Social isolation is one of the most serious problems faced by aging societies. According to the World Health Organization, social isolation is related to a lack of contact with family, friends, and others (1). The estimated prevalence of social isolation among older adults is as high as 88%, depending on the definition of isolation (2). Social isolation in old age leads to various negative health outcomes, such as anxiety and depression (3), poor sleep quality (4), disability (5), and dementia (6). One particular concern is that social isolation increases mortality rates (7, 8). A previous study reported that social isolation has a greater effect on mortality than established risk factors, such as smoking, alcohol consumption, and physical inactivity (9). Additionally, socially isolated individuals have higher suicide rates (10). Consequently, it is important to encourage older adults to maintain interactions with others and avoid the risk of social isolation.

However, the number of other individuals with whom older adults can interact decreases with age. As individuals age, their number of friends, relatives, and other social connections decreases (11), and the death of peers limits their social interactions (8). Moreover, some older adults prefer to be alone (12). A previous study revealed that solitude-seeking has more positive ramifications for older adults (13). In this case, in addition to preventing social isolation, there is a need to examine ways to maintain health even when isolated. However, evidence from this perspective is limited. Additionally, predictors of isolation, such as sex, age, and economic status, are mostly unchanged across interventions (2). Therefore, knowledge of how to prevent negative health outcomes in isolation would be beneficial.

Exercise is a lifestyle habit that reduces mortality risk. It improves physical function and quality of life, prevents frailty and sarcopenia, reduces noncommunicable chronic diseases, and reduces all-cause and cause-specific mortality, such as cardiovascular disease and cancer (14–17). Physical inactivity and sedentary behavior increase the risk of depression and all-cause and cardiovascular mortality among older adults (18, 19). Furthermore, exercise is beneficial for diseases common among older adults. Regular exercise can have anti-atherogenic effects on the vasculature, independent of its effects on traditional cardiovascular disease risk factors (20). Additionally, exercise interventions are beneficial for glycemic control and cardiovascular risk factors associated with diabetes (21). Exercise may thus reduce mortality via these mechanisms. However, to date, no studies to our knowledge have examined whether regular exercise reduces mortality, even among socially isolated individuals at a high risk of death.

Therefore, we aimed to ascertain whether regular exercise reduces the risk of mortality among socially isolated older adults living in the community. To address this question, this study aimed to determine the effects of the combination of social isolation and regular exercise on mortality among community-dwelling older adults.

After excluding 100 participants with missing data, a final analysis was performed on the complete datasets available for 735 participants (males = 303, females = 432; mean age [SD], 72.9 [5.1] years). In total, 132 (18.0%), 426 (58.0%), 27 (3.7%), and 150 (20.4%) participants were assigned to group 1 (not isolated with regular exercise), group 2 (not isolated without regular exercise), group 3 (isolated with regular exercise), and group 4 (isolated without regular exercise), respectively. The baseline characteristics of the patients are shown in Table 1. Isolated participants were more likely to be male. Participants who did not exercise regularly were slightly older than those who exercised regularly. The mortality rates in groups 1, 2, 3, and 4 were 6.1%, 9.2%, 7.4%, and 19.3%, respectively.

In the crude model, isolated individuals who did not exercise regularly had a significantly higher mortality rate [hazard ratio (HR), 3.62; 95% confidence interval (CI), 1.66–7.93] compared with those who were not isolated but did exercise regularly (Table 2). However, no significant association was noted in isolated individuals who exercised regularly (HR, 1.27; 95% CI, 0.27–5.98). In the adjusted model, only isolated participants without regular exercise showed a significantly increased mortality rate (HR, 2.48; 95% CI, 1.12–5.52). No significant association was noted in isolated participants who exercised regularly (HR, 1.25; 95% CI, 0.26–5.91). After excluding older adults who died during the first year, only isolated participants without regular exercise showed a significantly increased mortality rate (HR, 2.73; 95% CI, 1.17–6.36) in the adjusted model (Table 3).

This study examined the effects of the combination of social isolation, which increases mortality, and regular exercise, which decreases mortality. The results showed that, compared with participants who were not isolated and did engage in regular exercise, those who were isolated without regular exercise had a higher risk of mortality, whereas those who were isolated but engaged in regular exercise did not have an increased risk of mortality. Therefore, our study indicates that regular exercise reduces the risk of mortality among socially isolated older adults living in the community.

This study revealed that regular exercise had a positive effect on mortality risk, even in isolated individuals. This result is consistent with evidence from previous studies showing that exercise reduces mortality (16, 17). In particular, isolated individuals are at a higher risk of cardiovascular disease and death from cardiovascular disease than non-isolated individuals (28–30). It is possible that death from cardiovascular disease was reduced in our study, even in isolated older adults, because exercise can have anti-atherogenic effects on the vasculature (20) and improve cardiovascular disease (15). However, we cannot speculate on these relationships because no analysis was performed on the cause of death in this study owing to the limited sample size. Future analyses based on the cause of death will further clarify the relationship between social isolation, regular exercise, and mortality.

The prevalence of social isolation in this study was approximately 25%, similar to that previously reported for older adults in Japan (31). Preventing social isolation in old age is crucial because isolation leads to negative health outcomes, including death (7, 8). However, if the negative effects of isolation can be eliminated, we may be able to achieve a society that accepts an individual’s preference for isolation. The findings of this study may help improve the health of older adults who willingly isolate themselves. Furthermore, future analyses that consider socializing preferences may deepen our understanding of this area.

This study showed that promoting regular exercise in isolated older adults may be beneficial; however, the percentage of those who engaged in regular exercise was low (15.6%) compared with that of non-isolated older adults (23.7%). Healthy lifestyle habits, such as physical activity, increase with social connectedness (32, 33); those who are isolated may be less likely to develop exercise habits through social connectedness. Although network interventions or opinion leaders are effective in promoting healthy lifestyle habits, such as physical activity (34, 35), these interventions are likely to be ineffective for isolated individuals with few social connections. However, a previous review on the determinants of exercise reported insufficient evidence (36); thus, further investigations are required to determine how to promote the acquisition of exercise habits among socially isolated older adults. The following methods should be considered to promote regular exercise among isolated individuals. First, isolated older adults who exercise should be surveyed to determine the type of exercise they practice, and the types of exercise that are easy to practice in isolation should be promoted. For example, since ball sports require a partner, isolated older adults can practice walking, strength training, and swimming instead, as they can be practiced alone. Second, a mobile phone-based intervention may be beneficial because older adults who do not like socializing may not participate in face-to-face, classroom-type exercise classes. Indeed, the World Health Organization recommends the promotion of physical activity using digital technology for older adults (37).

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving humans were approved by the Ethics Committee of the Tokyo Metropolitan Institute for Geriatrics and Gerontology. The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

ME: Conceptualization, Data curation, Formal analysis, Investigation, Writing – original draft. HK: Conceptualization, Investigation, Writing – original draft. KIm: Conceptualization, Investigation, Writing – original draft. YF: Investigation, Writing – review & editing. KIh: Investigation, Writing – review & editing. HH: Investigation, Writing – review & editing. SO: Conceptualization, Funding acquisition, Supervision, Writing – review & editing.