Dr. Mercola is at it again with a new article: 25 Studies Prove This Reduces Your IQ.

The Fluoride Action Network is also urging communities to pressure their local water departments to place a warning on the water bill, stating that parents should not use fluoridated water to mix baby formula. In November 2006, the American Dental Association has issued this warning to its members—the dentists—but has failed miserably when it comes to actually warning parents...

"… We now have 25 studies that indicate that fluoride is associated with the lowering of IQ. A recent study from Ding et.al… [published] in the Journal of Hazardous Materials… found a lowering of IQ at low levels – not moderate, not high levels, (but) low levels...

They were able to correlate the level of fluoride in the urine, which is a measure of individual exposure to fluoride… there was a very strong linear correlation with lowering of IQ and fluoride concentrations in the urine; very strong.

… What parent in their right mind would put the interest of their children's teeth, even if it worked (to swallow fluoride), above the interest of their brains? It's literally a no-brainer isn't it?

… What Ding et.al showed was that for each one part per million of fluoride in the urine, there was a lowering of IQ by 0.59 IQ points. That doesn't sound much, but when you shift the IQ of the whole population… you will reduce the number of geniuses in your society, and you will increase the number of mentally handicapped. Believe me, in a competitive world, you want all the geniuses that you can get, and you want to minimize the number of children who have to be treated for mental development problems."

The link in the quote leads to another Mercola article which claims to amass 24 studies showing reduced IQ correlated with fluoridated water consumption. He says:

This brings the total to 24 IQ studies that have found exposure to fluoride associated with lowered IQ.

There's a footnote on that quote which leads to this IQ Studies page on the Fluoride Action Network website. There are two links there to "See the 24 published studies reporting an association of reduced IQ with high fluoride exposure." Both of those links go to the same page as the links themselves.

For an example of a study, here is a study by Xiang et al. in which fluoride urinary content was found to correlate with ingested fluoridated water quantities, and then fluoride urinary content was correlated to IQ.

Is there any concern to be had with drinking fluoridated water, especially the claim that it leads to reduced IQ in children? Furthermore, the claim is broadcast by Mercola as more or less universal (not just limited to children); is there credibility to fluoride's effects on IQ at any age?

  • In the Chinese data forming the basis of the Harvard metaanalysis, the fluoride concentration in natural waters may well be a surrogate for region and thus for cultural and economic differences, factors that are known to influence standardised IQ tests. It would be better to analyse data in say the UK or Eire where IQs are measured regularly in many school districts following nationally standardised protocols where there is some understanding of the other factors: this data is probably already available, and is probably unremarkable. – user15506 Sep 15 '13 at 10:47

There is an important point about the concentration of fluoride that Mercola doesn't address. The concentration of fluoride intentionally added to water in the US is according to the EPA between 0.7 - 1.2mg/L:

Optimally fluoridated community water systems add fluoride to a level between 0.7 – 1.2mg/L. Fluoride is sometimes naturally present in water at much higher levels, so the EPA established a Maximum Contaminant Level for fluoride of 4.0 mg/L (parts per million).

In the study you picked as example the fluoride levels in the high fluoride group were 2.47±0.79 mg/L and the control levels were 0.36±0.15 mg/L. The amount of fluoride that is added in the US is closer to the control group than the high fluoride group of this study. So you can't just transfer the conclusions of the study to the drinking water in the US, the amount of fluoride is different.

What I found surprising is that the study didn't spend much effort on eliminating other confounding factors, such as other toxic substances in the drinking water.

The National Research Council examined those studies in their report "Fluoride in Drinking Water: A Scientific Review of EPA's Standards"

The significance of these Chinese studies is uncertain. Most of the papers were brief reports and omitted important procedural details. For example, some studies used a modification of the Raven Progressive Matrix test but did not specify what the modifications were or describe how the test was administered. Most of the studies did not indicate whether the IQ tests were administered in a blinded manner. Some of the effects noted in the studies could have been due to stress induced by the testing conditions. Without detailed information about the testing conditions and the tests themselves, the committee was unable to assess the strength of the studies. Despite this, the consistency of the collective results warrants additional research on the effects of fluoride on intelligence in populations that share similar languages, backgrounds, socioeconomic levels, and other commonalities.

