No proof that a mother's intake of fluoride in pregnancy affects their child's IQ

"Pregnant women who drink fluoride-treated water may have children with lower IQs," the Mail Online reports.

Fluoride is a mineral that is naturally found at varying levels in the water supply of different countries and regions. Fluoride is known to strengthen tooth enamel and prevent tooth decay. For this reason some regions in the UK add fluoride to their water supply, particularly in the Midlands and northern parts of the country. Most toothpaste also contains fluoride, as do some foods.

This study assessed around 500 mothers and their children from 6 cities in Canada. The researchers estimated the mother's fluoride exposure when she was pregnant, and then looked at whether this was linked with their child's IQ when they reached 3 to 4 years old.

They found a 1mg increase in the mother's estimated daily fluoride intake through drinking water was linked with a 3.7 point lower child IQ score. The IQ scoring system uses a range of age-appropriate tests to assess understanding of language and other conceptual abilities. They aim to measure overall intelligence.

A higher IQ score indicates greater intelligence, with a score of 100 representing average intelligence and a score of 130 and above considered very advanced. They found a 1mg/L increase in the amount of fluoride found in a mother's urine was linked with a 4.5 point lower IQ score – for boys only. No link was found for girls.

Importantly this small study cannot prove that maternal fluoride exposure in pregnancy directly affected the child's IQ. The results may be being influenced by many hereditary, lifestyle and environmental factors.

Fluoride is well known to protect from tooth decay and, overall, this research does not provide convincing evidence of harms from fluoridated water. As such, pregnant women should not be concerned that they need to change their water drinking habits or avoid fluoridated dental products.

Where did the story come from?

This study was conducted by researchers from York University in Toronto, the University of British Columbia in Vancouver, and other universities and hospitals in Canada and the US.

This research was funded by the National Institute of Environmental Health Science. The Maternal-Infant Research on Environmental Chemicals Study, from where the data is taken, was supported by the Chemicals Management Plan at Health Canada, the Ontario Ministry of the Environment, and the Canadian Institutes for Health Research.

One of the authors declares a potential conflict of interest that they are serving as an expert witness in an upcoming case involving the US Environmental Protection Agency and water fluoridation.

The study was published in the peer-reviewed medical journal JAMA Pediatrics, and is freely available to read online.

The Mail Online's coverage takes the findings at face value without noting the limitations. Though the news website did provide commentary from an independent expert who was sceptical of the study's findings.

Dr Alastair Hay, a professor emeritus of environmental toxicology, was quoted as saying: "'A curious finding is that the link between maternal urine fluoride and IQ decrements is only seen in boys and not girls… I find these sex differences difficult to explain. With a neurotoxicant you might expect both sexes to be affected."

What kind of research was this?

This was a cohort study, which assessed data collected as part of the Maternal-Infant Research on Environmental Chemicals (MIREC) program. The MIREC study was set up on the grounds that many environmental chemicals can be found at very low levels (called "trace" levels) in different parts of the body, such as hair and urine. These chemicals' effect on health is often unclear, so this study aimed to assess this, focusing on the potentially most vulnerable groups – pregnant women and their babies.

In the current study the researchers have looked at the levels of fluoride in the mother's urine, and their self-reported intake of fluoridated water, and assessed whether this showed any relationship with their child's IQ.

The main limitation is that this study is observational. This means that women who drank more fluoridated water may have differed in other ways from those who drank less, and these differences may account for any differences in seen in their children. So you can never be sure that the fluoride levels are solely and directly responsible for any impact on the child's IQ. Many other hereditary, lifestyle and environmental factors could be involved.

What did the research involve?

The MIREC study recruited pregnant women from 10 Canadian cities in 2001. A subset of 610 children from 6 of the cities (Vancouver, Montreal, Toronto, Kingston, Hamilton and Halifax) had their development assessed when they were aged 3 to 4 years. These children were included in this study. About 40% of these children lived in areas where water was not fluoridated and 30% in areas where water was fluoridated. Fluoride exposure was unknown for 30%.

