Does consuming fluoridated water reduce IQ?

Question

Related:

• Is fluoride in water an industry waste product?
• Is water fluoridation harmful?

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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?

Answer 1

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.

Answer 2

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.