Caution advised on US fluoride-in-water study

Researchers are urging caution in response to an American report released overnight finding an association between fluoride intake and neurobehavioural development problems in some children.

The study was conducted as part of a specialised research program investigating environmental pollutants impacting maternal and infant health, particularly in low-income and predominantly Hispanic communities in Los Angeles. It was published today in the journal JAMA Network Open.

The study took ‘spot’ urine samples from 229 pregnant women during their third trimester and then asked them to assess their child’s neurobehaviour at 3 years of age, across 7 “syndrome scales”.

The results found women with higher levels of fluoride in their spot tests were more likely to report neurobehavioural problems in their child.

“Each 0.68mg per litre increase in fluoride levels in the pregnant women’s urine was associated with nearly double the odds of children scoring in the clinical or borderline clinical range for neurobehavioral problems at age 3, based on their mother’s reporting,” says lead author Ashley Malin, an epidemiologist at the University of Florida.

In a study of a specific group of people, the limitations matter

As a result of their study, the research group suggests US authorities may need to consider new recommendations limiting fluoride intake – from all sources – during pregnancy.

But much more research is required, given western health authorities overwhelmingly point to the health benefits of fluoride in preventing oral health diseases.

And while the authors emphasise several strengths in their data collection, they also acknowledge limitations, among them the use of spot sampling.

A spot sample is a one-off and natural variation or alteration due to food, beverage and water intake can give an imprecise urine profile in contrast to a more accurate 24hr test, which takes several samples over a day.

Although many participants were fasting before spot sampling, it’s also unclear whether water consumption was allowed as part of pre-test fasting. Daily tap water consumption behaviours were not taken either.

Crucially, the results cannot be applied beyond the study group, with the authors writing “given that the study sample resided in Los Angeles, California, and was predominately Hispanic, we do not know whether findings observed in this study are generalisable to other US populations or are nationally representative”.

That question around applicability extends globally, too.

“I think the results of this paper are interesting but not something to be alarmed about,” says Oliver Jones, an analytical chemist at RMIT University in Melbourne.

Jones has conducted water pollutant studies with several Australian water authorities.

“Several limitations in this paper mean the results should be treated very carefully,” Jones says.

“Fluoride … 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.”

On top of this, only 32 children – less than 14% of the study participants – scored in the “clinical” or “borderline clinical” range for neurobehavioural problems, but as these groups were combined, it’s unclear how many of those children’s symptoms were clinically significant.

Andrew Whitehouse, a leading autism researcher at the Telethon Kids Institute and University of Western Australia notes the use of parental observations from a small sample should be treated with caution.

“This small-scale study cannot determine anything beyond correlation. This should not change clinical advice, nor daily habits,” Whitehouse says.

“While parent reporting of child behaviours has important value, it can be highly subject to bias, which is something we must keep firmly in mind when interpreting these findings.”

Population details matter when reviewing topical research

For decades, communities have held concerns about health risks associated with fluoride intake.

Some are well-founded: prolonged, excessive fluoride exposure can risk the chemical building up in bones and teeth, leading to skeletal or dental fluorosis.

But the level of exposure needed for fluorosis is well beyond the average daily fluoride intake – that includes people who drink fluoridated tap water.

The US National Institutes of Health notes that acute symptoms like nausea, vomiting and other gastrointestinal symptoms can occur in events of severe fluoride toxicity, but such cases are usually due to rare accidents and are “virtually impossible to achieve from water or toothpaste containing standard levels of added fluoride”.

The benefits of fluoride intake – through diet and fluoridated drinking water – are supported by dental health experts and authorities.

In 2022, a large Australian study led by the University of Queensland found no link between water fluoridation and child brain development.

That research looked at fluoridated water exposure among Australian children in the first 5 years of life. Among nearly 2,700 study subjects – 10 times the number used in the US study – they found no association between fluoridated water exposure and emotional and behavioural development.

“Exposure measurement in the study was objective – residents [lived] in certain areas and we knew whether there was fluoridated or non-fluoridated [water] during the first 5 years for those children,” says that study’s lead author, oral epidemiologist Loc Do.

Like the US study, Do’s research combined test data with questionnaires – “[the methodology] can be considered somewhat comparable to what they use here”.

But he says having a large sample in the thousands can balance potential measurement errors between groups that might occur in small sample studies.

Single studies need to make up biggest parts of research

Scientific studies are complex and controversial subjects need to be treated carefully.

Globally, the benefits of fluoride intake have been consistently emphasised by health authorities as outweighing the disputed risks.

Adam Guastella is a clinical psychologist from the Sydney Children’s Hospital at Westmead and Sydney University’s Brain and Mind Centre where he leads a team investigating childhood neurodevelopment.

“We’re always interested in factors that contribute to healthy and positive outcomes for babies,” Guastella says.

“Looking at factors that can present risk is really important but when we do these sorts of studies, we need to really acknowledge the limitations of what we’re doing. When you’re looking at risk factors in pregnancy, it’s very, very hard.”

While singular studies can provide insight into important areas of health research, they are usually by researchers as part of holistic bodies of literature. Like other researchers Cosmos spoke to, Guastella emphasised the importance of clear communication of scientific research and the risks that the public may draw incorrect conclusions.

“It’s really important for people to know that these sorts of studies cannot tell us whether such factors cause neurodevelopmental delays or not,” he says.

“What I would have liked to have seen in this study, more clearly, is recognition of that, and more qualification of the of the findings that that that they are, at best, indicative for future research. I think that it’s really hard for lay people, and then for parents and for the community to read these findings and come to that conclusion.”

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