Drinking fluoridated water does not lower IQ, according to a new study by researchers from the University of Otago in New Zealand.

Many popular theories cast suspicion on the role of water fluoridation, with some people claiming that fluoridated water is associated with a range of adverse health outcomes.

Fluoride is routinely added to drinking water in the US and other countries as a supplement to safeguard against tooth decay. However, some people object to the compulsory nature of water fluoridation.

Also, some of the concern around water fluoridation stems from conspiracy theories relating to the end of World War II. These include suggestions that the Nazi regime secretly fluoridated water supplies in an attempt to damage the pineal gland of their citizens, which some people think promotes docility in humans.

The proximity of conspiracy theories to the issue has made the debate over health risks of water fluoridation contentious. However, in 2012, researchers from Harvard University in Cambridge, MA, cast doubts on the health benefits of fluoride in water supplies.

They conducted a review of studies looking at the effects of water fluoridation on children and found that children living in high-fluoride areas "had significantly lower IQs than those who lived in low-fluoride areas."

Fluoride, the researchers said, is a chemical "with substantial evidence of developmental neurotoxicity."

However, these findings are challenged by a new study published in the American Journal of Public Health.

The Dunedin Multidisciplinary Study and water fluoridation

Drawing data from a large study of 1,000 people born in Dunedin in New Zealand during 1972-1973 - the Dunedin Multidisciplinary Study - researchers from the University of Otago compared the IQs of study participants who grew up in suburbs with and without fluoridated water. They also took into account to what extent the participants were exposed to fluoride toothpaste or tablets while growing up.

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Dr. Broadbent suggests that studies finding an association between water fluoridation and reduced IQ tend to have used poor research methodology with a high risk of bias.

The IQ scores for 992 participants were examined between the ages of 7-13. Of these people, 942 were tested again at age 38. The scores of tests assessing verbal comprehension, perceptual reasoning, working memory and processing speed were also available to the Otago researchers.

The team controlled the results for factors that are known to influence IQ variation in childhood, such as the socioeconomic status of the parents, birth weight and breastfeeding, as well as achievement in secondary and tertiary education, which are thought to influence adult IQ.

Lead author Dr. Jonathan Broadbent describes the team's findings:

"Our analysis showed no significant differences in IQ by fluoride exposure, even before controlling for the other factors that might influence scores. In line with other studies, we found breastfeeding was associated with higher child IQ, and this was regardless of whether children grew up in fluoridated or non-fluoridated areas."

Dr. Broadbent suggests that studies finding an association between water fluoridation and reduced IQ tend to have used poor research methodology with a high risk of bias. Speaking to Medical News Today, he said of the Harvard study: "The authors stated that each of the articles reviewed had deficiencies, in some cases rather serious. It is a meta-analysis based on poor quality research."

He adds that the Dunedin Multidisciplinary Study, by comparison, is world-renowned for the quality of its data and rigor of its analysis.

In conclusion, Dr. Broadbent says:

"Our findings will hopefully help to put another nail in the coffin of the complete canard that fluoridating water is somehow harmful to children's development. In reality, the total opposite is true, as it helps reduce the tooth decay blighting the childhood of far too many New Zealanders."