Experts Question Evidence Linking Fluoride to Lower IQ

Pauline Anderson

March 06, 2020

The conclusion of a recent, widely publicized draft report linking higher fluoride exposure to decreased intelligence quotient (IQ) is not satisfactorily supported by the evidence, an expert committee of the National Academies of Sciences, Engineering, and Medicine (NAS) has determined.

The 2019 draft report, which was based on a large, systematic review by the National Toxicology Program (NTP) and was reported by Medscape Medical News, concluded that there is evidence of an association between fluoride exposure and cognitive neurodevelopmental problems in children.

Dr David Savitz

There are a number of concerns about the methods the NTP used to evaluate studies in its review, committee chair David A. Savitz, PhD, professor and chair, Department of Epidemiology, Brown University School of Public Health, Providence, Rhode Island, told Medscape Medical News.

Most notably, it was not clear that the evidence presented in the report's tables and other materials warrants the authors' conclusion about fluoride exposure and IQ, said Savitz.

However, he added, this does not mean that the review's conclusion is incorrect.

"Although we found problems with the review, we're not saying the conclusion is wrong or right; we're saying that the report doesn't provide the trail of evidence and logic that would be desired to reach a conclusion," he said.

Major Public Health Advance

Fluoride, which comes from fluroine, is an abundant element found naturally in the environment. It has been proven to prevent tooth decay. As a result, it has been added to community water supplies for decades and is present in some consumer products, including toothpaste.

The Centers for Disease Control and Prevention has recognized community water fluoridation as one of the greatest public health achievements of the 20th century.

However, some experts claim fluoride exposure can have adverse effects on health. A 2006 review of the literature by the NAS concluded that long-term exposure to fluoride can cause enamel fluorosis and weakening of bone that could increase fracture risk.

At the time, studies of the potential neurotoxicity from fluoride exposure lacked sufficient detail, making a definitive conclusion impossible. The NAS determined that the consistency of the results on neurotoxicity warranted further investigation.

Since publication of the 2006 report, there have been several epidemiologic studies of fluoride exposure and neurodevelopmental and cognitive effects. This prompted the NTP to take a closer look.

In a systematic review conducted in 2016, the NTP, which operates under the auspices of the National Institute of Environmental Health Sciences of the National Institutes of Health, found a low to moderate level of evidence that learning and memory deficits occur in nonhuman mammals exposed to fluoride.

In its 2019 systematic review, the NTP identified 149 relevant published human studies, 339 experimental animal studies, and 60 in vitro/mechanistic studies for analysis.

Neurodevelopmental Hazard?

In its 2019 report, the NTP concluded that the available evidence showed "a consistent pattern" of findings that high exposure to fluoride is associated with lower IQ in children and that fluoride is "presumed to be a cognitive neurodevelopmental hazard to humans."

However, the authors of the NTP review also point out that this association is primarily based on high levels of fluoride exposure ― levels >1.5 ppm in drinking water.

Regarding the potential impact of lower, more typical levels of fluoride exposure ― 0.03 to 1.5 ppm ― "effects on cognitive neurodevelopment are inconsistent, and therefore unclear," they note.

The limited evidence for cognitive effects in adults makes it impossible to determine whether fluoride exposure in adults is associated with cognitive effects, the NTP investigators noted. In addition, they concluded that there is not enough evidence from animal studies to draw conclusions about fluoride's potential cognitive effects.

The authors of the NTP report also noted that the mechanisms underlying fluoride-associated cognitive neurodevelopmental effects are not clear.

The NTP subsequently asked the NAS to conduct an independent evaluation of the its 2019 monograph. The NAS convened a committee that included experts in public health, systematic reviews, epidemiology, environmental science, and neuropsychology to determine whether the NTP's research supported its conclusions.

"We were not doing an independent evaluation of the evidence to see if we agree or disagree; we were evaluating what they had done and mainly asking the question of [whether their research] did or did not support the conclusions they drew," said Savitz.

No Meta-Analysis

The NAS committee flagged a number of problems, including potential selection bias. The review relied on the Fluoride Action Network to identify potential studies, and the process by which this was done was unclear, the committee notes.

This, said Savitz, may have resulted in the identification of only a subset of studies and may not have included some of the "harder to find studies" ― for example, those that were translated from Chinese.

The committee also pointed out an apparent inconsistent application of risk-of-bias criteria across studies in the review. For example, some studies did not include information on blinding techniques.

"Because failure to blind examiners might result in a high risk of bias of study results and conclusions, NTP should consider this aspect more carefully when assessing the risk of bias of human studies," the NAS report notes.

In addition, the NAS committee points out that the NTP review did not compare studies that had similar parameters. For example, the studies that were compared utilized different measures of fluoride exposure, different analytic techniques, and different means of assessing neurodevelopmental and cognitive outcomes at different life stages.

The NAS committee found many cases in which the NTP's evaluation of confounding "was insufficient, difficult to understand, or applied inconsistently across studies."

The committee also noted that the NTP did not conduct a meta-analysis.

"Given that meta-analysis is a useful tool for aggregating and summarizing data and analyzing comparable studies, the committee strongly recommends that NTP reconsider its decision not to perform one," the NAS report authors write.

New Review in the Works

Savitz noted that there is keen public interest in whether exposure to fluoride is harmful and, if so, at what level it is harmful.

"We thought it was important that the authors circumscribe more clearly where the bounds of inference are here, especially because a lot of people are very concerned about this balance between the perceived benefits of fluoridation and the risks," he said.

Now that the committee's evaluation is complete, it's up to the NTP to "judge what they feel is the right way to proceed," said Savitz. "It's entirely their call; it's back in their hands."

Commenting on the NAS report findings, the NTP told Medscape Medical News that it plans to issue a revised, final report later this year after an additional expert review.

"As we anticipated, the Academies offered insightful and helpful suggestions for improving the NTP report. We will be revising the report to add additional details that were recommended for clarifying the NTP's analysis and conclusions.

"We are sincerely grateful for the work done by the Academies and for the carefully considered comments offered by the committee chairman and each of the expert reviewers," NTP spokesperson Christine Bruske Flowers said in an e-mail.

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