a source of fluoride)—during pregnancy translated to an average 3.66-point IQ drop for boys, the study shows.

“I would even argue that two IQ points would be something, at the population level, that we should be concerned about,” Till says.

Christakis stresses that this is only one study, and an observational one at that—meaning it looked only at associations between fluoride intake and IQ, rather than randomly assigning some women to ingest fluoride, then tracking their children’s development compared to a control group. (There’s no ethical way to complete such a study, Christakis says.) Nevertheless, Christakis says this particular study was stronger than some that came before it, since it tracked mothers and children over time and used objective fluoride measurements through urine.

Additional studies are needed, but a true conclusion could be a long time coming, leaving women who are currently or soon-to-become pregnant at an impasse. “The real question that needs to be asked is, ‘What would you advise a pregnant woman to do today, based on what you know now?'” Christakis says. “That’s a personal decision; it’s different from a public-policy decision. I’m not advocating, on the basis of this study, that we should necessarily change public-policy—but I would minimize exposure to fluoride” during an individual pregnancy.

This story has been updated to clarify study design.

Write to Jamie Ducharme at jamie.ducharme@time.com.

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