Introduction

With the rapid aging of the global population, cognitive impairment disorders such as dementia and mild cognitive impairment (MCI) have emerged as major public health challenges [1]. Currently, a new dementia case occurs every three seconds worldwide, and the global dementia population is estimated at more than 55 million [2]. The global prevalence of MCI, a transitional state between normal cognition and dementia, is estimated at 22.7% among older adults [3] and is continuing to rise [4]. Cognitive decline has profound impacts on both individuals and society. Decreased ability to perform activities of daily living compromises the independence and quality of life of older adults [5], ultimately leading to an increased risk of mortality [6]. Furthermore, families and society face escalating burdens of medical services, informal care, and social care, resulting in substantial socioeconomic costs [4,7]. Therefore, an urgent need exists to identify risk factors for cognitive decline and explore modifiable factors to prevent and slow its progression.

Rowe and Kahn's Model of Successful Aging proposes [8] that successful aging encompasses not only the absence of disease and disability but also the maintenance of physical and cognitive function and engagement in social and productive activities. In this context, activity engagement among older adults is considered essential for healthy aging beyond mere leisure activities. According to previous research [9], the activity engagement of older adults can be categorized into physical, cognitive, and social activities. Physical activity includes all bodily movements involving energy expenditure, including strength training, aerobic fitness, balance exercises, household chores, and gardening [10]. Cognitive activity comprises mentally stimulating activities that challenge cognitive abilities, including reading, playing chess or card games, and solving crossword puzzles or sudoku [11]. Social activity involves relationships, interactions outside the home, and participation in various organizations. This includes formal activities (religious gatherings, social clubs, cultural/sports groups, alumni associations, volunteering, political/civic organizations) and informal activities (interactions with friends, relatives, and neighbors) [12]. These diverse types of activity engagement enhance quality of life and promote health, playing particularly crucial roles in protecting and enhancing cognitive function among older adults [10,13-15]. Although activity engagement positively influences cognitive function through mechanisms including enhanced cognitive reserve, stress alleviation, and improved cardiovascular health [16], different activity types may operate through distinct pathways. However, research that examines pathways through which each activity type affects cognitive function in older adults is limited.

Depressive symptoms are a significant risk factor that accelerates cognitive decline in older adults [17]. Longitudinal panel analyses indicate that more severe depressive symptoms accelerate cognitive decline, with these effects particularly pronounced in patients with dementia [18]. Depression causes sustained hypersecretion of cortisol, which promotes atrophy and dysfunction in the hippocampus and other essential brain regions that affect emotion regulation and memory. Additionally, this process triggers neuroinflammatory responses that contribute to cognitive decline [19,20]. Prolonged elevation of cortisol accelerates neurodegeneration and negatively affects prefrontal cortical and limbic structures, exacerbating depressive symptoms and worsening impairment across various cognitive domains [20,21].

Meanwhile, activity engagement has been reported to have positive effects on reducing depressive symptoms. Physically active older adults have a reduced risk of depression [14]. Cognitive activities such as reading newspapers or books alleviate depressive symptoms, and social activities, including participation in community organizations and educational programs, are also associated with reduced depressive symptoms [22,23]. These relationships suggest that depressive symptoms likely serve as an important mediating variable in the relationship between activity engagement and cognitive function. That is, there may be an indirect pathway whereby activity engagement reduces depressive symptoms, and this reduction in depression subsequently leads to improved cognitive function. Identifying these mediating pathways would enable the development of integrated intervention strategies utilizing the 'activity-depressive symptoms-cognition' relationship. By designing activity programs beyond simple cognitive stimulation and simultaneously helping prevent or manage depressive symptoms, we can anticipate synergistic effects and achieve multiple health outcomes with a single intervention. However, most research has been limited to single activities or has analyzed multiple activities in aggregate. Studies designed to examine how physical, cognitive, and social activities—each with distinct characteristics—affect cognitive function by alleviating depressive symptoms are rare. Given that approximately 10%-16% of older adults worldwide experience clinically significant depressive symptoms [24], systematically analyzing the extent to which each activity type contributes to cognitive function by improving depressive symptoms is essential for effectively preventing cognitive decline.

This study aims to examine the mediating effect of depressive symptoms in the relationship between activity engagement and cognitive function among older adults. Given that each activity type has unique characteristics and purposes, we seek a more nuanced understanding by analyzing both the direct and indirect effects of depressive symptoms on each activity type. We aim to clarify the pathways through which activity engagement influences cognitive health and to elucidate the mediating role of depressive symptoms.

Methods

Study design and participants

This study conducted a cross-sectional secondary analysis using data from the 2022 core dataset of the Health and Retirement Study (HRS). The HRS is a nationally representative longitudinal panel study of U.S. adults aged 50 and older, initiated in 1992 and conducted biennially [25]. The HRS is funded by the National Institute on Aging and administered by the University of Michigan. All data used in this study were obtained from the HRS website, https://hrs.isr.umich.edu. The inclusion criteria for the present study were participants aged 65 years or older who responded to the items for activity engagement, cognitive function, and depressive symptoms. After excluding missing data on these variables, the current study includes a total of 2,741 older adults.

Measurements

Activity engagement

The frequencies of engagement in the three types of activities (physical activity, cognitive activity, and social activity) were assessed using items from the HRS Psychosocial and Lifestyle Questionnaires. Physical activity was measured using three items: playing sports or exercising, walking for 20 minutes, and maintenance or gardening. Cognitive activity was measured using five items: doing word games, playing cards or chess, writing, knitting, and working on a hobby or project. Social activity was measured using six items: volunteer work with children, charity work, educational course, sport or social club, nonreligious organization, and community arts group. These items were validated in a previous study [9] through exploratory factor analysis (EFA) and confirmatory factor analysis (CFA), which supported a three-factor model and confirmed construct validity. Each item was originally rated on a 7-point Likert-type scale in the HRS (1 = daily, 2 = several times a week, 3 = once a week, 4 = several times a month, 5 = at least once a month, 6 = not in the last month, and 7 = never). However, because the original 7-point scale failed to satisfy the assumption of normality and the midpoint response categories were infrequently endorsed, the previous study [9] that categorized these three activity domains using HRS items suggested collapsing the scale to three response options (1 = never, 2 = not in the last month to several times a week, and 3 = daily). Accordingly, the present study recoded the original responses into a 3-point scale for analysis. The mean scores for each subscale ranged from 1 to 3, with higher scores indicating greater levels of activity engagement. In the validation study [9], the reliability of the instrument, measured using Cronbach’s alpha, was .68 for physical activity, .72 for cognitive activity, and .89 for social activity. In the present study, Cronbach's alpha coefficients were .60 for physical activity, .61 for cognitive activity, and .80 for social activity, indicating acceptable internal consistency (≥.60).

Cognitive function

Cognitive function was assessed using a modified version of the Telephone Interview for Cognitive Status (TICS) [26]. The TICS includes an immediate word recall test (0-10 points), a delayed word recall test (0-10 points), a serial 7s subtraction test (0-5 points), a backward counting test (0-2 points), naming tasks (0-2 points), and recall of the date, as well as the names of the president and vice president (0-6 points). The total score ranged from 0 to 35, with higher scores indicating better cognitive functioning.

Depressive symptoms

Depressive symptoms were measured using an 8-item short form of the Center for Epidemiologic Studies–Depression Scale (CES-D) [27]. Participants reported whether they had experienced each of the following feelings for much of the time during the past week: (a) felt depressed, (b) felt everything they did was an effort, (c) had restless sleep, (d) were happy, (e) felt lonely, (f) enjoyed life, (g) felt sad, and (h) could not get going. Each item was coded as binary (yes = 1 or no = 0), and the two positively worded items (i.e., d and f) were reverse-coded before the total score was calculated. Total scores ranged from 0 to 8, with higher scores indicating more severe depressive symptoms. The scale demonstrated satisfactory internal consistency, with Cronbach’s alpha of 0.84 in the prior validation study [27] and 0.80 in the present study.

Covariates

Covariates included age, sex, marital status, educational attainment, satisfaction with household income, and presence of chronic diseases. Educational attainment was categorized as 12 years or less (high school or less) and 13 years or more (college or higher). In the HRS, satisfaction with household income was assessed using a 5-point Likert scale (1 = completely satisfied, 2 = very satisfied, 3 = somewhat satisfied, 4 = not very satisfied, 5 = not at all satisfied). For the present study, responses were dichotomized into ‘satisfied’ (1 to 3) and ‘not satisfied’ (4 to 5). Chronic diseases were classified into the following five categories: cardiometabolic diseases (hypertension, diabetes, and high cholesterol), cardio-cerebrovascular diseases (heart condition and stroke), cancer, arthritis or osteoporosis, and others (lung disease and incontinence).

Data analysis

Descriptive statistics were used to summarize participants' general characteristics, activity engagement, cognitive function, and depressive symptoms. Independent t-tests were conducted to examine differences in cognitive function according to general characteristics. Pearson's correlation analysis was performed to explore relationships among types of activity engagement, cognitive function, and depressive symptoms. Simple mediation analyses using Model 4 of the PROCESS macro developed by Hayes [28] were conducted to test whether depressive symptoms mediated the association between types of activity engagement and cognitive function. The significance of the mediation effects was estimated through a bootstrapping procedure with 10,000 samples and 95% boot confidence intervals (CIs). A mediating effect was considered statistically significant if the 95% boot CI for the indirect effect did not include zero. All analyses were performed using IBM SPSS Statistics 29 (IBM Corp., Armonk, NY, USA) and the PROCESS macro v3.5 for SPSS.

Ethical considerations

The original HRS study protocol was approved by the institutional review board (IRB) at the University of Michigan, and all participants provided written informed consent [25]. The HRS dataset used in this study is publicly available and fully de-identified to protect participants' privacy. The present study was reviewed and determined to be exempt by the IRB at Kyungpook National University.

Results

General characteristics of participants

A total of 2,741 participants were included, with a mean age of 75.0 years (range: 65-101), and 58.8% were female (Table 1). Among the participants, 60.2% were married or partnered, 57.7% had a college degree or higher, and 83.3% were satisfied with their household income. Regarding chronic diseases, cardiometabolic diseases (e.g., hypertension, diabetes, and high cholesterol) were most prevalent (80.5%), followed by arthritis/osteoporosis (70.0%), other diseases (39.5%), cardio-cerebrovascular disease (34.9%), and cancer (20.4%).

Descriptive statistics of activity engagement, depressive symptoms, and cognitive function

The mean scores for activity engagement were 1.97 for physical activity, 1.64 for cognitive activity, and 1.36 for social activity (Table 1). The mean depressive symptoms score was 1.23, and the mean cognitive function score was 21.94.

Differences in cognitive function by general characteristics

Table 1 presents differences in cognitive function according to general characteristics. Cognitive function was significantly higher in the 65-74 age group than in the ≥75 age group (t=10.27, p<.001). Married or partnered participants showed higher cognitive function than those who were never married, separated, divorced, or widowed (t=7.39, p<.001). Regarding educational attainment, participants with a college degree or higher had significantly higher cognitive function than those with a high school education or less (t=−19.22, p<.001). Those satisfied with their household income showed significantly higher cognitive function than those who were not satisfied (t=5.99, p<.001). Participants with cardiometabolic diseases (t=−8.07, p<.001), cardio-cerebrovascular diseases (t=−4.10, p<.001), or arthritis/osteoporosis (t=−2.89, p=.004) had significantly lower cognitive function compared to those without these conditions.

Correlations between types of activity engagement depressive symptoms, and cognitive function

As shown in Table 2, cognitive function was significantly positively correlated with physical activity (r=.20, p<.001), cognitive activity (r=.29, p<.001), and social activity (r=.22, p<.001), and negatively correlated with depressive symptoms (r=−.21, p<.001). Depressive symptoms were negatively correlated with engagement in engagement in physical activity (r=−.21, p<.001), cognitive activity (r=−.12, p<.001), and social activity (r=−.19, p<.001).

Mediating effect of depressive symptoms in the relationships between types of activity engagement and cognitive function

To examine the mediating effect of depressive symptoms on the relationships between physical, cognitive, and social activity engagement and cognitive function, covariates that were significantly associated with cognitive function were included in the analysis. These covariates were age, marital status, educational attainment, satisfaction with household income, cardiometabolic diseases, cardio-cerebrovascular diseases, and arthritis or osteoporosis. Prior to testing the mediation models, the assumptions of linear regression were assessed. Each of the regression models with physical, cognitive, and social activity engagement as independent variables was statistically significant (F=73.93, p<.001; F=88.80, p<.001; F=74.25, p<.001, respectively). The assumption of normality was satisfied based on normal probability plots of standardized residuals. Durbin-Watson statistics ranged from 1.95 to 1.96, indicating no autocorrelation and confirming that residuals were independent. Variance inflation factors (VIFs) ranged from 1.03 to 1.18, suggesting that multicollinearity was not a concern. These results confirmed that the assumptions for linear regression were met, and the data were suitable for mediation analysis.

The mediation analysis results for each activity type are presented in Table 3 and visually summarized in Figure 1. Following Baron and Kenny’s three-step procedure, the mediating role of depressive symptoms was examined. For physical activity, step 1 showed a significant association with depressive symptoms (β=–.14, p<.001). In step 2, physical activity had a significant direct effect on cognitive function (β=.10, p<.001). In step 3, when both physical activity and depressive symptoms were included as predictors of cognitive function, both remained significant (β=.08, p<.001; β=–.14, p<.001, respectively). These results indicate that depressive symptoms partially mediate the relationship between physical activity and cognitive function. To confirm the mediating effect, a bootstrapping analysis was conducted, which showed that depressive symptoms significantly mediated the relationship between physical activity and cognitive function (β =.02; 95% boot CI=.01 to .03).

The mediation analysis for cognitive activity also satisfied the three-step criteria. Cognitive activity was negatively associated with depressive symptoms (β=–.08, p<.001) and positively associated with cognitive function (β=.21, p<.001). When both cognitive activity and depressive symptoms were included in the model, both cognitive activity (β=.20, p<.001) and depressive symptoms (β=–.13, p<.001) remained significant. These results suggest a partial mediating effect of depressive symptoms, which was statistically supported by the bootstrapping analysis (β=.01; 95% boot CI=.01 to .02).

Consistent results were observed in the analysis of social activity. Social activity was significantly associated with fewer depressive symptoms (β=–.13, p<.001), and with better cognitive function (β=.10, p<.001). When both social activity and depressive symptoms were included in the model, both remained significant predictors of cognitive function (β=.09, p<.001; β=–.14, p<.001, respectively). These findings indicate a partial mediating effect of depressive symptoms in this relationship. The bootstrapping analysis confirmed a statistically significant indirect effect (β=.02; 95% boot CI=.01 to .03).

Discussion

This study explored the pathways linking three activity domains—physical, cognitive, and social—to cognitive function, a focus on depressive symptoms as a mediating factor. Analyses revealed positive associations between each activity type, cognitive function, and depressive symptoms. Notably, partial mediation emerged for all activities, demonstrating that cognitive benefits arise through two complementary channels: direct effects on cognitive function and indirect effects mediated by improvements in depressive symptoms. This comprehensive examination of differential mediating pathways across activity types provides critical insights for designing integrated interventions that optimize mental health and cognitive outcomes in aging populations.

The findings of this study revealed that engaging in physical, cognitive, and social activities each has positive effects on cognitive function, consistent with previous research. Physical activity influences cognitive enhancement through neurobiological pathways, including increased cerebral blood flow [29], reduced white matter loss [30], and prevention of hippocampal atrophy [31], as demonstrated in large-scale longitudinal studies [32,33]. Studies conducted in multiple countries have confirmed that cognitive activity enhances memory, numeracy, and verbal fluency while reducing MCI risk [15,32,34]. These effects can be attributed to neuroplasticity, the brain's ability to form new neural connections and build cognitive reserves in response to mental challenges [35,36]. Social activity also contributed significantly to maintaining cognitive function. Korean research has shown that forming social networks through activities such as participating in religious groups is essential for maintaining cognitive function in later life [37]. One example is that older adults with higher initial levels of social participation had relatively slower rates of cognitive decline [38]. Interacting effectively with individuals beyond the boundaries of one’s long-standing familiar relationships requires considerable cognitive effort [39], and communication during social interactions also helps maintain cognitive function [40].

A key finding of this study is that depressive symptoms play a crucial mediating role in the interactions between engaging in physical, cognitive, and social activities and cognitive function. This finding suggests that activity engagement influences cognitive function via dual pathways: a direct pathway from activities to cognitive function, and an indirect pathway whereby activities reduce depressive symptoms, and this decrease in depressive symptoms then enhances cognitive function. Following Baron and Kenny's three-step procedure, step 1 of this study confirmed that all activity types effectively reduced depressive symptoms. These findings are consistent with longitudinal studies from the UK, China, and Thailand showing that physical, mental, and social activities significantly reduced depressive symptoms [22,23,33,41]. This reduction in depression occurs through enhanced self-esteem, strengthened social support, and improved self-efficacy and quality of life based on activity engagement [42-47]. Notably, altruistic activities such as volunteering can reduce depression rates by up to 43% [48]. Step 2 of this study confirmed that all three activity types had direct effects on cognitive function. Step 3 confirmed that depressive symptoms had partial mediating effects across all three activity types.

This study’s findings indicate that physical activity indirectly promotes cognitive health by alleviating depressive symptoms. Path analysis of longitudinal data from the UK [33] found that increased physical activity was significantly associated with improvements in global cognition, episodic memory, and numeracy. Physical activity also had significant effects on depressive symptoms, which, in turn, were associated with cognitive decline. This illustrates the mediating role of depressive symptoms. Furthermore, a study using structural equation modeling in the United States [49] found that physical activity affected depressive symptoms, and depressive symptoms affected cognition, confirming a significant indirect effect from physical activity through depressive symptoms to cognition. Physical activities such as exercise promote neuroplasticity processes related to depression, reduce inflammatory levels, enhance resilience to physiological stress, and improve self-esteem, social support, and self-efficacy [44]. These improved mood states and reduced depressive symptoms can ultimately enhance cognitive function.