Researchers assessed the mother's fluoride exposure in 2 ways. First they assessed the level of fluoride in the mother's urine at 12, 19 and 33 weeks of pregnancy. Secondly mothers completed questionnaires on their consumption of tap water as well as tea and coffee (usually made with tap water). The mother's postal codes were matched with records from their local water treatment plant. Fluoride levels were measured daily at plants where fluoride was directly added to the water, and weekly or monthly if it was not added. From this information the researchers estimated mothers' daily fluoride intake. For these analyses they excluded mother-child pairs where the mother reported not drinking tap water or living outside of the water treatment plant areas.

Child's IQ was assessed at age 3 to 4 years using an accepted test (the validated Wechsler Preschool and Primary Scale of Intelligence). Researchers analysed whether there was a link between the children's score and the 2 estimates of maternal fluoride exposure.

Analyses were adjusted for confounding factors that might affect results, including:

• child gender
• ethnicity
• mother's age at pregnancy
• mother's educational level
• number of other children in the child's family
• aspects of the home environment (including secondhand smoke)
• how the mothers' bodies were estimated to metabolise fluoride (mainly through looking at frequency of urination)

Researchers had complete data for 512 mother-child pairs for the analysis looking at maternal urinary fluoride, and for 400 pairs for the analysis estimating maternal fluoride intake.

What were the basic results?

On average mothers' urine contained 0.41mg of fluoride per litre (mg/L) during pregnancy. It was higher among the 30% of women living in areas with fluoridated water (0.69 mg/L) compared with those in areas with non-fluoridated water (0.40 mg/L).

After adjusting for confounders, researchers found a link between levels of fluoride in a mother's urine and the IQ of boys at age 3 to 4 years. An increase of 1mg/L in urine fluoride concentration was linked with a 4.5 point lower IQ score.

The sons of mothers with the highest levels of urinary fluoride (the top 10% of levels) in pregnancy had an IQ an average of 3.14 points lower than sons of mothers with the lowest amount of urinary fluoride (the lowest 10%). There was no link between mothers' urinary fluoride level in pregnancy and daughters' IQ at 3 to 4 years old.

In the analyses that estimated mothers' fluoride intake based on tap water consumption, the estimated average daily fluoride intake was 0.39mg per day. Again intake was higher for mothers who lived in areas with fluoridated water (0.43mg/day) compared with those in non-fluoridated areas (0.30mg/day).

They found that that a 1mg per day increase in mothers' estimated fluoride intake was linked with a 3.66 lower IQ score for both boys and girls.

How did the researchers interpret the results?

The researchers conclude: "In this study, maternal exposure to higher levels of fluoride during pregnancy was associated with lower IQ scores in children aged 3 to 4 years. These findings indicate the possible need to reduce fluoride intake during pregnancy."

Conclusion

The findings from this study have the potential to alarm expectant parents, particularly the researchers' suggestion that women need to reduce their fluoride intake in pregnancy. This would be an impractical suggestion for most people, who aren't taking fluoride directly, simply drinking tap water and brushing their teeth.

The findings need to be interpreted carefully. First, this is an observational study that cannot prove that the mother's fluoride exposure in pregnancy is directly responsible for the child's later IQ. Many hereditary, environmental and lifestyle factors could influence the child's IQ. Although the researchers have tried to adjust for potential confounders, it's very difficult to account for all the things that may be having an influence.

The estimates of maternal fluoride exposure, both through urine fluoride concentration and daily water intake, may include inaccuracies.

We also do not know anything about the children's intake of fluoride. Children living in the same places where their mothers did while pregnant will have been similarly exposed to fluoride through water, as well as from other sources such as toothpaste as they grow.

The researchers found a few points decrease in IQ score with each 1mg increase in fluoride exposure (per litre urine concentration or per daily intake) – but very few women in this sample actually had fluoride exposure as high as this. So this small sub-group may be unduly influencing the results.

There is also the question of why the reported effect on IQ was only seen in boys in one of the analyses. There seems no obvious reason why the impact on boys and girls would be different, and this result should be seen as very tentative.

Considerable past research has been conducted into the safety of fluoride, including those conducted by the UK government and other international organisations. Overall, these studies all found that fluoride was not associated with significant health risk, while clearly reducing tooth decay.