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BMJ. 2007 October 27; 335(7625): 840–841.
PMCID: PMC2043442
Fluoridation

Interpreting the Newcastle fluoride bioavailability study

Stephen T Holgate, MRC clinical professor of immunopharmacology1 and Trevor A Sheldon, professor and pro-vice chancellor2

We are surprised that the chief dental officer and chief medical officer for England in their full response to Cheng et al consider that the Newcastle study on bioavailability “contributed to a better understanding of the health effects of water fluoridation.”1 2 The researchers themselves urged caution when interpreting the results.3 It is disappointing that such senior public health officials make the error of assuming that “no statistically significant differences in bioavailability between artificially and naturally fluoridated water” has any meaning when the study was too small to find scientifically important differences.

Interestingly, despite the small size, the Newcastle study did report a significant difference in the relative bioavailability of fluoride in drinking water (plasma Fp%) at three hours (27%) and eight hours (36%) follow-up (mean difference in Fp% (0-8)=35, 95% confidence interval 5.9 to 64.5).3 However, the authors removed one of the 20 data points, which they determined was an outlier because one subject had much larger values than others. This manoeuvre reduced the statistical significance below the critical value. The trend of increased bioavailability in artificially fluoridated water, however, remained in all plasma comparisons (tables 5-7).3 Discarding an outlier (removing 5% of the data) to eliminate an “inconvenient” significant result is not best practice and raises doubts about the validity of the inferences.

Given the weaknesses in the study design and analysis it is surprising that these senior health officials should state that “as a result, we may continue to have confidence in the safety of fluoridation.”

Notes

Competing interests: None declared.

References

1. Cheng KK, Chalmers I, Sheldon TA. Adding fluoride to water supplies. BMJ 2007;335:699-702. (6 October.) [PMC free article] [PubMed]
2. Electronic response. Cockcroft B, Donaldson L. Adding fluoride to water supplies. 2007. www.bmj.com/cgi/eletters/335/7622/699#177837.
3. Maguire A, Moynihan PJ, Zohouri V. Bioavailability of fluoride in drinking water—a human experimental study. Department of Health. Newcastle: University of Newcastle, 2004. www.ncl.ac.uk/dental/research/diet/fluoride_bioavailability_report.htm. [PubMed]

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