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Does fluoride affect IQ?

Travis Dixon

Does fluoride affect IQ? The research surprised me.

You might have seen the debate about fluoride and IQ in the news recently. I didn’t know much about the subject, so I decided to spend a few hours deep diving into the research.

Fluoride has been used since the 1930s to help protect teeth. It’s a mineral that strengthens tooth enamel reducing the chance of cavities. In many places, it’s added to community drinking water. However, there has been a long debate about the possible negative effects of fluoride. Relevant to psychology is the potential link between fluoride and intelligence (IQ).

I have personal experience with fluoride. I’m the youngest of four brothers. My mum gave my two oldest brothers fluoride tablets to strengthen their teeth. However, by the time I was born in 1983 the topic, to quote mum, “started to be controversial…” and she “…didn’t want to take any risks” so I was never given fluoride. From her recollection, there are no differences in the number of cavities in our family (I was too scared to ask about IQ differences).

My mum was probably influenced by poster’s like this 1970s NZ ad promoting fluoride tablets.

However, anecdotal data from personal experience is only one way of knowing in psychology. Before I can form an opinion on the topic, I need to read the research from peer-reviewed studies. Google Scholar is great for this.

Searching Google Scholar for “does fluoridation of water affect IQ?” reveals a number of studies. To avoid bias, I read the first three as a sample. The first study listed was conducted in 2003 by Xiang et al. and they concluded that “in endemic fluorosis areas, drinking water fluoride levels greater than 1.0 mg/L may adversely affect the development of children’s intelligence.” (Xiang et al. 2003) Similarly, the next study Google Scholar listed was by Lu et al. in 2000 who found “…exposure of children to high levels of fluoride may … carry the risk of impaired development of intelligence.” (Lu et al. 2000). The third study also concluded that “Fluoride in the drinking water was significantly (negatively) related with the IQ of children.” (Kundu et al. 2015). At first glance, this seems strong evidence for the effects of fluoride on IQ in kids.

However, individual studies can provide us with some insights. But they’re not enough to base strong conclusions on. If I read further, I’m sure there are other studies showing opposite or nil effects. Indeed, the results from the fifth study on my Google scholar search “…suggest that exposure to fluoride reduces the prevalence of dental caries, but no association was found to the intelligence of children.” (Soto-Barreras et al. 2015).

More recent studies suggest the findings are mixed or inconclusive. This is when meta-analyses are helpful. A meta-analysis is when psychologists take lots of studies and pool the data to look for overall trends. By adding “meta-analysis” to my earlier search question, the first listed was Tang et al.’s 2008 study, which found that “…those who live in a fluorosis area have five times higher odds of developing low IQ than those who live in a nonfluorosis area or a slight fluorosis area.” (Tang et al. 2008). Qualitative analysis also  found “…a consistent and strong association between the exposure to fluoride and low IQ.”

So far the evidence seems to be strong for a link between fluoride and IQ. However, there are a few important questions to ask about the research. Firstly, these are correlational studies and correlation does not mean causation. Just because there is a “strong association” it doesn’t mean that fluoride is causing changes in IQ. Are there other possible factors involved? Here’s one: the fluoride in the drinking water in some of these studies is endemic, meaning it’s occurring naturally. It hasn’t been added. This means there’s less control over the amount of fluoride in the water. It might not be fluoride that’s the issue, but perhaps the amount of fluoride in the water. Similarly, if there’s less control of fluoride, could there be other minerals or chemicals in the water that are causing the decrease in IQ? Maybe there are other uncontrolled variables.

A lot studies showing that fluoride is linked with lower IQ come from places in China and India where the fluoride is naturally occurring and at levels higher than recommended by countries that add fluoride to the water. (wikicommons)

This is why it’s important to control for such variables to isolate the relationship to just IQ and fluoride. This was attempted by researchers from the famous “Dunedin study” in New Zealand. In a 2015 study, they used data on people born in 1971-72 in Dunedin. They measured their IQ before 5 years old, repeatedly between 7-13 years old and again at 38 years old. They compared IQ levels of those who lived in an area of Dunedin who had fluoridated water with those that lived in a non-fluoridated water area. They also compared those that took fluoride tablets and used fluoride toothpaste, and those that didn’t. They also controlled for variables such as “…sex, socioeconomic status, breastfeeding, and birth weight (as well as educational attainment for adult IQ outcomes.)” From their analysis, they found “…No clear differences in IQ because of fluoride exposure…” (Broadbent et al. 2015).

This is good evidence suggesting there is no risk in fluoride in drinking water and it’s more recent (2015) than the earlier studies in China. It also compares added fluoride, as opposed to endemic (naturally occurring) fluoride. One might think the case is settled, but just last year in 2023 another study was published whose opening line claims “Many uncertainties still surround the possible harmful effect of fluoride exposure on cognitive neurodevelopment in children.” In this meta-analysis of 33 studies, most “…suggested an adverse effect of fluoride exposure on children’s IQ, starting at low levels of exposure.” (Veneri et al. 2023) The case is not closed, it seems.

Correlational studies give us a good indication of a potential relationship. They’re more convincing if there’s a logical causal hypothesis that can explain the relationship. So why might fluoride affect IQ? One hypothesis is that fluoride has the capacity “…to accumulate in brain regions responsible for memory and learning.” (Veneri et al. 2023). It would be difficult and ethically dubious to experiment with fluoride and the human brain, which is where animal studies are useful. In 2023, researchers experimented on how fluoride can affect learning, memory and the hippocampus in mice. Their study found that fluoride did impair learning and memory and was associated with a reduction in brain cell density in the hippocampus. However, this was only at levels much higher than are recommended for adding to community water supplies. Again, it appears that the risks are there but only at high levels.

Places like Juneau (in blue) that start or stop fluoridation offer valuable opportunities to study the effects.

So do the rewards outweigh these risks? The argument for fluoride in drinking water is that it saves money by reducing cavities and thus dentist bills. But is it actually effective in reducing cavities? While this is not psychology, it is relevant to the debate. In 1945, Grand Rapids city in Michigan USA became the first American city to fluoridate its water and it saw a reduction in tooth decay. This led to more cities following suit. More recently in 2007, the city of Juneau in Alaska stopped adding fluoride to their drinking water. Follow-up studies showed a “…substantial increase in caries-related procedures and treatment costs (e.g. fixing cavities),” (Source) after they stopped adding fluoride. This was especially the case with low-income families. Many believe fluoridated water to be one of the greatest health initiatives of the 21st century. Similarly, the NZ Ministry of Health, argues that it’s a safe and effective way of reducing tooth decay.

But not everyone agrees. In New Zealand, for example, Fluoridefree.org.nz started in 2003 to campaign against the fluoridation of water. They have 350 medical health professionals in support, including 21 dentists.  However, in 2019 a survey of 218 NZ dentists showed that 84% support fluoridation in the water.

So where are we at? Does fluoride affect IQ? I’m left with the answer to nearly every question in psychology – it’s complicated. It definitely seems like there’s a risk, but there are numerous factors involved that complicate the issue including the level of fluoride being used.

My Conclusion

I’m pleased I now understand both sides of the debate. If I hear someone promoting fluoridation, I’ll understand why. Similarly, if I hear someone advocating for removing fluoride, I’ll also understand where they’re coming from. I like to presume positive intent and both sides are coming from a good place – they want to do what’s best for kids.

If you’re concerned, there seem to be simple solutions. If your water is fluoridated and you’re worried, you can use water filters. If it’s not and you want fluoride, you can use fluoride tablets.

References

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