Kevin Lomangino is the managing editor of HealthNewsReview.org. He tweets as @KLomangino.
Multiple news outlets are covering a new study conducted in Mexico on fluoride exposure during pregnancy and its association with lower childhood IQ.
Water fluoridation is common across the U.S. and the potential to stoke unwarranted fear is great. If you are reading or writing about this study, here are four questions you should be asking yourself:
This was an observational study that can only show associations between fluoride and IQ — not that fluoride exposure caused a change in IQ. Newsweek’s headline — “CHILDREN’S IQ COULD BE LOWERED BY MOTHERS DRINKING TAP WATER WHILE PREGNANT” — goes beyond what the study can tell us, as does Readers Digest’s “If You Drink This Type of Water During Pregnancy, Your Child’s IQ Could Suffer.”
Remember: If fluoride “could” cause these problems it just as easily “could not” cause them.
It’s impossible, based on the results of this one study, to know if fluoride “lowered” IQ or whether children with lower IQ in Mexico are simply more likely to have higher fluoride levels for some unknown reason. CNN gets it right with “Fluoride exposure in utero linked to lower IQ in kids, study says.” Canada’s National Post also nails it with “Researchers urge caution over study linking fluoride exposure in pregnancy to lower IQs in children.”
Learn more by reading our primer: Observational studies: Does the language fit the evidence? Association vs. causation
The study authors describe a long list of items that could have influenced their results, including the methods they used to collect urine samples (one-time collection rather than the more rigorous 24-hour method) and lack of information about other factors (e.g. exposure to neurotoxins like arsenic) that might impact childhood cognition. They also don’t know how the mothers in the study were exposed to fluoride or at what levels. In addition, they said, “we cannot rule out the potential for uncontrolled confounding due to other factors, including diet, that may affect urinary fluoride excretion and that may be related to cognition.”
The other point to consider is that this is a single study conducted in an area that doesn’t have fluoridated water and which might not reflect U.S. circumstances. As CNN appropriately observes, “a study done in New Zealand, a country that has community water fluoridation similar to the United States, found no connection between fluoridated water and IQ.”
This is an area where all of the stories I read could have done better. The researchers reported a drop of 2.5 IQ points for every 0.5 mg/dL increase in fluoride levels (average fluoride levels in these children were around 0.8 mg/dL). Obviously, any association of fluoride with reduced cognition is worrisome, but it’s important to get specific. Is a 2.5-point difference meaningful or even noticeable? None of the stories explained. Susan Molchan, MD, a psychiatrist and former NIH researcher, said “2.5 points is probably not enough to make a difference in school performance for example when you consider the range of a normal IQ would be 90-110. What’s the difference if you’re 100 or 97.5? And how one performs on an IQ test is just one small piece of how one will do in school/life—so it is a good question on how clinically or functionally significant this is.”
CNN repeated the CDC’s claim that water fluoridation is “one of the 10 greatest public health achievements of the past century.” CTV said that water fluoridation should be acknowledged as “a public health success story.” But none of the stories dug into the evidence supporting these claims, much of which comes from observational studies conducted before 1975. Just as these types of studies can’t definitively prove harm from fluoridated water, they can’t prove definitively that water fluoridation caused reductions in cavities. In fact, many didn’t account for circumstances that have clear potential to impact the effectiveness of fluoridated water — for example the introduction of fluoride toothpaste or the amount of sugar consumed by study participants. A recent systematic review of the evidence conducted by the Cochrane Collaboration found no evidence that fluoridation reduces cavities in adults and very limited evidence for a benefit in children.
There is very little contemporary evidence, meeting the review’s inclusion criteria, that has evaluated the effectiveness of water fluoridation for the prevention of caries. The available data come predominantly from studies conducted prior to 1975, and indicate that water fluoridation is effective at reducing caries levels in both deciduous and permanent dentition in children. Our confidence in the size of the effect estimates is limited by the observational nature of the study designs, the high risk of bias within the studies and, importantly, the applicability of the evidence to current lifestyles.
The Cochrane authors point out that cross-sectional studies — considered a lower form of evidence because they only examine data at a single time point — have been conducted more recently but were excluded from their analysis. They also acknowledge that it may be difficult or impossible to obtain gold-standard clinical trial evidence demonstrating the benefits of water fluoridation. The practical obstacles to conducting such studies are significant. Nonetheless, they say that more contemporary studies, with more rigorous designs, and needed to determine whether water fluoridation is beneficial under current circumstances and to what extent.
Sometimes mountains of observational data are enough to guide sound public health recommendations and action. Observational studies documenting the harms of tobacco are an example of this. Still, the four questions raised in this piece are important for journalists and the public to consider.
Molchan believes the emerging data should prompt reflection about community fluoridation policies. “Why go through the expense of adding a chemical to the whole water supply if there is no known benefit, especially if there are questions about its safety?” she asks.
That seems to be theme running through this entire topic: questions, but few answers, on the benefits and risks of water fluoridation. Journalists who explore these questions carefully will help readers understand what’s at stake without spreading needless fear.
Note: We’ve previously examined coverage of research linking fluoride in water with attention deficit hyperactivity disorder.