health & science

A curious case of fluoride and pregnancy

Advertisements are being seen country-wide claiming a link between fluoride and lower IQ. However, scientists are saying hold your anti-fluoride horses until better studies are completed.

Billboards, newspaper advertisements and radio advertising are telling New Zealanders 'Fluoride lowers kids’ IQs'.

The advertisements in Auckland, Wellington, Hamilton, Palmerston North and Hastings, in the New Zealand Herald, 20 community newspapers and on Newstalk ZB were placed by Fluoride Free New Zealand and are funded through donations.

They’re based on a recently published Canadian study which claims “fluoride exposure during pregnancy was associated with lower IQ scores in children aged three to four years”.

Fluoride Free New Zealand’s national coordinator Mary Byrne calls the study the best published study on fluoridated water and health effects. She’s irked that since the removal of a Dunedin billboard, her organisation's advertisements are getting more media attention than the study.

“We’re finding it a little bit annoying we’re having to pay to get this information out to pregnant women when we feel this is the responsibility of the Ministry of Health.”

The advertisements may sound scary but going beyond the billboard message and digging into the study paints a not-quite-so-alarming, but interesting picture.

However, the study’s methods and statistical analysis have raised scientific eyebrows and caused some gnashing of statistical teeth.

The study has been labelled “weak” and said to contain “false” claims, however, many still think it's an “interesting” study, worthy of further research. 

How was the study done?

Mothers, some living in Canadian cities with a fluoridated water supply and some not, gave three urine samples during their pregnancy. These samples were tested for fluoride levels.

Twice during pregnancy they were asked to recall tap-water and other water-based beverages they drank through the day which might contain fluoride, such as tea or coffee.

When the children were aged between three and four they were tested for IQ using a performance and verbal IQ test.

What were the results?

For children whose mothers lived in an area without fluoride in the water while pregnant, the average IQ was 108.07. For children whose mothers lived somewhere with fluoride, the average IQ was slightly higher, 108.21.

There is a difference between boys and girls. Boys’ average IQ was 1.53 points lower in areas where fluoride was in the water supply, while girls’ was 1.61 points higher.

Changes this small are considered statistically insignificant and these numbers weren't adjusted for variables such as education and socioeconomic status. 

A statistically significant difference was shown when IQs were compared based on the urine sample results of mothers who had a higher consumption of fluoride. This was a smaller portion of approximately 600 mothers involved in the study. These results for the higher levels of fluoride exposure were adjusted for education and socioeconomic status. 

A one milligram increase in fluoride intake by mothers correlated with an IQ drop of 3.66 points in all children. 

For boys, this was 4.49 points lower.

Boys versus girls

University of Leeds professor of environmental toxicology Alastair Hay has said he has a range of concerns about the paper, including the difference between boys and girls.

“A curious finding is that the link between maternal urine fluoride and IQ decrements is only seen in boys and not girls. And the IQ decrement is not present for verbal IQ in boys. Whilst the authors are just reporting what they found I find these sex differences difficult to explain. With a neurotoxicant you might expect both sexes to be affected.”

It’s not clear whether the decision to split the results into boys and girls was intended prior to data being gathered, or was done later.

Billboards around New Zealand are saying fluoride lowers kids’ IQ. Photo: Farah Hancock

Nottingham Trent University professor Thom Baguley calls the claim made in the paper false. 

“This is an example of subgroup analysis – which is frowned upon in these kinds of studies because it is nearly always possible to identify some subgroup which shows an effect if the data are noisy. Here the data are very noisy.”

University of London professor Rick Cooper points out the children's IQ scores include what are referred to as outliers. 

“The male sample includes a couple of extremely low IQ scores (below 70 would be special needs, but they have two boys with IQ in the 50s). The results would be more convincing if it were shown that they did not depend on these children.”

The study itself does not speculate on why the IQ scores for boys are lower.

Are the measures of fluoride accurate?

Another concern raised regarding the study is how accurate the calculations of fluoride are The study itself states the methods used to estimate the fluoride intake of the mothers have not been validated.

Professor Hay sees the lack of validation in the study as a crucial flaw. Fluoride doesn’t stay in the body for a very long time so urine samples could be hit or miss.

“For a substance with a short half-life, such as fluoride, urine concentrations vary hugely and are really only representative of the last drink.”

Open University professor of applied statistics Kevin McConway points out urine samples could be unrepresentative of normal intake if a mother had drunk a bottle of fluoride-free water prior to the test. Swallowing toothpaste before the test may have also skewed results. Basing results on just three tests over an entire pregnancy could give inaccurate results.

McConway also questions the validity of the self-reported consumption of drinks twice during pregnancy. This relies on memory, and: “... couldn’t take into account the actual brand of tea (because the fluoride content varies), and didn’t take into account any fluoride from food (which depends on exactly what was eaten) or from swallowing some toothpaste after teeth-cleaning.”

The study also didn’t look at any fluoride consumption between birth and the IQ tests.

Should you avoid tap water while pregnant?

The study finds an association between fluoride and IQ, but not a causation. Association in research generally means something is worth delving into further. Unfortunately until that happens there's not a definitive answer.

The editor of JAMA Pediatrics Dimitri Christakis, the journal which published the paper, suggests: "I would advise them [pregnant people] to drink bottled water or filtered water because it is not a particularly odious thing to do and actually does reduce the risk.”

In an unusual move he also published a letter in the journal explaining why it had decided to publish the study, saying the decision was not easy.

"... scientific inquiry is an iterative process. It is rare that a single study provides definitive evidence. This study is neither the first, nor will it be the last, to test the association between prenatal fluoride exposure and cognitive development."

On the other hand, University of Leeds professor Alastair Hay said while the study is interesting, he didn’t see it as a reason to alter food or fluid intake until the limitations of the study were addressed. 

Most scientists think more "appropriately-designed studies" need to be done to see if the results are replicated.

Past studies have been done on the topic with varying results. A Mexican study looking at around 200 mother-child pairs found lower IQ was associated with higher fluoride levels. A Dunedin study found no association between fluoride exposure and IQ.  

A Ministry of Health spokesperson said it had no comment to make on the Canadian study.

“The position of the Ministry of Health and the international health authorities which have endorsed community water fluoridation is not based on any single study.”

It reviews scientific evidence every 10 years “or earlier if a large, well-designed study is published that appears to shift the balance between health benefit and risk”.

“ … the Ministry does not consider that any information on CWF [community water fluoridation] published in recent years has shifted that balance between health benefit and risk.”

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