The major criticisms of this study and our responses are listed below.
1. This study doesn’t prove cause and effect
Of course not! No epidemiological study proves cause and effect. Observational, epidemiological studies are optimal for testing many important questions in public health and they are often used to infer causality.
2. Criticisms about the study’s methodology
- The effect of residual confounding: As with all observational studies, the results are subject to confounding by unmeasured factors. “Many potential confounders were considered and unless unmeasured factors are identified and shown to be associated with both fluoride exposure and IQ, these criticisms are speculative.”
- Reliance on P values: “Our conclusions did not ‘simply rely on P values.’ We established our hypotheses a priori, carefully interpreted effect estimates and confidence intervals, conducted a comprehensive set of sensitivity analyses, acknowledged the limitations of our study, and considered the results of other studies before concluding that pregnant women should reduce their fluoride intake.”
- Concerns about “imprecise results”: Firstly, the fitted regression line is covariate-adjusted, whereas individual data points in the figure are not, and secondly, individual data points close to the regression line may represent numerous overlapping cases. Also, the outliers in the data set have low influence but good leverage because their inclusion improves the estimate’s precision. Nevertheless, the study included secondary analyses without the two very low IQ scores and the fluoride effect remained.
3. Aim of this study
The study did not set out to compare IQ scores in children who lived in fluoridated vs non-fluoridated communities. It used a biomarker of fluoride exposure that accounts for all sources. However, the study did find consistent IQ decrements associated with fluoride intake and water fluoride concentrations.
4. This is just one study.
This is definitely not just one fluoride-neurotoxicity study; it is in fact consistent with other high-quality research. More than 50 studies have shown an association between higher fluoride exposure and lower IQ. Five mother-infant cohort studies have reported that higher prenatal fluoride exposure led to lower IQ scores in children. Please see this article for more details.
5. It was only 3 or 4 IQ points.
This was not a subtle finding. A decrement of 4.5 IQ points among many boys can have a substantial effect on the population distribution of IQ scores. In fact over a population, this decline is enough to double the number of intellectually handicapped children and halve the number of intellectually gifted children. With 14% of Canadian women living in fluoridated regions having urinary fluoride of at least 1mg/L, we would expect even larger IQ decrements for some children. Also, keep in mind that Australia has a higher fluoride concentration than Canada.
6. Concerns about reported IQ/sex differences.
Male individuals are often more susceptible to toxicants, including prenatal fluoride exposure in some animal studies. Moreover, the National Toxicology Program (2016) called for sex-based analyses for fluoride. This study therefore planned to test sex differences based on the evidence that male mitochondria may be more sensitive to fluoride.
Blendoe et al suggest that the IQ differences are explained by sex differences. Sex differences in IQ scores do not explain the exposure-response association that was observed in boys using urinary fluoride concentrations.
7. Cultural biases of IQ tests
The study acknowledged that IQ tests have been criticised as having cultural biases. However, this study did not compare groups that differed by culture and controlled for ethnicity and city.
8. Other requested results not reported
The study’s authors endeavoured to make their analyses as transparent as possible and have posted supplemental material on the Open Science Framework. Access to the data to validate our findings is available to independent researchers who adhere to the MIREC Biobankpolicies outlined here: http://www.mirec-canada.ca/en/research/.
9. Similarities to criticisms of lead studies
It is important to be aware that many similar criticisms of this study such as bias, imprecision, unmeasured confounding and subtle effects were raised about studies of low level lead toxicity.
10. Any studies that showed fluoride diminishes children’s intellectual abilities should be interpreted cautiously.
The study’s lead researchers agreed, however pointed out that we should also take precautions to protect children’s brains from a potential toxicant, especially when there is no benefit of fluoride for pre-erupted teeth.
11. We need more studies
Of course, we always need more studies.
For more details: