Monday, September 16, 2013
Will your drinking water poison you?
The addition of fluoride to drinking water has remained controversial so we are greatly indebted to the massive and very thorough literature review below -- which summarizes the available evidence on the question. And what it finds is that there is no cause for alarm.
Scientific studies very rarely find exactly zero differences between two groups. Zero effect is however recognized if the difference is very small. SOME differences will arise due to random variations alone. And the results below show a tiny difference and hence signify zero real difference between the groups, meaning that no concern about fluoride is warranted.
An important source of random variation in the study is that IQ testing is just not fine-grained enough to recognize true differences of less than one point. And it was a difference of less than half of one point that was found in the work below. Large differences in IQ score are highly diagnostic of many things but tiny differences are simply unreliable as predictors. The authors are clearly not familiar with the psychometrics of IQ research. IQ tests are not a magic black box. They have to be used and interpreted with care.
I note that statistical significance was achieved for the results reported. Statistical significance is only a first condition for work to be taken seriously, however. It shows that the results are not due to just one source of random variation: small sample size. Since the sample below was large, that finding is essentially irrelevant
The authors below seem to think that they have found something alarming. After all the work they did, I suppose they would. They have lost perspective. These are resoundingly negative results
The journal abstract:
Developmental Fluoride Neurotoxicity: A Systematic Review and Meta-Analysis
Anna L. Choi
Background: Although fluoride may cause neurotoxicity in animal models and acute fluoride poisoning causes neurotoxicity in adults, very little is known of its effects on children’s neurodevelopment.
Objective: We performed a systematic review and meta-analysis of published studies to investigate the effects of increased fluoride exposure and delayed neurobehavioral development.
Methods: We searched the MEDLINE, EMBASE, Water Resources Abstracts, and TOXNET databases through 2011 for eligible studies. We also searched the China National Knowledge Infrastructure (CNKI) database, because many studies on fluoride neurotoxicity have been published in Chinese journals only. In total, we identified 27 eligible epidemiological studies with high and reference exposures, end points of IQ scores, or related cognitive function measures with means and variances for the two exposure groups. Using random-effects models, we estimated the standardized mean difference between exposed and reference groups across all studies. We conducted sensitivity analyses restricted to studies using the same outcome assessment and having drinking-water fluoride as the only exposure. We performed the Cochran test for heterogeneity between studies, Begg’s funnel plot, and Egger test to assess publication bias, and conducted meta-regressions to explore sources of variation in mean differences among the studies.
Results: The standardized weighted mean difference in IQ score between exposed and reference populations was –0.45 (95% confidence interval: –0.56, –0.35) using a random-effects model. Thus, children in high-fluoride areas had significantly lower IQ scores than those who lived in low-fluoride areas. Subgroup and sensitivity analyses also indicated inverse associations, although the substantial heterogeneity did not appear to decrease.
Conclusions: The results support the possibility of an adverse effect of high fluoride exposure on children’s neurodevelopment. Future research should include detailed individual-level information on prenatal exposure, neurobehavioral performance, and covariates for adjustment.
Posted by John J. Ray (M.A.; Ph.D.).