Review of the Draft NTP Monograph Systematic Review of Fluoride Exposure and Neurodevelopmental and Cognitive Health Effects (2020) / Chapter Skim
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4 Human Evidence
4 Human Evidence
Pages 33-45
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From page 33... ...
This chapter provides the committee's assessment of NTP's evaluation of the human evidence in the monograph. LITERATURE SEARCH In the monograph, NTP clearly displayed the results of the literature search and screening process in a PRISMA flow diagram, a widely accepted framework for reporting a screening process and the ultimate number of included studies.
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From page 34... ...
The committee emphasizes that its comments regarding FAN are aimed only at evaluating bias; they are not intended to discourage stakeholder input into the systematic-review process, and the committee acknowledges and encourages the important contributions of FAN and other stakeholder organizations in this process. STUDY INDEPENDENCE The unit of analysis in a systematic review is a study, not a report or a publication.
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From page 35... ...
, NTP states that "the main confounder missing for evaluating dementia is smoking status." However, the relative risks are low in many studies of smoking and dementia, and this suggests that smoking is unlikely to
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From page 36... ...
Specifically, the potential confounders that are important in studies where high exposures are due primarily to naturally occurring fluoride in drinking water might differ from those in studies that involve intentionally fluoridated water. For example, arsenic and fluoride may co-occur in some areas that have naturally occurring fluoride, but the co-occurrence might be less common in areas where fluoride exposures come only from intentionally fluoridated water.
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From page 37... ...
Many neurobehavioral or cognitive assessments require direct interaction with children and interpretation of their responses to test items, so preconceived assumptions about the effects of a specific exposure can result in a biased interpretation in which children assumed to be members of a high-exposure group are classified as more deficient in the outcome. Many of the cross-sectional and case–control studies reviewed by NTP include children from different areas of residence that have different magnitudes of exposure.
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From page 38... ...
Three of the 18 studies that NTP classified as having low risk of bias used traditional English-based standardized intelligence tests and were accurately classified as having low risk of bias on the basis of the outcome criterion. In some cases, NTP classified studies as having low risk of bias when the measure of the neurodevelopmental and cognitive outcome was seriously flawed.
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From page 39... ...
, and incorrect comparisons of observed prematurity rates with national expected rates. ANALYZING THE DATA NTP states that all 13 studies of childhood IQ that NTP rated as having a low risk of bias identified at least some evidence of an association of fluoride exposure with neurodevelopmental and cognitive effects (NTP 2019, p.
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From page 40... ...
Exposure misclassification because of migration in and out of high-fluoride areas could be a concern in some cross-sectional studies but would likely (albeit perhaps not in all cases) bias results toward the null, not toward the positive associations identified
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From page 41... ...
Although the desire to provide a simple summary of a complex array of evidence is understandable, such claims imply that the studies provide an array of clearly comparable results and that all suggest an adverse effect of fluoride on neurodevelopment or cognition. In fact, many of the studies provide results that are based on multiple indicators of fluoride exposure, assess multiple measures of cognition and neurodevelopment at different ages, use multiple statistical approaches to characterize the relationship between fluoride exposure and health outcomes, and address markedly different magnitudes of fluoride exposure.
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From page 42... ...
Informative evaluations can then be made by comparing study results within the categories. For example, if the methodologically strongest studies tend to show clearer associations than the methodologically weaker ones, the evidence could be interpreted as providing greater support for a possible adverse effect than if the reverse were found.
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From page 43... ...
Finally, the committee agrees with NTP's decision to base its conclusions primarily on studies that have a lower risk of bias given the previous discussion regarding NTP's risk-of-bias evaluations. However, its focus on the lower riskof-bias studies of childhood neurodevelopment outcomes and what seem to be the highly consistent findings across all these studies might give the impression that NTP has artificially increased the confidence in its conclusion regarding this outcome.
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From page 44... ...
(2012) considered only a subset of the studies included in the systematic review, so NTP's claim of minimal publication bias would be strengthened by adding recent papers to the meta-analysis and constructing a new funnel plot.
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From page 45... ...
2019. Draft NTP Monograph on the Systematic Review of Fluoride Exposure and Neurodevelopmental and Cognitive Health Effects.
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