Pregnant women exposed to fluoride might be putting their unborn children's intelligence at risk, new research has found.
The findings are so controversial, the editors of the journal that published it have gone to extreme lengths to make sure they're not making a mistake on the scale of that which befell The Lancet in 1998, when it published a fraudulent paper by disgraced former doctor Andrew Wakefield which gave rise to the anti-vaxxer movement.
Researchers in Canada measured the fluoride concentration in the urine of more than 500 mothers, and found the more there was, the lower their sons' IQ scores were at age three. For every 1mg/litre, IQ scores were on average 4.5 points lower. No such link was found for their daughters.
They also recorded how much tap water, coffee and tea 400 mums drank, and found a link between fluoride intake and lower IQs for both their sons and daughters - about 3.7 points per milligram.
Findings hard to swallow
Dimitri Christakis, editor-in-chief of publishing journal JAMA Pediatrics, said it was not an easy decision to publish the study, considering the potential negative impacts it could have should it be flawed in some way.
"My initial inclination was, 'What the hell!?'" he said in audio released alongside the study.
"The traditional teaching when I was going through residency and my early professional career was that fluoride is completely safe, all these people that are trying to take it out of the water are nuts, it's the best thing that's happened for children's dental health and we just need to push back and get it into every water system."
Activists here in New Zealand and abroad have long fought against fluoridation of water supplies, saying the benefits to dental health - if they exist - are outweighed by concerns it could have adverse effects. An advertising campaign in Dunedin last year, run by advocacy group Fluoride Free NZ, linked fluoridated water to lowered children's IQ. Complaints about the posters were initially partially upheld by the Advertising Standards Authority, then ruled not upheld on appeal.
"I thought it was junk science," said Dr Christakis. "One of the things we struggled with as an editorial staff was, we certainly would not want to be publishers of the Wakefield study and be the article that pulled fluoride out of water and led to decreased dentition. But... science is an iterative process. It's not perfect."
The Wakefield study, published in 1998, claimed a link between the MMR vaccine and autism. It was later withdrawn, the journal's editor calling it "utterly false", and Wakefield was struck off the medical register. Research since then has failed to find any link between vaccines and autism whatsoever.
Fluoride Free NZ national coordinator Mary Byrne said the study "must spell the end for water fluoridation".
"A warning should be urgently be sent to the 22 councils that are still fluoridating advising them to implement a moratorium. A child has only one chance at brain development," she told Newshub, saying a 2015 review found no evidence fluoride had any effect on tooth decay rates.
That review found it was effective for children, but little evidence it did much for adults.
Byrne said the new research also confirms the finding of a 2017 UK "hich found an across-the-board six IQ-point drop with increased fluoride exposure during pregnancy". That study was criticised by Hamilton scientist Ken Perrott of pro-fluoride group Making Sense of Fluoride, who wrote that it based its "unsatisfying" findings on "scant" information and couldn't prove causation.
But the new study has its critics. Dr Stuart Ritchie of the Institute of Psychiatry Psychology and Neuroscience at King's College London said the way the study was designed would be likely to result in false positives.
"Overall, I think the findings here are pretty weak and borderline. They might be interesting as part of a larger set of studies on this question, but alone they shouldn't move the needle much at all on the question of the safety of fluoride."
Neuroscience professor Grainne McAlonan, also at King's College London, said variations in water fluoride levels between areas with a fortified supply and those without only vary by about 0.3mg - far less than the researchers say is needed to affect IQ.
"If you look at the figures, it's only the individuals which have the very highest levels of fluoride whose children have any IQ lowering, and it's a pretty small drop in IQ score with a range of error that includes the average of the non-fluoridated children's IQ."
She also said IQ tests contain degrees of error that might render the results insignificant.
Born vs the unborn - are the effects different?
The Fluoride Free NZ poster campaign mentioned earlier showed a child drinking water out of a glass. The study didn't say whether exposure to fluoride after birth might be detrimental, as children's own exposure to the substance wasn't measured - only their mother's.
"It is possible from a public health standpoint that fluoride in the water is beneficial to children after they're born, with no known harms," said Dr Christakis.
"Or it could be that it's also harmful after they're born," added paediatrician Dr Frederick Rivara. "The brain of the infant we know is still developing."
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Both doctors noted while the amount of fluoride in mothers' urine seemed to have an effect on boys' intelligence, no such link was found for girls.
"We do see that the brain development of boys and girls is different," said Dr Christakis.
"As much as we like to think of ourselves as being one species, we know autism is higher in boys than girls, ADHD is higher in boys. In fact, even in the animal models... the effect is seen more in male rats than female rats."
They also noted that as directly experimenting with fluoride on children would be unethical, the study can only find a correlation between the substance and lowered IQ - which doesn't necessarily mean fluoride is the cause.
"Are there other unmeasured confounders here which are coming into play?" asked Dr Rivara.
Fluoride's proven track record
In areas where the water supply is fluoridated, studies have shown rates of tooth decay and spending on dental treatment goes down. The Ministry of Health says while fluoride occurs naturally, it recommends councils fortify the supply up to between 0.7 and one part per million.
"This is the optimal amount that provides protection against tooth decay, and is recommended by the World Health Organization."
The Supreme Court last year ruled councils are allowed to fluoridate their water supply, despite activists saying it breached the Bill of Rights because it amounted to medicating people against their will. The court said it was no different to chlorinating the water or adding iodine to salt, and the benefits outweighed the risks.
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A 2014 report commissioned by then Prime Minister's Chief Science Advisor Sir Peter Gluckman and the Royal Society of New Zealand found fluoridation worked for dental health, and had no adverse effects. It said the levels of fluoride reported in previous studies linking fluoride to children's IQ were "up to 20 times higher than any that are experienced in New Zealand" and failed to take other factors into account, such as nutrition and socio-economic status.
A long-running New Zealand study found no links between fluoride exposure in childhood and IQ at 38 either, the report said.
'I hope I don't regret it'
JAMA Pediatrics published an editorial note alongside the new study, aware that fluoridation is a controversial topic that has attracted a lot of attention from non-scientists. It said the study "is neither the first, nor will it be the last, to test the association between prenatal fluoride exposure and cognitive development. We hope that purveyors and consumers of these findings are mindful of that as the implications of this study are debated in the public arena."
Dr Christakis said it's the only time he's ever written such a note, and hopes it will deflect criticism.
"I'm certain if we didn't take the paper, someone else would... I hope I don't regret it."
Where to from here?
Dr Christakis said his advice to pregnant women would be - if they can afford to - only drink bottled or filtered water.
"It's not a particularly odious thing to do and potentially does reduce the risk."
Dr Rivara said pregnant women should "maybe" think twice about drinking tap water, if their supply is fluoridated, but others shouldn't be concerned.
"I hope that this will result in (US health agency) National Institutes of Health funding some good research on this topic. I think that this is a really concerning study, to me. I don't think that people should stop using fluoridated water overall. Pregnant women, maybe... We need to really look at this."
The Ministry of Health told Newshub it would not be changing its guidelines in light of the new research.
"There is a large body of international scientific evidence regarding the safety, efficacy and cost-effectiveness of community water fluoridation," a spokesperson said.
"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."