Introduction
Chronic diseases are the leading cause of the global disease burden.1 With the ageing of the population, the number of people, particularly women, living with multiple chronic conditions (multimorbidity) has continually increased.2 Tackling multimorbidity is a key challenge faced by healthcare systems and services traditionally designed around a single condition.3 Current approaches for the management and prevention of multimorbidity have been people-centred, involving integrated health services, which emphasise treating individuals as whole persons.2 4 Emerging evidence shows that the nature and impact of the social relationship with which people live and how it shapes the development of chronic conditions should be considered in these approaches.5–9
Social relationships have been associated with many individual mental and somatic chronic conditions (eg, depression, hypertension, cardiovascular disease and cancer), as well as all-cause and cause-specific mortality.10–12 For example, a meta-analytical review across 148 studies indicated that individuals with adequate social relationships have a 50% greater likelihood of survival than those with poor or insufficient social relationships.10 Another meta-analysis showed that loneliness and social isolation were associated with a 30% increase in the risk of cardiovascular disease.11 A longitudinal study in Korea found that loneliness, social disengagement and group-level segregation were all associated with all-cause mortality.12 Despite these findings, there is little evidence on the association between social relationships and multimorbidity.13 We are only aware of cross-sectional studies investigating the link of physical multimorbidity with social participation and loneliness.13 14 However, no study has directly shed light on the association between social relationships and multimorbidity, particularly the accumulation of multimorbidity (the transition from a healthy state or only one condition to multimorbidity).
Social relationships have been defined diversely, with three broad categorisations that assess different aspects: structural, functional and qualitative.6 The associations between the structural (eg, social network size and marital status) and functional (eg, received and perceived social support and loneliness) aspects of social relationships and cognitive performance, morbidity and mortality have been estimated in many studies and populations6 7 15; however, the role of qualitative indicators (eg, subjective rating of satisfaction or quality) is largely unknown. Furthermore, most previous studies have examined only individual types of social relationship satisfaction or quality (eg, satisfaction with the partner, family members, friends, work and social activities),16–21 not considering the relative importance and combined effects of these types in relation to the development of individual chronic conditions and accumulation of multimorbidity. To examine the relative importance of each social relationship type, it is important to compare the associations between individual relationship types and the development of multimorbidity using the same analytical model.22
Social relationships change over time. Previous studies have typically assessed these relationships at only one time point, assuming that they remain constant over time.23 Little attention has been paid to the satisfaction with social relationships measured at multiple time points and accounted for as a time-varying variable in relation to the development of individual chronic conditions and multimorbidity.24 In addition, previous evidence suggests that the magnitude of the influence of social relationships on the risk of mortality is comparable with well-established risk factors (eg, obesity and alcohol intake).6 10 However, much remains to be learnt about the extent to which social relationship satisfaction contributes to multimorbidity.
To address these important gaps in the literature, we used longitudinal data spanning two decades to investigate whether satisfaction levels of a single social relationship type and the combined effect of multiple types are associated with the accumulation of multimorbidity in a national cohort of Australian women from midlife to early old age. We also quantified the potential contribution of other risk factors (socioeconomic status, health behaviours and menopausal status) to the association.