Cognitive activity also affected depressive symptoms, which are associated with cognitive decline, illustrating the mediating effect of depressive symptoms. This is consistent with results from a large-scale longitudinal study in China showing that depressive symptoms partially mediated the effects of leisure and mental activities on cognitive function [41]. A longitudinal study of older adults in Taiwan observed a tendency for depression scores to decrease with greater participation in activities such as reading newspapers or books [22]. Applying education and training programs such as brain education programs, computerized cognitive training, and cognitive enhancement group training to older adults increases self-esteem, life satisfaction, and quality of life, reducing depression [42,43,45-47]. These results suggest that cognitive activity provides intellectual stimulation, and the sense of achievement and purpose gained through learning and problem-solving processes alleviates depression and ultimately improves cognitive function.

Similarly, in the present study, social activity contributed indirectly to cognitive health in later life through pathways that alleviate depressive symptoms. A Chinese longitudinal study showed that social activity participation improves cognitive function directly and indirectly by reducing depressive symptoms [23]. When older adults stay socially active, they experience fewer depressive symptoms because these activities strengthen their social connections and provide emotional support [50]. Activities where people help others, like volunteering, are potent and can cut depression rates by up to 43% [48]. A large-scale cross-sectional study from Japan showed a cascading process where reducing the frequency of social contact increases depressive symptoms, and this increase in depression mediates atrophy of brain regions related to cognitive function, ultimately leading to cognitive decline [51]. Considering these linkages between activity engagement, depressive symptoms, and cognitive function, there is a need to provide activity programs that are tailored to maximize cognitive function by reflecting participants' functional levels, social isolation levels, and quality of life.

The significant mediating effects of depressive symptoms across multiple activities emphasize the importance of mental health maintenance and management of the pathways between activity engagement and cognitive function. Based on the findings of this study, activity programs for older adults with depressive symptoms should be designed not merely to encourage participation but to strategically reduce depressive symptoms by incorporating psychosocial components such as emotional support, enhancement of self-esteem, and promotion of social connectedness. This comprehensive approach requires community-based aging policies that combine infrastructure for activity engagement with structured mental health services such as depression screening, counseling, and referrals. Korea's 5th Health Plan (2021-2030) [52] identifies physical activity, mental health care, and dementia prevention as core national priorities, and the Ministry of Health and Welfare has implemented various initiatives, including senior employment programs, volunteer activities, and leisure welfare facilities [53]. Some Korean communities have already established systems that screen for depression within senior welfare facilities and link high-risk individuals to professional counseling and preventive interventions such as horticultural therapy [54]. These integrated models connecting activity programs with depression management can be effective strategies to optimize cognitive preservation by improving both neurobiological and psychological health in older adults.

While previous studies have focused on the relationships between individual activities and depressive symptoms or cognitive function, this research categorized activity types into physical, cognitive, and social activities and simultaneously analyzed and compared mediating effects within the same sample, thereby comparing the pathways through which each activity type affects cognitive function through depressive symptoms. To our knowledge, this is one of the first studies to simultaneously examine and compare the mediating role of depressive symptoms across three activity types using HRS data. This comprehensive approach contributes to a deeper understanding of the multifaceted effects of activities in later life and the development of evidence-based intervention strategies. This study has several limitations. First, the cross-sectional study design makes it difficult to infer causal relationships among activity engagement, depressive symptoms, and cognitive function. Therefore, future research using a longitudinal design is warranted to confirm the structural nature of these pathways. Second, there are limitations in the measurement instruments. Activity engagement and depressive symptoms were measured using self-reported instruments, which may be subject to memory errors or recall bias. In particular, only the frequency of activity engagement was measured, which ignores the contributions of intensity, duration, and qualitative aspects.

Conclusions

This study examined the mediating effect of depressive symptoms in the relationship between activity engagement and cognitive function among older adults using a nationally representative sample from the U.S. The results showed that older adults participating frequently in physical, cognitive, and social activities had significantly higher cognitive function. Notably, these relationships operated not only through direct pathways but also through indirect pathways by reducing depressive symptoms. In other words, activity engagement exhibited complex mechanisms that directly improve cognitive function and indirectly contribute to cognitive health by reducing depressive symptoms. These findings highlight the need for an integrated approach to promote cognitive health in community-dwelling older adults by developing multimodal intervention programs that combine physical, cognitive, and social activities while simultaneously addressing mental health conditions such as depressive symptoms.

Figure 1.

Mediating effect of depressive symptoms in the relationship between types of activity engagement and cognitive function.

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