Supplementary Material for: 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 two 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%CI: 1.01, 3.02) times higher accumulated dental caries experience by age five years than in children born to high-income 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 five 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.## 引言 口腔健康的决定因素在早期便已形成，并受社会经济地位（Socioeconomic Status，简称SES）差异的影响。目前尚不明确生命早期的SES是直接影响儿童口腔健康，还是通过游离糖摄入等决定因素间接发挥作用。本研究采用边际结构模型（Marginal Structural Modelling）法，以出生时的家庭收入与2岁时的游离糖摄入为研究变量，探究上述决定因素影响儿童口腔健康的路径。 ## 方法 本研究使用SMILE队列研究收集的数据。SMILE是一项基于人群的澳大利亚母婴配对出生队列研究，自新生儿出生起便通过问卷与临床评估开展前瞻性随访。分娩时收集的区域与个体层面因素为背景混杂因素。将分娩时的家庭收入（低/中/高）与2岁时的游离糖摄入（低/中/高）分别作为主要暴露因素与中介变量，以探究分娩时SES影响口腔健康的路径。本研究采用因果推断方法（Causal Inference）结合边际结构模型（Marginal Structural Modelling），估算了家庭收入对儿童龋病患病情况的控制直接效应，以及游离糖摄入对龋病患病情况的直接效应与中介效应。本研究构建了因果有向无环图（Causal Directed Acyclic Graph）以指导数据分析，并采用稳定逆概率治疗权重（Stabilized Inverse Probability of Treatment Weighting）对基线混杂因素进行均衡，以模拟随机分组效果。 ## 结果 与出生于高收入家庭的儿童相比，分娩时家庭收入较低的儿童在5岁时的累计龋病患病情况要高出1.65倍（95%置信区间：1.01, 3.02）。高游离糖摄入对5岁时儿童的龋病患病率（1.55倍，95%置信区间：1.03, 2.32）与累计患病情况（2.64倍，95%置信区间：1.36, 5.15）均具有显著的直接效应。家庭收入对口腔健康的影响有一部分是通过游离糖摄入介导的。 ## 结论 出生时的社会经济地位差异以及游离糖摄入等近端决定因素，可通过直接与间接途径影响口腔健康。及时且妥善地应对这些差异，有助于缓解口腔健康领域的不公平现象。