2 | METHODS

The LOTUS study (fLuOridaTion for adUltS) was a retrospective cohort study using routinely collected National Health Service (NHS) dental treatment claims (FP17) data, submitted to the NHS Business Services Authority (NHS BSA), between 22nd November 2010 and 21st October 2020. Data were collected in a range of NHS primary dental care settings, including: general dental practices, community dental services, domiciliary settings, prisons, urgent/out-of-hours and specialized referral-only services.

The primary objective was to evaluate the impact of water fluoridation on NHS invasive dental treatments (restorations and extractions). Secondary objectives were to evaluate the impact on oral health (caries experience [DMFT] and missing teeth), social inequalities in oral health, and to determine cost-effectiveness and return-on-investment.

Approval for NHS BSA to select the cohort and de-identify the data was given by the Health Research Authority Confidentiality Advisory Group (20/CAG/0072) and the study received ethical approval from North East, Tyne & Wear South Research Ethics Committee (20/NE/0144). The study protocol was published in 2021.¹²

Treatment claims for individual courses of treatment from different settings were linked together by NHS BSA to create the longitudinal cohort, using the following selection criteria:

• Claims relating to a unique individual (confirmed by 1: 1 NHS number and NHS BSA ID match)
• 12 years and over on 22nd November 2010
• Valid English postcode at first dental visit
• Attended at least twice between 22nd November 2010 and 21st October 2020. This was to provide least two data points on location of residence (to assign fluoridation exposure).

6.4 | Interpretation

This was a pragmatic, observational study with limitations outlinedabove. Therefore, it is important to triangulate our findings with ‘interlocking evidence’.²⁹ Our results support the hypothesis that water fluoridation appears to be producing less dramatic impacts on oral health in contemporary UK populations than in historical studies. This finding is echoed by the recently published NIHR CATFISH prospective cohort study in UK children.¹³ The majority of recent studies including adults that have reported relatively large absolute effects come from contexts which may have higher levels of dental disease and unmet treatment needs, including those studies conducted in Brazil,^{26,30,31,32,33,34,35,36} or in high-need population subgroups such as young offenders or Army recruits.^37,38 Recent studies more applicable to the UK general population (from Australia, the US, Sweden and South Korea) have demonstrated inconsistent effects across age groups,³⁹ and/or absolute differences so small that whether they are meaningful is debateable.^40-42 Before conducting our analysis, as an online public engagement activity, we invited patient, public and professional stakeholders to consider the minimum reduction in invasive dental treatments over 10 years they would consider clinically or practically meaningful. Contributors held a wide range of views, indicating this is a highly subjective judgement. However, based on their feedback, the majority would not have considered a relative reduction of 3% as being meaningful.

6.5 | Implications

Both the present study and the recent CATFISH study of the effectiveness of water fluoridation in children support the conclusion that existing schemes in the UK remain cost-saving for the public sector.¹³ In high-income countries with widespread access to topical fluorides, water fluoridation may now represent a classic ‘prevention paradox’; a preventive measure which can bring benefits for populations and services but ‘offers little to each participating individual’.⁴³ This is in contrast to the historic studies which identified absolute differences in caries severity of 50%–60%.^2,3 The costs of new water fluoridation programmes are extremely variable and depend on local water supply configurations.⁴⁴ Any cost estimates for new water fluoridation programmes, or significant investment in existing schemes, should be viewed in the context of our estimate that receipt of optimal water fluoridation (>/= 0.7 mg F/L) over 10 years reduced costs of dental treatment to the NHS by 5.5% per person.

For context, a 2009 estimate for a new water fluoridation programme for Greater Manchester and Merseyside (similar population size to the optimally fluoridated population of England) would be £55.8 m (+/? 30%) in 2023 prices^.44,45 Such significant investment would need to be carefully evaluated against the projected lifespan of the dosing equipment and civil infrastructure and cost-recovery may not be guaranteed in increasingly low caries generations. Breaks in the supply of optimally fluoridated water and sub-optimal dosing are common, and would result in smaller impacts on NHS treatment costs.¹⁶ More fundamentally, whether the case for water fluoridation can be based solely on the potential for reductions in NHS dental service utilization and NHS savings, rather than health gains that are meaningful to patients and the public, may need to be considered by stakeholders. Participatory consultation methods using contemporary estimates of effect, for example, through citizen juries, may be beneficial to explore these issues further.

There is no doubt that population-level, ‘mass preventive’ interventions for dental caries are still required. Dental caries remains almost universal by adulthood, even in populations that have had access to fluoride toothpastes and fluoridated water from birth.⁴⁶ However, in high income countries, we may be reaching the limit of what can be achieved through fluorides alone. A dose–response relationship between free-sugars and dental caries is evident at all levels of intake above zero and fluorides merely attenuate this relationship.⁶ Average consumption of free sugars in the UK is more than double the recommended level for adolescents, and is almost double for adults.⁴⁷ The discovery of water fluoridation made an unparalleled contribution to oral health in the 20st century. In the 21st century, greater impact may be achieved by advocating for upstream, policy level action to address the commercial determinants of health and create supportive food environments.⁴⁸