A Commentary on

Goodwin M, Emsley R, Kelly M P et al.

Evaluation of water fluoridation scheme in Cumbria: the CATFISH prospective longitudinal cohort study. Public Health Res 2022; https://doi.org/10.3310/SHMX1584

GRADE Rating:


This study was a well-designed, welcome addition to the evidence base but was observational and under GRADE’s scoring system is classed as low quality.


The CATFISH study is an important contemporary investigation into CWF and follows the 2015 Cochrane review1 which highlighted the need for high-quality contemporary evidence. The re-introduction of a CWF scheme in Cumbria presented the opportunity to evaluate the impact and cost-effectiveness of CWF on two cohorts of children over a five-year period.

Main findings

CWF was found to have a statistically significant, but modest, effect on caries in children who received it from birth. The modest reduction in dental caries observed in the older cohort was statistically non-significant. In both groups, CWF was shown to be a potentially cost-effective intervention, with 77% probability of cost-effectiveness in the birth cohort and 68.3% probability in the older cohort. Overall, the study highlighted positive impacts on oral health inequalities; however this was statistically non-significant. The study described the reduction of decay experience for those in fluoridated areas as a “modest beneficial effect”.

CATFISH was well designed but encountered unforeseen circumstances that could have impacted on the study’s findings and the reported strengths of association

One water plant was affected by flooding, resulting in sub-optimal fluoride dosing levels for over 12 months. This plant was supplying some of the more deprived communities, affecting the ability to assess the effect of CWF on health inequalities, which was an important variable under investigation. When adjusted for dosing, the authors observed statistically significant differences in both cohorts; however they acknowledge that these were post-hoc analyses which were not accounted for in the power calculations.

The COVID-19 pandemic was disruptive to conducting final clinical assessments. A lack of access to schools led to 67% of participants (of those originally recruited) in the non-CWF group receiving their final examinations, compared with 78% of participants in the CWF group. Such attrition could lead to biases in the final results.

Examiners were calibrated according to British Association for the Study of Community Dentistry (BASCD) standards to record caries at the dentine level. While this gives an indication of treatment need in both groups, this represents an advanced level of disease and the study did not explore the effect of fluoridation on early caries or enamel lesions.

Follow-up was for 5 years which may not have allowed sufficient time to detect the presence or absence of caries at this advanced level. Further, in the birth cohort, the follow-up time did not allow for assessment of the effects of CWF on the permanent dentition.

The estimated effect size used in the power calculations was predicated upon on a higher caries rate than found in the populations in this study. This may have resulted in the study being underpowered to detect differences between fluoridated and non-fluoridated areas.


The study complements existing evidence, showing that water fluoridation can be a cost-effective method for reducing caries prevalence, even with near ubiquitous topical fluoride use.

No significant difference in decay experience between deprivation quintiles was observed, though this may have been affected by issues such as interrupted fluoride dosing and the reduced power to detect differences in caries at the dentinal level. Health policy decisions are not contingent on the findings of one publication alone. CATFISH is a welcome, contemporary addition to the evidence base on the effects of water fluoridation.