So the existing studies don't seem to be good enough to come to a conclusion yet. And the level of fluoride they examined is significantly higher than the level that is added to drinking water in the US. Those studies don't provide sufficient evidence to conclude that the fluoridated drinking water would have an effect on the IQ of children.

  • 3
    Do you know if all of Mercola's listed studies fall under the umbrella of "these Chinese studies"? Also, do we know what the typical fluoridation levels actually are, not just the EPA recommendation? I think this would improve the answer. I just picked the study I did as an example; I have no idea if it's the strongest of the listed 24 or what. Lastly, it might be helpful to add whether there is credible evidence that these higher levels do reduce IQ. Even if it's not a concern in the US, the claim would hold under a certain subset of fluoridation levels. Thoughts? – Hendy Dec 4 '11 at 15:03
  • Fabian, the burden of proof falls upon proponents and municipalities, not opponents. What the NRC committee found is that further research is needed. Not to discontinue fluoride but to continue using it. The evidence is enough to question the practice of fluoridation and to suspend it until we know for certain what level of fluoride in water is safe, if any at all. The committee urged the EPA to significantly lower acceptable levels of fluoride added to water and called for further research on the subject. Three members of that committee, Robert Isaacson, Kathleen Thiessen and Hardy Limeback, – user8322 Aug 24 '12 at 22:34
  • @whirleegirl: In your (now deleted) answer, you claim 180m of the 200m people drinking fluoridated water live in the US. This sounded suspicious, and Wikipedia agrees: It is 204m of 405m worldwide - 50% not 90%. – Oddthinking Aug 25 '12 at 3:13
  • @whirleegirl: You argue the burden of proof is on the proponents. I think that is fair. They have shouldered that burden - there are 5 chapters in that (independent) report discussing the scientific evidence already collected. Presumably, at some point it is acceptable to say "Okay, we have gathered years of evidence, evaluated the benefits and the risks and, from a public health stance, it is worth it." Many authorities (EPA, NRC, etc.) believe that point has well and truly been passed. – Oddthinking Aug 25 '12 at 3:20
  • @Oddthinking The WHO (pdf), too. They even provide a recommendation for controlling levels at 1.5mg/L. – jozzas Aug 28 '12 at 6:07

A recent carefully conducted study clearly says there is no relationship

Fabian's answer addresses the consensus and the problems with the reported methods (or their lack) of the Chinese studies quoted by the anti-fluoride movement. But a recently reported and carefully conducted study from New Zealand specifically studies the reported link between intake of fluoride and intelligence and concludes there is no link at all.

From the pubmed abstract (my emphasis):

Objectives. This study aimed to clarify the relationship between community water fluoridation (CWF) and IQ. Methods. We conducted a prospective study of a general population sample of those born in Dunedin, New Zealand, between April 1, 1972, and March 30, 1973 (95.4% retention of cohort after 38 years of prospective follow-up).

Results. No significant differences in IQ because of fluoride exposure were noted. These findings held after adjusting for potential confounding variables, including sex, socioeconomic status, breastfeeding, and birth weight (as well as educational attainment for adult IQ outcomes). Conclusions. These findings do not support the assertion that fluoride in the context of CWF programs is neurotoxic. Associations between very high fluoride exposure and low IQ reported in previous studies may have been affected by confounding, particularly by urban or rural status.

The full paper is in the American Journal of Public Health but is paywalled.

The key conclusion is that, when careful consideration of potential confounding variables is done, there is no relationship between fluoride intake and IQ.

It is very easy for shoddily conducted studies to throw up false relationships and this carefully conducted study finds no relationship and suggests what some of the potential confounders in previous studies might have been.

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