Opponents of water fluoridation are quick to cite a 2019 Canadian study (Green et al.) to claim that fluoride affects the IQ of 3 and 4 year old children. A new peer-reviewed analysis refutes the validity of this study, showing that the Canadian researchers relied on invalid measures of both fluoride exposure and IQ. For years, opponents have promoted this and other Canadian studies that have misused the same data set.
The new analysis, authored by Guichon, Cooper, Rugg-Gunn, and Dickinson, details the serious weaknesses in those studies. It cautions that claims about fluoride and IQ based upon them “should be considered unacceptable for legal and policy purposes.”
Significant Flaws
After analyzing data from the Maternal-Infant Research on Environmental Chemicals (MIREC) study, Green et al said they found an association between fluoride exposure and IQ. The primary outcome of this study was supposed to show whether there are differences in IQ resulting from fluoride exposure. The authors compared the average IQ score between fluoridated and non-fluoridated Canadian cities, found that the scores were nearly identical, but did not highlight this. Instead, they conducted a subsequent analysis that the MIREC study was not designed to support. MIREC gathered “spot urine samples,” which the Green et al. authors claimed could measure fluoride exposure of individual pregnant women.
The Guichon paper cites the conclusions of experts that spot samples of urinary fluoride are not a valid measure of an individual’s exposure to fluoride. In addition, the measurement of the children’s IQs was viewed as unreliable. The paper questions the reliability of the IQ scores reported for the 3 and 4 year olds in the study because best practice calls for the tests to be administered by the same person, known as an examiner. In this instance, the IQ tests in each city were administered by different examiners. In that case, Green et al. should have provided an assessment of the impact of using different test examiners. They did not. For these reasons, the Guichon paper concluded that MIREC data “cannot be used to make claims that fluoride exposure affects IQ.”
Reliability
In addition to Guichon, the authors of this paper include Andrew Rugg-Gunn and Colin Cooper. Rugg-Gunn is a British scientist and international expert in the measurement of fluoride intake. Cooper is a British psychologist who has authored more than a dozen books, chapters or scientific articles about human intelligence and related topics. The fourth co-author, James Dickinson, is an epidemiologist and professor of Family Medicine at the University of Calgary, in Canada.
The reliability of fluoride-related studies is an important issue because fluoride is widely recognized as a crucial way to prevent tooth decay, the most common chronic disease for both children and adults. Water fluoridation is recommended by the American Academy of Pediatrics, American Dental Association, American Public Health Association, and other leading health, medical and dental organizations.
The flaws of a research paper often do not come to light until long after publication. Journals should hold authors to high standards, for example, by following the lead of BMC Public Health, which retracted a fluoride-IQ study after examining serious concerns about the methods used in this study.
The Weight of the Evidence
Beyond the new analysis, the safety of fluoridation was further demonstrated by a 2023 meta-analysis of eight studies that reported “fluoride exposure relevant to community water fluoridation is not associated with lower IQ scores in children.”
There has been only one fluoride-IQ study to follow individuals over a 30-year period, testing their IQs at multiple ages to determine if a link existed. That New Zealand study found no link between fluoride and IQ scores. Analyses of data from Australia, Denmark, Spain, and Sweden also have revealed no association between fluoride and cognitive deficits.
Here in the U.S., the Centers for Disease Control and Prevention continues to report that fluoridated water is safe and effective, reducing tooth decay by 25% for children and adults. Millions of people all around the world receive significant dental health benefits because their tap water contains fluoride at the recommended level to prevent tooth decay.
While new research on fluoride and fluoridation is important, to be considered a sound contribution the study design of that research must include valid measures of exposures and health outcomes.
