You will be seeing headlines claiming a link between community water fluoridation (CWF) and diabetes. Or even that fluoridation can predict an increase in the prevalence of diabetes. But they are misleading
These articles report results from a recently published study indicating that in the majority of situations CWF is linked to a decrease in diabetes prevalence. But many of these articles, and especially those from anti-fluoride activists are making opposite claims.
Why the confusion? Well, the study used modelling to relate a number of factors to the prevalence of diabetes. According to the model’s prediction CWF using fluorosilicic acid and sodium fluorosilicate is related to a decrease in diabetes prevalence. However, the saving clause for anti-fluoride activists is that the model predicts an increase in diabetes prevalence when the least common fluoridation chemical, sodium fluoride, is used.
A 1992 survey found that only 9% of the US population received water fluoridated with sodium fluoride – compared with 63% for fluorosilicic acid and 28% for sodium fluorosilicate. I got the latest figures from a fluoridation engineer at the US Center for Disease Control. The current figures are 75% for fluorosilicic acid, 13% for sodium fluorosilicate and 7% for sodium fluoride.
In New Zealand only on water treatment plant for a small community uses sodium fluoride.
So this subheading by the Fluoride Action Network (FAN) is completely misleading – “Regression analyses suggest association between increases in consumption of fluoridated water and type 2 diabetes.” The only way anti-fluoride propagandists can make mileage out of this study is by deliberately ignoring the results indicated for over 90% of the population!
Perhaps supporters of CWF should be the ones reporting and promoting this study – arguing that CWF could reduce diabetes prevalence! However, I would not push that idea on the basis of a single report. This study has a number of deficiencies – and recommendations should not be based on individual cherry-picked studies anyway.
This is the paper reporting the study:
Fluegge, K. (2016). Community water fluoridation predicts increase in age-adjusted incidence and prevalence of diabetes in 22 states from 2005 and 2010. Journal of Water and Health.
Here are some of the problems I see with it.
Insufficient consideration of confounders
It is a modelling study looking for correlations between selected parameters. Such studies often suffer from little or no consideration of important confounders. Statistically significant correlations can disappear when such confounders are later included. For example, consider my criticism of the Malin and Till (2015) ADHD study – see ADHD linked to elevation not fluoridation and ADHD link to fluoridation claim undermined again.
Fluegge included obesity prevalence and leisure time physical inactivity as confounders but more could have been considered. One that sticks out like a sore thumb to me is the community size. It could be that the sodium fluoride data he used could be acting as a proxy for community size as these days sodium fluoride is usually only considered for small water treatment plants.
Adjustment of fluoride exposure data
Fluegge compared his model prediction for diabetes prevalence using two different measurements of fluoride exposure – drinking water fluoride concentration (ppm) and an adjusted estimate of fluoride intake (mg/day). His estimation was made from per capita domestic water deliveries per county. I find this questionable as the proportion of water consumed will vary by location where there are different requirements for things like lawn and garden watering, car washing, swimming pools etc.
Whereas the drinking water fluoride concentration showed a negative correlation with diabetes prevalence (the prevalence decreased with increasing fluoride concentration), the adjusted exposure values showed a positive correlation (the prevalence increased with increasing fluoride concentration). He declared the second correlation more “robust” but his reasons seem more related to confirmation bias than any proper analysis.
Fluegge seems completely unaware that sodium fluoride is now only rarely used as a fluoridating chemical. He even suggests a possible policy outcome of his research could be switching from sodium fluoride to fluorosilicic acid!
He refers to Hirzy et al. (2013) claiming it showed cost savings from using sodium fluoride but critiques Hirzy for not including consideration of effects on diabetes prevalence. He seems completely unaware that Hirzy’s paper was discredited and he had to withdraw its claims about cost savings.
This suggests to me that Fluegge is not familiar with fluoridation research. In fact, his very brief publication history indicates his interest is more associated with cherry-picking various health measures to find fault with by using statistics and modelling.
How reliable is the modelling?
I have drawn attention to possible problems with poor selection of confounders and lack of familiarity with the fluoridation literature. But there may also be problems with the modelling methods used.
I do not have the modelling skills or time to delve into his model in any depth but note there has been some controversy about another modelling paper he was involved in.
He co-authored a paper with his brother claiming a link between glyphosate and ADHD. This created some controversy because it was rejected by the journal and then published by mistake. So the journal had to retract the paper. You can read about it at Retraction Watch – A mess: PLOS mistakenly publishes rejected ADHD-herbicide paper, retracts it.
The paper was rejected because it did not satisfy the standards of experimental and statistical analysis required, or describe these in enough detail. Also because the conclusions were not presented in an proper way or supported by the data.
OK, we should not discredit future work because an earlier paper was rejected, even for the given reasons. Authors can learn from their mistakes. But it does ring warning bells. With this history, I would prefer a deeper critique of the methods used and the reliability of his conclusions.
These questions just underline my warning that one should never base policies, or final interpretations, on single papers – especially cherry-picked ones. Conclusions should be based on a more complete reading of the scientific literature.
So, always take headlines with a grain of salt. In this case they will be completely misleading – especially if promoted by anti-fluoride activists.