In this national cross-sectional study in China, we found a positive association between CF and physical function in middle-aged and older individuals. This finding showed that those who had better CF were more likely to have a lower rate of impairments in physical function. It was particularly interesting that we demonstrated the mediating role of depressive symptoms on the relationship between CF and physical function in elderly individuals. Moreover, the joint effects of CF and depressive symptoms on physical function in later life were observed.
Better CF was associated with better physical function later in life. This finding was consistent with a previous study showing that early life experience can influence the health of individuals in later life [27]. This could be for several reasons. First, repeated stress in early life can cause long-term health problems, including an increasing prevalence of chronic diseases in later life, which may act as stressors of impaired physical function [28]. Second, individuals with worse CF have an increased risk of being exposed to a low socioeconomic status in adulthood, which may also be associated with impaired physical function [29]. Third, poor CF may increase the likelihood of engaging in risky behaviour in adolescence and adulthood such as smoking; and drinking, which are also possibly associated with impaired physical function [27]. Theoretically, evidence also supports that adverse environments, such as experiencing fewer positive friendship experiences may lead to changes in gene expression and cause functional and structural changes in the brain, autonomy and immune system [30]. This finding further indicated that community-based intervention programs promoting children’s friendship opportunities (e.g., interaction with friends, teamwork, and club activities) may improve the long-term physical function of individuals.
Our findings have further confirmed the mediating effect of depressive symptoms on the association between CF and the levels of physical function in middle-aged and older adults. On the one hand, a previous large study demonstrated that middle-aged and older adults in China with adverse CF were at higher risk of depressive symptoms as they aged [14]. Children who grew up in environments lacking friendships tended to be lonely and more vulnerable to psychological and emotional trauma[14]. On the other hand, depressive symptoms were demonstrated to be significantly associated with a higher prevalence of impaired physical function among middle-aged and older adults in China [31]. The reason is that people with depressive symptoms were more likely to close themselves off, which may lead to impaired role performance and interfere with self-care and mobility, as well as loss of opportunities to be socially and physically active leading to decreased physical function [32, 33]. Therefore, developing appropriate interventions for prevention and controlling the development of depressive symptoms in adults (e.g., through child companionship, social interaction, and cultivation of interests) may help reduce the decline in physical function in later life.
It is important to note that the increased risk of impaired physical function depends on the presence of adverse CF in patients with depressive symptoms, as the impaired physical function risk among positive CF participants with depressive symptoms did not increase in our findings. The potential mechanism behind the joint effect of adverse CF and depressive symptoms is unknown. This may be explained by several reasons. First, as both adverse CF and depressive symptoms are associated with the volume of the hippocampus, one might speculate that the observed excess impaired physical function risk may be related to reduced hippocampal volume [30, 34]. Second, adverse CF is associated with less physical activity, which may add to the decline in physical activity levels induced by depressive symptoms, resulting in an excess risk of impaired physical function [35]. Third, middle-aged and older adults with depressive symptoms may experience secondary psychological trauma in the process of recalling adverse CF experiences, which may aggravate depressive symptoms and result in an excess risk of impaired physical function [34, 36]. Therefore, middle-aged and older adults with coexisting adverse CF and depressive symptoms were at higher risk for impaired physical function.
Based on the results of the joint effect of depressive symptoms and CF on physical function, we further found that depressive symptoms were more strongly associated with physical function than CF in middle-aged and older adults. Compared with CF, both depressive symptoms and physical function were measured in 2015 and were less susceptible to memory bias, so the results obtained are more accurate and not weakened. Moreover, previous studies have reported a variety of mechanisms linking depression and physical function [37, 38]. Via behavioural mechanisms, depressed individuals are less likely to pay adequate attention to personal health and follow treatment regimens and are more likely to engage in risky behaviours such as smoking, drinking and physical inactivity [39]. Other contributing mechanisms could be multiple pathophysiological pathways, such as inflammation and oxidative stress, that have been found to be increased in both persons with depression and in those with poorer physical function [37].
Although our hypothesis of causality between CF and physical function is supported by life-course theory, we cannot rule out the possibility of reverse causality given our cross-sectional study design. If the relationship is indeed bidirectional, such that CF affects depressive symptoms leading to impaired physical function, and conversely, physical function affects depressive symptoms leading to more severe friendship problems, then small effects in the short term may snowball into larger practical effects over time.
Policy Implications
Our results have several health policy implications for China. First, instead of focusing on the health of specific age groups, health care policies in China should consider their long-term impacts over the life cycle. For example, the fact that childhood health matters to achieve successful ageing, including avoidance of disability, maintenance of high physical function, and sustained engagement in social and productive activities, implies that interventions to improve elderly health should take into consideration the events in the life course, beginning in childhood. The CDC's comprehensive approach to preventing adverse childhood experiences uses multiple strategies from the best available evidence [40]. The fifth of these strategies is interventions to strengthen primary care through screening, referral and support to identify and address childhood exposure to bad friendships, thereby mitigating immediate and long-term harm and advancing trauma-informed care for children, adolescents, and adults with a history of adverse exposure to CF [41, 42].
Limitations And Strengths
Our study has several limitations. First, this study used the Baron and Kenny's classic mediation model as the primary analysis. The well-recognized analytical limitations include that this approach cannot handle exposure-mediator interactions (interaction between CF and depressive symptoms in this study). This is inevitable and may alter our findings. Therefore, further research is needed. Second, the association between CF and physical function may vary across developmental stages, from early childhood to adolescence. For example, preadolescents and adolescents may experience different friendship deficits, which may have different consequences for physical function in later life. Third, it may happen that participants with depressive symptoms report their childhood experiences differently than those without depressive symptoms. For example, depressed individuals may tend to remember negative events, which could have an impact on our results. Fourth, we were not able to control for people’s behavioural problems in childhood, genetic factors or friendship problems in adulthood that might confound the relationship between worse CF and physical function in the regression analysis. Fifth, CF in our study was measured in middle-aged and older adults, and hence was subject to recall bias. Previous research has reported the reliability of retrospective measures, which cannot be simply replaced by prospective measures [43]. Nevertheless, further studies are needed to evaluate whether prospective and retrospective CF have different associations with impaired physical function. Given the lack of longitudinal studies that span from childhood to the elderly in China, our current findings would be valuable when assessing the influence of childhood experience on later life. Finally, although we combined three items to measure the status of CF, no other details of childhood friendship questions were measured in CHARLS. We did not consider the number, quality and long-term nature of friends, all of which have been found to be associated with worse friendship outcomes[13]. Therefore, it is necessary to further investigate the effects of various CF patterns on physical function later in life.
Despite these limitations, this study has some strengths. For instance, to the best of our knowledge, this study is the first to analyse the association between CF and physical function in middle-aged and aged adults. We validated depression as a risk factor for physical function in middle-aged and older adults, and further extended the findings to childhood. Moreover, this study was a large-scale nationwide sample, data were derived from diverse regions of China, and consistent and standardized survey methods were used, so our data are representative of the middle-aged and older population in China.