Income inequality, social capital and self-rated health and dental status in older Japanese
Highlights
► Explores whether social capital attenuates the associations between income inequality and health outcomes in Japan. ► Income inequality was significantly associated with poor self-rated health and poor dental status. ► The association between income inequality and self-rated health was partially explained by social capital. ► On the other hand, social capital did not attenuate the strong association of income inequality and dental status.
Introduction
A recent meta-analysis showed that income inequality affects mortality and self-rated health (Kondo, Sembajwe, et al., 2009). There are several possible pathways linking income inequality and health (Kawachi, Fujisawa, & Takao, 2007). First, societies with high levels of income inequality have higher proportions of people living in poverty, and poverty is harmful for health (Shaw, Dorling, & Smith, 2006). Second, more unequal societies have higher levels of psychological stress caused by social comparisons, which in turn may have detrimental effects on health. Indeed, social-evaluative threats, one of the main causes of stress (Dickerson & Kemeny, 2004), are more common in more unequal societies (Wilkinson & Pickett, 2009). Third, income inequality erodes social capital and social capital is associated to health (Kawachi, Kennedy, Lochner, & Prothrow-Stith, 1997).
Although there is a growing body of evidence that social capital is associated with various health outcomes (Islam et al., 2006, Kim et al., 2008), only a few non-ecological studies have examined the contribution of social capital to the association between income inequality and health, whilst considering contextual effects and undertaking multi-level analysis (Celeste et al., 2009, Ichida et al., 2009, Kim and Kawachi, 2007, Subramanian et al., 2001). Social capital is defined as those features of social organizations, such as civic participation, norms of reciprocity, and trust in others, which facilitate cooperation for mutual benefit (Putnam, 1993). Social capital is embedded in communities and individuals (Kawachi et al., 1997). Multilevel modeling has been used to evaluate contextual community-level effects of social capital, controlling for compositional (individual-level) effects of social capital (Kawachi, Subramanian, & Kim, 2008). Of the previous studies assessing the relative effects of social capital and income inequality on health, most have used only community-level social capital (Celeste et al., 2009, Kim and Kawachi, 2007, Subramanian et al., 2001). In addition, a study by Ichida et al. (2009) evaluated community-level contextual effects of social capital adjusting for individual-level compositional effects, though it used only cognitive social capital.
Most of the cross-sectional studies on social capital and health have used self-rated health as an outcome variable. Few studies have used physical indicators of health as an outcome (Islam et al., 2006, Kim et al., 2008). In terms of function of older people, the number of remaining teeth is an important indicator of physical health. Dental health among older people is associated with impacts on daily living, particularly eating difficulties and nutritional deficiencies (Locker, 1992, Nowjack-Raymer and Sheiham, 2003, Nowjack-Raymer and Sheiham, 2007, Sahyoun et al., 2003, Sheiham et al., 2001, Tsakos et al., 2010a, Walls and Steele, 2004) while periodontal disease and tooth loss are associated with mortality (Abnet et al., 2005, Appollonio et al., 1997, Shimazaki et al., 2001) and chronic conditions such as hypertension (Franek et al., 2009, Tsakos et al., 2010b, Volzke et al., 2006), cardiovascular disease (Buhlin et al., 2003, D’Aiuto et al., 2006, Holmlund et al., 2006, Meurman et al., 2004) and metabolic syndrome (D’Aiuto et al., 2008, Shimazaki et al., 2007).
The number of remaining natural teeth and self-rated health can be considered as complementary measures of health in older people as they reflect past and current exposures to health risks. Self-rated health, a key measure of health in older people has been used in epidemiological studies because it predicts future health outcomes (Moller, Kristensen, & Hollnagel, 1996) including mortality (Idler & Benyamini, 1997). Current self-rated health reflects current health status (Solomon, Kirwin, Ness, O’Leary, & Fried, 2010). On the other hand, the number of remaining natural teeth is a historical measure of health and reflects the accumulation of exposures to social determinants of dental health throughout the life-course (Poulton et al., 2002). No previous studies on the association of health with social capital or income inequality have simultaneously used self-rated health and dental status.
This study was therefore planned with the objective of examining whether individual- and community-level social capital attenuated the associations between income inequality and two disparate health outcomes, self-rated health and dental status (number of remaining natural teeth).
Section snippets
Study population
The present analysis is based on data from the Aichi Gerontological Evaluation Study Project (the AGES Project), an ongoing Japanese prospective cohort study (Aida et al., 2009, Kondo, 2010, Kondo et al., 2009a). The AGES Project investigates factors associated with health related to functional decline or cognitive impairment among individuals aged 65 years or over. The AGES Project sample was restricted to people who did not already have physical or cognitive disability, defined as receiving a
Results
Table 1 shows the demographic distribution and univariate association between self-rated health, number of remaining teeth and covariates. Communities with higher income inequality had increased risks of poor self-rated health (OR = 1.39) and poor dental status (OR = 1.86). Community level mistrust was not significantly associated with self-rated health and dental status. Communities with higher levels of non-volunteering had increased risks of poor self-rated health (OR = 1.57) and poor dental
Discussion
This study showed that income inequality in communities was significantly associated with poor self-rated health (OR = 1.39) and poor dental status (OR = 1.86) of older Japanese. Income inequality was a major contributor to the variation in dental status between communities (50% reduction of MOR), but not to self-rated health. The association between income inequality and dental status remained significant when social capital variables were included in the analyses. On the other hand, the
Acknowledgments
This study used data from the Aichi Gerontological Evaluation Study (AGES). The survey was conducted by the Nihon Fukushi University Center for Well-being and Society as one of their research projects. The study was supported in part by a grant of Strategic Research Foundation Grant-aided Project for Private Universities from Ministry of Education, Culture, Sport, Science, and Technology, Japan (MEXT), 2009-2013.
The authors thank Drs Tatsuo Yamamoto, Tomoya Hanibuchi, Yukinobu Ichida, Hiroshi
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