The individual and contextual pathways between oral health and income inequality in Brazilian adolescents and adults

Abstract

We evaluate the association between income inequality (Gini index) and oral health and in particular the role of alternative models in explaining this association. We also studied whether or not income at the individual level modifies the Gini effect. We used data from an oral health survey in Brazil in 2002–2003. Our analysis included 23,568 15–19 and 22,839 35–44 year-olds nested in 330 municipalities. Different models were fitted using multilevel analysis. The outcomes analysed were the number of untreated dental caries (count), having at least one missing tooth (dichotomous) and being edentulous (dichotomous). To assess interaction as a departure from additivity we used the Synergy Index. For this, we dichotomized the Gini coefficient (high vs low inequality) by the median value across municipalities and the individual income in the point beyond which it showed roughly no association with oral health. Adjusted rate ratio of mean untreated dental caries, respectively for the 15–19 and 35–44 age groups, was 1.12 and 1.16 for each 10 points increase in Gini scale. Adjusted odds ratio of a 15–19 year-old having at least one missing tooth or a 35–44 year-old being edentulous was, respectively, 1.19 and 1.01. High income inequality had no statistically significant synergistic effect with being poor or living in a poor municipality. Higher levels of income inequality at the municipal level were associated with worse oral health and there was an unexplained residual effect after controlling for potential confoundings and mediators. Municipal level income inequality had a similar, detrimental effect, among individuals with lower or higher income.

Introduction

Income inequality rather than low income, is said to be a major health hazard in industrialised countries (Wilkinson, 1996). This idea was conceived mainly based on three findings: the weak inter-country ecological association between income and health, when analysis is restricted to rich western countries; the strong association between individual income and health within these countries, despite good material conditions of life; and the inter-country ecological association between income inequality and health among rich western countries (Wilkinson, 1992).

Different mechanisms to explain income inequality effects on health have been described (Kawachi and Kennedy, 1999, Lynch et al., 2000). In developed countries, the effects of income inequality are often said to be through relative income. Individuals at lower socioeconomic positions may be at higher risk of diseases, through stress induced ill-behaviour and physiological effects of chronic stress (Wilkinson, 1997). On the other hand, it may be that relative position is nothing but a marker of cumulative exposure of adverse material conditions (Lynch et al., 2000). At contextual level, income inequality could operate through decreasing social capital and under-investment in social infrastructure (Kaplan et al., 1996, Kawachi and Kennedy, 1999, Lynch et al., 2000). Much of the research conducted so far have not empirically and simultaneously tried to assess the importance of those mechanisms. Therefore, it has been difficult to assess the relative importance of each pathway.

Absolute income appears to be a powerful risk factor for many diseases. However, relative and absolute income may act through different mechanisms and they may interact. In places where the income inequality is greater, the effect of absolute poverty may be stronger. It has been claimed that this interaction between income inequality and poverty has not been extensively assessed (Subramanian & Kawachi, 2006) and results were still not conclusive (Henriksson et al., 2007, Subramanian and Kawachi, 2006). Three studies analysed the interaction of income inequality with mean income at ecological level (neighbourhood areas in Sweden, provinces in Italy, and among 107 countries) and found contrasting results (Materia et al., 2005, Moore, 2006, Stjarne et al., 2006). When low income (at individual level) and high income inequality (at ecological level) were risk factors, there was a synergistic interaction (Dahl et al., 2006, Daly et al., 1998, Henriksson et al., 2007, Kahn et al., 2000, Kennedy et al., 1998, Lochner et al., 2001, McLeod et al., 2003, Osler et al., 2002, Subramania et al., 2001, Subramanian and Kawachi, 2006). Some gaps in the literature can be identified. Firstly, we are aware of only one attempt to assess this interaction in a middle-income country, i.e., Chile, (Subramanian, Delgado, Jadue, Vega, & Kawachi, 2003), and this reported a statistically non significant interaction. Secondly, about half of the studies that assessed this interaction were performed in the USA. It is important to study this issue in developing countries, where relatively greater income inequality as well as higher absolute poverty rates are both often present. Thirdly, cross-level interaction has been assessed as a departure from multiplicative, but not additivity of effects, despite the latter approach being more in line with biological plausibility (Rothman, 1986).

Outcome-specific associations have been claimed to be more informative when studying health effects of income inequality because they can elucidate which specific behavioural and physiological pathways are affected (Lynch et al., 2004). In this regard, oral health has specific and known pathways that can be affected by material and psychosocial factors.

Between the 1980s and 2003, 12 year-old children in Brazil have experienced a 61.7% decline in the number of teeth decayed, missing or filled and the prevalence of caries free individuals increased from 3.7% to 31.1% (Narvai, Frazao, Roncalli, & Antunes, 2006). However, the burden of disease has increasingly been concentrated in smaller groups (Narvai et al., 2006). The reasons why such groups have benefited less from improvements in dental health are warranted and the contextual effect of income inequality may have a role.

The social environment and economic conditions (e.g. income inequality) are considered distal determinants of oral health (Holst et al., 2001, Petersen, 1990) that influence proximal determinants, such as diet (sugar consumption) and oral hygiene (with fluoridated toothpaste) (Fejerskov, 1997, Holst et al., 2001, Petersen, 1990). Specifically, but in line with general explanations, places with high income inequality may not be supportive of oral healthy behaviours and/or may under-invest in public health policy – including fluoridated water or widespread access to fluoridated toothpaste. Although behavioural and material circumstances of life have an established strong relation to dental caries, there is also some evidence regarding psychosocial factors (Holst et al., 2001). For instance, it has been reported that chronic stress, instead of acute stress, may decrease stimulated saliva flow (Hugo et al., 2008), and coping abilities are associated with tooth loss (Sanders, Slade, Turrell, Spencer, & Marcenes, 2007). Stress may further influence health-related behaviour negatively.

So, the aims of this study were to evaluate the association between income inequality and oral health in Brazil, to assess the role of alternative models that could explain this association and to assess whether income levels modify the income inequality effect.