Effect of Early Life Income and Sugars Intake on Child Oral Health: Marginal Structural Modelling Using a Birth Cohort Study

Introduction: Determinants of oral health are formed early and influenced by variations in socioeconomic status (SES). It is unclear whether early life SES influences child oral health directly or indirectly through determinants such as intake of free sugars. This study applied the marginal structural modelling approach to household income at birth and free sugar intake to investigate pathways those determinants influence child oral health. Methods: We used data collected in SMILE, a population-based birth cohort study of Australian mother/newborn dyads, who have been followed-up prospectively since birth with questionnaires and clinical assessment. Area- and individual-level factors collected at childbirth were background confounders. Household income at childbirth (low/medium/high) and free sugar intake at age 2 years (low/medium/high) were used as primary exposure and mediator to investigate pathways through which SES at childbirth influences oral health. By applying the causal inference approach and using marginal structural modelling, we estimated the controlled direct effect of household income and the direct effect and mediating effect of intake of free sugars on dental caries experience. We developed a causal directed acyclic graph to guide the analysis. The baseline confounders were balanced using a stabilised inverse probabilities of treatment weight, mimicking randomisation. Results: Low household income at childbirth was associated with 1.65 (95% confidence intervals [CI]: 1.01, 3.02) times higher accumulated dental caries experience by age 5 years than in children born to highincome households. High intake of free sugars had strong direct effects on both the prevalence (1.55 [95%CI: 1.03, 2.32]) and cumulative experience (2.64 [95% CI: 1.36, 5.15]) of dental caries by age 5 years. Proportions of effects of income were mediated by intake of free sugars. Conclusion: Socioeconomic variations at birth and immediate determinants such as intake of sugars, directly and indirectly, influence oral health. Timely and appropriate addressing of those variations may limit inequity in oral health.