Social relationships and inflammatory markers: An analysis of Taiwan and the U.S.

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Abstract

We evaluated the association between two aspects of social relationships and six inflammatory markers in Taiwan and the U.S. These two countries share similar levels of current life expectancy, but exhibit important differences in social structure. The data comprised population based samples from Taiwan (aged 53+; n = 962) and the U.S. (aged 35–86; n = 990) collected between 2003 and 2009. Circulating levels of interleukin-6 (IL-6), C-reactive protein (CRP), fibrinogen, and soluble forms of intercellular adhesion molecule 1, E-selectin, and IL-6 receptor (sIL-6R) were measured in fasting blood samples. A social integration score was based on marital status, contact with family and friends, church attendance, and other social participation. A perceived social support index was based on questions regarding the availability of care and support from family and friends. Linear regression models tested the association between these two measures and each inflammatory marker controlling for sociodemographic characteristics, obesity, medication use, and baseline health status. After adjusting for potential confounders, social integration had a significant but weak inverse association with CRP in Taiwan. Perceived social support was significant in two of 12 models, and the coefficient was positive (i.e., higher support was associated with higher CRP and sIL-6R in the U.S.). We found no evidence that the coefficients for social relationship measures varied by sex or age. Our results yielded limited evidence of a weak association between two dimensions of social relationships and six inflammatory markers in Taiwan and the U.S. Given that the literature suggests a strong link between social relationships and mortality, and that inflammation plays an important role in the leading causes of death, we had expected to find consistent and moderately strong associations between social relationships and inflammatory markers. The small effect sizes and lack of robustness across markers were surprising.

Highlights

► Prior studies suggested that inflammation may mediate the strong link between social relationships and mortality. ► Our results yielded weak evidence of a link between social relationships and six inflammatory markers in Taiwan and the U.S. ► Social integration had a significant, but weak inverse association with CRP in Taiwan. ► We found no evidence that higher perceived support was associated with lower inflammation. ► Given the proposed role of inflammation, the small effect sizes and lack of robustness across markers were surprising.

Introduction

A recent meta-analysis concludes that the link between social relationships and mortality is as strong or stronger than other well known risk factors such as smoking, physical activity, and drug treatment for hypertension (Holt-Lunstad, Smith, & Layton, 2010). Yet the physiological and behavioral pathways through which social relationships affect survival are not well understood. Among studies that examine the link between social relationships and physiological parameters, markers of immune function and/or inflammation yield the most consistent effects (Kiecolt-Glaser et al., 2010, Uchino, 2006). Chronic inflammation is a significant predictor of mortality and plays an important role in the development of cardiovascular disease and other conditions (Kiecolt-Glaser et al., 2010, Pearson et al., 2003), although it remains unclear whether this association is causal. Prior studies of social relationships and inflammation are based largely on samples of U.S. or European populations. Most rely on a small number of subjects, examine only one inflammatory marker, or include only one dimension of social relationships.

Measures of social relationships are typically based on the existence of social ties, or on the function or perceived quality of those networks (Cohen, 1988). Holt-Lunstad et al. (2010) finds that measures of social integration that incorporate multiple components (e.g., marital status, network size, network participation) yield the strongest association with mortality. Among measures of the quality of social relationships, perceived social support and loneliness also exhibit notable associations with mortality (Holt-Lunstad et al., 2010). The two main pathways through which social relationships may affect inflammation—and in turn, health—involve behavioral and psychological processes (Cohen, 1988, Uchino, 2006). Social relationships may influence health behaviors and health care by providing information and tangible resources or by indirect means and may also have psychological effects on affect, self-esteem, personal mastery, sense of life purpose and perceptions of stress.

Differences in social structure across cultures could contribute variation in both the nature of social relationships and how those relationships affect health. Americans favor self-expression and independence in relationships with others, whereas Chinese value interdependence and social harmony (Markus & Kitayama, 1991). Social ties with family are likely to be more prevalent in Taiwan compared with the U.S. Divorce rates are lower than in the U.S. Older Taiwanese are more likely than their U.S. counterparts to live with or in close proximity to their married children or other relatives. In Taiwan, adult sons are expected to provide financial support for their elderly parents.

This study uses data from large population based samples of adults in Taiwan and the U.S. to examine associations between social relationships and six inflammatory markers: interleukin-6 (IL-6) and its soluble receptor (sIL-6R), C-reactive protein (CRP), fibrinogen, soluble intercellular adhesion molecule 1 (sICAM-1), and soluble E-selectin (sE-selectin). Given the complexity of the immune system, no single marker can adequately capture inflammatory processes. Our analyses allow us to determine the consistency of patterns across multiple measures of inflammation, for different dimensions of social relationships, and between countries. Prior studies of the link between social relationships and inflammation have focused on IL-6, CRP, or fibrinogen, while the other markers considered here have rarely been examined. Given that the six inflammatory markers are biologically related, we anticipate similar associations. However, we hypothesize that the associations with inflammation will be stronger for integration than for perceived support given its stronger association with mortality (Holt-Lunstad et al., 2010). Taiwan and the U.S. provide interesting contexts for investigating these questions because they experience similar levels of life expectancy and patterns of chronic disease, but as noted above, differ substantially in social structure, particularly with regard to self-other relations. Although these differences in the social environment may lead to variations in the estimates between the two countries, we expect to find significant associations between social relationships and inflammation in both populations.

Section snippets

Taiwan

The Taiwan data came from the 2006 wave of the Social Environment and Biomarkers of Aging Study (SEBAS) and the 2003 wave of its parent study, the Taiwan Longitudinal Study of Aging (TLSA). TLSA began in 1989 with a nationally representative sample of persons aged 60 and older; younger refresher cohorts were added in 1996 and 2003. In 2000, a random sample of those interviewed in the 1999 TLSA was selected for SEBAS; the oldest cohort and urban residents were oversampled. Among the 1497

Results

Descriptive statistics for all variables in the analysis are shown in Table 1. The mean ages for the two study samples reflected the younger range of the U.S. sample (35–86) compared with the Taiwan sample (53–97). As noted earlier, the educational distributions also differed: the majority of the Taiwan sample, but less than 1% of the U.S. sample, had six or fewer years of education; the percentage with post-graduate education was less than 2% in Taiwan but nearly one-quarter in the U.S. The

Discussion

After evaluating the relationship between two dimensions of social relationships and six inflammatory markers in the U.S. and Taiwan, we found only a few statistically significant associations. Given that the literature suggests a strong link between social relationships and mortality, and that inflammation plays an important role in the leading causes of death, we had expected to find consistent and moderately strong associations between social relationships and inflammatory markers. The small

Role of funding

The study sponsors played no role in the study design; in the collection, analysis, or interpretation of data; in the writing of the report; or in the decision to submit the paper for publication.

Conflicts of interest

None of the authors have a conflict of interest that could inappropriately influence the results of this study.

Acknowledgments

We thank Christopher Coe and Elliot Friedman at University of Wisconsin-Madison for their helpful comments on this paper.

This work was supported by the National Institute on Aging (grant numbers R01 AG16790, R01 AG16661, P01-AG020166); and the Eunice Kennedy Shriver National Institute of Child Health and Human Development (grant number R24HD047879).

Funding for the TLSA came from the Taiwan Department of Health, the Taiwan National Health Research Institute [grant number DD01-86IX-GR601S], and

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