Is fluoride exposure in pregnancy linked to child neurobehaviour?

A new study has found a link between fluoride intake during pregnancy and neurobehavioural problems among children living in an area with fluoridated water.

The observational study, by the University of Florida, USA, included 229 pairs of predominately Hispanic mothers and children and followed the children to age three.

The authors measured the levels of fluoride in the mothers’ urine at a single time point during the third trimester and found that higher levels were linked to an increased risk of child neurobehavioural problems at age three.

They found that a 0.68 milligram per litre increase in fluoride exposure was associated with nearly double the chance of a child showing neurobehavioural problems in a range considered close to or at a level to meet the criteria for clinical diagnosis.

“There are no known benefits to the foetus from ingesting fluoride,” said Ashley Malin, PhD, an assistant professor of epidemiology at the University of Florida’s College of Public Health and Health Professions and College of Medicine and lead author of the study. Malin conducted the research in part as a postdoctoral scholar at the Keck School of Medicine.

“Children exposed to an additional 0.68 milligrams per litre of fluoride in the womb were 1.83 times more likely to show behavioural problems considered to be clinically significant or borderline clinically significant. Specifically, children exposed to more fluoride had more problems with emotional reactivity, somatic complaints (such as headaches and stomachaches), anxiety and symptoms linked to autism,” said Keck School of Medicine in a statement.

“No association was found with several other neurobehavioural symptoms, including ‘externalising behaviours’ such as aggression and attention problems.”

While the study cannot show cause and effect, the authors say these findings suggest there may be a need to establish recommendations for limiting fluoride exposure during the prenatal period.

An inflated number?

Associate Professor Matt Hopcraft, CEO of the Australian Dental Association Victorian Branch, co-founder of SugarFree Smiles and Honorary Fellow at the University of Melbourne, said, “This new study purports to show an association between prenatal fluoride consumption and child neurobehaviour at age 36 months. The study is based on a small sample of parents (229 mother-child pairs), with only 32 children (14%) reaching the threshold for total problems on the Preschool Child Behaviour Checklist. However, this number is inflated by including children who scored in the borderline clinical range with those who scored over the clinical threshold — and the paper does not report the numbers in each group. This means it is possible that the associations found are for children who have not reached the clinical threshold of the neurobehaviour measure.

“There are also associations between a number of demographic factors (such as parental income and maternal education) and fluoride exposure, but there is no data presented on whether there is also an association between these factors and neurobehaviour. Blood lead levels are also reported to be associated with increasing the magnitude of the effect of urinary fluoride on CBCL scores, but it is not clear if there is also a direct association between blood lead levels and neurobehaviour.

“Importantly, as the authors note, fluoride exposure is a single measure at one point in time during the third trimester — even allowing for the fasting (and it is not clear from the methodology whether the sample was taken during the fasting period or not) it cannot be considered representative of the total prenatal fluoride exposure.”

Not a cause for alarm?

Oliver Jones, Professor of Chemistry at RMIT University, said, “I think the results of this paper are interesting but not something to be alarmed about. Several limitations in this paper mean the results should be treated very carefully.

“Firstly, the paper notes an ‘association’ between the fluoride concentration of a mother’s urine and the behaviour of her child at three years of age. An association between two factors does not mean the two factors are linked or that another factor (that was not tested for) is the cause of said association. The previous Canadian and Mexican studies that the authors refer to also had limitations that restrict the conclusions drawn from them.

“Secondly, according to the paper, the participants in this study were mostly from one specific group — ‘Hispanic women of low socioeconomic status living in urban Los Angeles’. This means the results can’t be extended to other populations. Fluoride also has a short half-life in the human body and urine concentrations vary hugely so taking a single measurement is not accurate, yet this paper also used the ‘spot’ method of urine collection, rather than the more accurate 24-hour method, so one can’t be sure of the actual fluoride levels the children were exposed to.

“Thirdly, if fluoride really did cause problems with brain development in children, we would expect to see this effect consistently worldwide, but we don’t. In fact, at least two recent studies found that maternal fluoride levels were actually associated with better cognitive scores in children.

“I could go on, but to summarise — fluoride is a naturally occurring substance found in water and some foods. It is not toxic at the levels we are exposed to and there is no robust evidence of any relationship between fluoride exposure and IQ level or behaviour issues. This study does not change that.”

Sample size

Professor Loc Do, a Professor of Dental Public Health in the School of Dentistry at The University of Queensland, said, “Exposure to fluoride in this study was measured by spot urinary fluoride analysis, which is highly variable within a person even during a day. This major shortcoming was even acknowledged in a paper, of which Malin was a co-author. Even 24-hour urinary fluoride measurement (a much more reliable exposure measurement) cannot measure chronic fluoride exposure in individuals (Rugg-Gunn et al 2011). An inexplainable observation when those with the highest income or education had markedly high mean urinary fluoride levels, pointing to the unreliability of this measure in this study.

“The outcome measure has high measurement error, requiring relatively high literacy and close attention to the child. Those with the highest income or education (also with the highest mean urinary fluoride in this study) could observe and report more problems about their child than others, leading to assessment bias.

“The sample was very small (only 229) and not adequate to balance expected measurement errors. Even a few individuals with biased measurements could completely alter the overall findings.

“The paper did not cite all the literature that contradicted their findings. An Australian nationwide study with a large population-based sample (2682 children) did not find any effect of early life exposure to fluoride on child neurobehavioural development and executive functioning. The exposure measurement of fluoride exposure in this study was objective and fixed. It used two sophisticated instruments of child neurobehavioural development and executive functioning.”

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