We found a linear association between obesity and dental caries in bivariate analyses; however, this association disappeared after accounting for the effect of deprivation. Furthermore, we showed that the association between obesity and dental caries was not uniform; rather, the strength and direction of this association varies by modifying characteristics such as deprivation, ethnicity and lone parenthood. The findings of our study are relevant to oral health policies aiming to reduce dental caries and obesity through the common risk factor approach.
We did not establish an independent association between prevalence of childhood obesity and dental caries in English local authorities. Previously, an official PHE report suggested a ‘weak to moderate’ association between dental caries and obesity prevalence in children . This report, however, did not examine the independent association between obesity and dental caries after taking into account the effect of confounding variables such as deprivation. Nevertheless, the lack of an association between childhood obesity and dental caries in this study could be explained in a number of ways. Most notably, consumption of free sugar is a risk factor both dental caries and obesity; however, obesity could also occur as a result of excessive consumption of non-cariogenic foods. This may raise questions about use of the common risk factor approach in tackling dental decay, a point which has been highlighted by other authors . Our findings regarding the influence of deprivation, the prevalence of lone parenthood and white ethnicity on the association between obesity and dental caries prevalence suggests that a more nuanced understanding of dietary intake patterns in different population groups is required which, in turn, would inform prevention approaches to both obesity and caries.
Previous studies have rarely assumed the possibility of non-linear association between childhood obesity and dental caries. Using fractional polynomial modelling, we did not find a non-linearity in the association between childhood obesity and dental caries. It should be noted, however, that the findings of our study were obtained from analyses of area-level rather than individual-level data. To our knowledge, one study at least has shown a U-shaped association between dental caries and weight with those being either underweight or obese reporting higher dental caries than normal weight children . It has been proposed that some of the inconsistencies in the literature regarding the association between children dental caries and obesity could be due to dismissing the non-linear shape of the association between two health conditions . Again, having a good understanding of this association will enable a more tailored approach to understanding risk factors and developing obesity and caries prevention approaches for children of different weight status.
While obesity was not significantly associated with dental caries in our fully adjusted models, we showed that deprivation, lone parenthood and ethnicity moderated the association. Of particular note is that obesity was more strongly related to the prevalence of dental caries in the least deprived areas while there was a weak or no association in more deprived areas. The interrelationship between obesity, dental caries and socioeconomic status has been a common observation although the reasons behind this are complex and poorly understood . In concordance with our findings, others have also showed a more pronounced association between obesity and dental caries in more affluent children  and countries [11, 12]. This notion has important policy implications; a population approach that is successful in reducing both obesity and dental caries prevalence may have a greater impact on caries in the less deprived communities compared with the more deprived communities and may inadvertently widen oral health inequalities. The risk that some public health interventions may increase inequalities in health outcomes is well-documented  and should be a significant consideration in designing health policy.
We also showed that ethnicity moderates the association between obesity and dental caries. Unlike areas with a predominantly white population, in areas with a majority non-white population there was an inverse association between obesity and dental caries. There is evidence to show that children of white ethnicity have a higher dietary intake of sugar compared to other ethnic groups in the UK, which may partly explain this finding .
Our study, for the first time, used national data from England to examine the link between obesity and dental caries at local authority level. While there have been many cross-sectional studies to examine the link between obesity and dental caries, ecological studies, which enable exploration of associations on a larger scale, have been scarce. The findings of our study, however, should be interpreted in the light of limitations of an ecological study design and may not be extrapolated at the sub-population or individual level.