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Is Artificial Fluoride Absorbed to the Same Extent as Natural Fluoride?
1 Professor of Health Sciences and Deputy Vice Chancellor, University of York, Heslington Hall, Heslington, York YO10 5DD, UK; tas5{at}york.ac.uk To the Editor: Maguire et al.(2004, 2005) investigated the bioavailability and absorption of fluoride from naturally and artificially fluoridated drinking water, looking also at the influence of water hardness. There was a 16% greater increase in plasma fluoride (AUC at 8 hrs) when participants drank artificially fluoridated compared with naturally fluoridated water. The study, however, with 20 participants, had the power (retrospectively calculated), at the 5% level, only to detect a 33% difference in output of fluoride. The mean difference of plasma AUC at 8 hrs was 237 ng F.min/mL (95% CI: –49.7, 524.2), a standardized difference of 0.7. To have an 80% power to detect this as statistically significant at the 5% level would have needed a sample size of around 65 participants (85 participants for 90% power). The power of their study to test for any interaction effect with water hardness was even lower. A small difference in bioavailability would of course be a scientifically important discovery, given that the chemistry is thought to be identical, and thus it is surprising that such a small study was conducted. While being potentially useful as a pilot to test the methods, the published study is grossly statistically underpowered, and potentially important findings using plasma Fp% (Maguire et al., 2004), another measure of relative bioavailability, were omitted. The study should not be used as the basis of confident or policy-useful judgments about the relative bioavailability of fluoride and interactions with the hardness of the water. Better and larger studies should be conducted. REFERENCES
The authors reply3 a.maguire{at}newcastle.ac.uk In our study of the bioavailability of fluoride in drinking water (Maguire et al., 2005), the standard deviation of plasma AUC values at 8 hrs across 100 sets of observations (20 subjects by 5 water samples) was 719 ngF.min.mL–1; the standard deviation of the difference in mean AUC values between fluoridated and non-fluoridated samples was 680 ngF.min.mL–1. The estimated difference in mean plasma AUC between artificially and naturally fluoridated water of 237 ngF.min.mL–1 corresponds to a standardized difference of either 0.33 or 0.35, depending on the measure of variability used. The study had 87% power to detect an effect size of 0.7, assuming a Type 1 error rate of 5%. The alternative measure of plasma fluoride, Fp%, is closely related to the measure of plasma fluoride described in the paper and represents the AUC value for fluoridated or non-fluoridated samples divided by the AUC for reference water, expressed as a percentage. In our paper, the AUC for reference water was included as a covariate in the analysis of AUC values for test waters. With no evidence base to suggest what difference in bioavailability should be considered clinically important, it is difficult to make authoritative statements about the adequacy of the sample size. In consequence, interval estimates were reported throughout the paper for all measures of bioavailability. REFERENCE
Journal of Dental Research, Vol. 87, No. 1,
8 (2008)
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