However, the study has been slammed by some groups who say it is scientifically flawed and reveals blatant examiner bias
The Otago researchers had been testing the contentious claim that exposure to levels of fluoride used in community water fluoridation is toxic to the developing brain and can cause IQ deficits. Their findings were published this week in the American Journal of Public Health.
No significant differences
The research followed nearly all aspects of the health and development of around 1,000 people born in Dunedin in 1972-1973.
Lead author Dr Jonathan Broadbent said the team focused on study members’ fluoride exposure during the first five years of their lives, as this is a critical period in brain development, after which IQ is known to be relatively stable.
They examined the group’s average IQ scores between the ages of seven and 13 years and at age 38, as well as subtest scores for verbal comprehension, perceptual reasoning, working memory and processing speed.
The team controlled for childhood factors associated with IQ variation, such as socio-economic status of parents, birth weight and breastfeeding, and secondary and tertiary educational achievement, which is associated with adult IQ.
“Our analysis showed no significant differences in IQ by fluoride exposure, even before controlling for the other factors that might influence scores,” said Broadbent.
“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.”
Called to question
But the US-based Fluoride Action Network pressure group has called into question the research methods used by Broadbent’s team, accusing the lead author a pro-fluoridation activist with an axe to grind and claiming the study was inconclusive at best.
"Even if this study was high quality science, which it is not, it could not cancel out over 100 animal and 45+ human studies showing fluoride can cause brain deficits. Broadbent's research has serious weaknesses,” said Dr Paul Connett, executive director of the Fluoride Action Network.
Among other criticisms, FAN accused Broadbent of falsely criticising 27 previous studies linking fluoride to children's lower IQ, and implying they didn't adjust for any potentially confounding variables like lead, iodine, arsenic, nutrition, parent's IQ, urban/rural and fluoride from other sources.
It added that of the four factors Broadbent did adjust for, most were only crudely controlled, while the study’s small sample size of non-water-fluoridated subjects (99 compared to 891 water-fluoridated subjects) meant it has low ability to detect an effect.
"Broadbent is one of New Zealand's leading political promoters of fluoridation. He is a dentist not a developmental neurotoxicologist," says Connett.
"This single weak study is hardly sufficient to outweigh the substantial body of evidence showing fluoride's potential to harm the developing brain at relatively low exposure levels."
For his part, Broadbent has countered such accusation by saying that similar studies by opponents of fluoridation used poor research methodology and have a high risk of bias.
“In comparison, the Dunedin Multidisciplinary Study is world-renowned for the quality of its data and rigour of its analysis,” he said.
“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.”