This study conducted a systematic review and synthesis of existing evidence examining the relationships among functional ability including ADL and IADL, depressive symptoms, and/or social networks in older adults.

This systematic study sought to elucidate the interconnections between functional ability, psychological depression, and social networks in older populations, with the goal of identifying critical factors relevant to everyday functioning. A systematic literature search was conducted using the keywords “older adults,” “functional ability,” “depression,” and “social networks” to identify studies published between January 2015 and January 2025 in PubMed, Google Scholar, and MEDLINE. Specifically, this study extracted only those papers published for 10 years, but within the most recent five years. A total of 5,866 records were initially identified across the databases. Following title and abstract screening, 5,369 articles were assessed for full-text eligibility. After full-text review and application of the exclusion criteria, 20 studies were ultimately included in the systematic review and analysis, as illustrated in Fig. 1.

The study selection process is illustrated in the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA) diagram shown in Fig. 1. The inclusion criteria for this study were as follows: (a) studies involving older adults aged 65 years or older; (b) studies examining functional ability, depression, and/or social networks; (c) quantitative or qualitative empirical studies; (d) articles published in peer-reviewed international journals; (e) publications written in English; and (f) studies published between 2015 and 2025. Studies were excluded if their primary objective was not related to ADL/IADL, depression, or social networks, or if they were non-human studies. Additional exclusion criteria were as follows: (a) conference abstracts, editorials, commentaries, or letters; (b) case reports or single-case studies; (c) studies lacking relevant outcome variables; and (d) duplicate publications across databases.

The methodological quality of the included studies was assessed independently by two reviewers using established critical appraisal tools appropriate to each study design. For observational studies, including cross-sectional and longitudinal cohort studies, a modified version of the Newcastle–Ottawa Scale was applied (Carra et al., 2025). This tool evaluates study quality across three domains: selection of study groups, comparability of groups, and assessment of outcomes or exposures. For qualitative and mixed-methods studies, the Critical Appraisal Skills Program (CASP) checklist was used to assess methodological rigor, clarity of research aims, appropriateness of study design, data collection, analytical procedures, and relevance of the findings (Duval et al., 2023). Studies involving methodological or predictive model development, including machine learning-based depression screening studies, were evaluated using adapted quality criteria focusing on sample representativeness, transparency of model development, validation procedures, and interpretability of results. Each study was assigned an overall quality rating (high, moderate, or low) based on predefined thresholds. Any discrepancies between reviewers were resolved through discussion, and when necessary, by consultation with a third reviewer. No studies were excluded solely on the basis of quality assessment.

Owing to the heterogeneity in study designs, populations, and outcome measures across the included studies, a meta-analysis was not performed. Instead, a narrative synthesis approach was adopted in accordance with PRISMA 2020 recommendations. The included studies were first categorized by study design (cross-sectional, longitudinal cohort, qualitative/mixed-methods, and methodological studies). Subsequently, findings were synthesized thematically based on key domains of interest, including functional capacity, depressive symptoms, and social networks. For quantitative studies, the direction and consistency of associations between key exposures (e.g., ADL/IADL impairment, social isolation, physical activity patterns) and outcomes (e.g., depression, frailty, quality of life) were systematically compared. Longitudinal studies were given greater interpretive weight when temporal relationships or risk trajectories were reported. Qualitative and mixed-methods studies were analyzed using a thematic aggregation approach to identify recurring patterns in older adults. Methodological and predictive model studies were summarized descriptively, with emphasis placed on analytical validity and clinical applicability. Results were presented using structured summary tables detailing study characteristics and quality assessments, accompanied by narrative synthesis to integrate findings across studies.

As shown in Table 1, the methodological quality of the included studies was moderate to high. Longitudinal cohort studies generally demonstrated higher quality scores due to robust study design, adequate follow-up duration, and appropriate control of confounding variables. Cross-sectional studies showed moderate quality, with limitations primarily related to causal inference and potential residual confounding. Qualitative and mixed-methods studies met most CASP criteria, particularly in terms of methodological rigor and relevance, although transferability was occasionally limited. No study was excluded on the basis of low methodological quality.

A growing body of evidence demonstrates a close association between functional ability and depressive symptoms among older adults. Research involving community-dwelling older populations has shown that greater impairment in functional ability is significantly associated with higher scores on the geriatric depression scale, indicating that age-related declines in functional capacity constitute an important contributor to depression in later life (Altun and Bulca Acar, 2025). Consistent findings have been reported in several studies, which revealed that poorer physical health status is strongly linked to elevated levels of depressive symptoms. Diminished physical health, negative self-rated health perceptions, and increased limitations in IADL were all associated with higher depression severity (Jahn and Cukrowicz, 2012). Beyond these cross-sectional associations, longitudinal and mechanistic studies suggest a reciprocal relationship between ADL and depression. A study identified two feedback pathways whereby functional dependence heightens vulnerability to depressive tendencies, while depressive symptoms subsequently exacerbate functional decline (Greer et al., 2010). This bidirectional relationship indicates a self-reinforcing cycle in which declining functional independence and worsening mental health mutually influence one another. Supporting this perspective, Barry et al. (2011) reported that depressive symptoms significantly increased the risk of subsequent ADL impairment among older adults, further emphasizing the dynamic and interdependent nature of physical and psychological functioning in aging populations. A review of recent data on the relationship between ADL and depression is shown in Table 2.

The studies summarized in Table 2 collectively illustrate how functional ability, social resources, and psychosocial factors converge to shape mental health and well-being in older adults. Recent large-scale observational and longitudinal studies emphasize that aging-related outcomes cannot be sufficiently explained by physical or psychological factors alone but rather emerge from their interaction within broader social and behavioral contexts. Several studies highlight the central role of functional status and physical activity in mental health outcomes. Altun and Bulca Acar (2025) demonstrated that variations in functional capacity and psychosocial well-being among older adults attending healthy aging centers were closely related to patterns of health service utilization, suggesting that functional independence and psychosocial health cluster together in real-world aging populations. Similarly, Liu et al. (2025) showed that functional disability was a strong predictor of depression in older adults, to the extent that ADL-related variables could be effectively incorporated into interpretable machine learning models for depression screening. These findings reinforce the notion that impairments in daily functioning are not merely physical limitations but also key indicators of psychological vulnerability. The importance of physical activity as a behavioral pathway linking functional health and depression is further supported by Kim et al. (2025) and Morikawa et al. (2025). Using national survey data, Kim et al. (2025) found that lower levels of physical activity were associated with higher depressive symptoms, while Morikawa et al. (2025) demonstrated that not only the amount but also the timing of daily physical activity influenced the onset of depressive symptoms over time. Together, these studies suggest that disruptions in daily activity patterns may represent an early behavioral marker of declining mental health in older adults. Social and psychosocial resources emerged as additional critical determinants of well-being. Dakua et al. (2023) reported that higher levels of social capital were associated with better psychological well-being among older adults left behind by migrant children, underscoring the buffering role of social networks in contexts of family separation and social vulnerability.

Beyond depression and social support, broader dimensions of psychosocial and functional aging were addressed in studies focusing on quality of life and frailty. Sahin et al. (2025) showed that long-term survivors treated with immune checkpoint inhibitors experienced substantial variability in health-related quality of life and psychosocial outcomes, indicating persistent functional and emotional needs even after successful medical treatment. Valeriano-Paños et al. (2025) further linked motor and cognitive decline—assessed through gait variability and working memory—to early frailty, suggesting that functional deterioration spans both physical and cognitive domains and may precede more overt disability. Taken together, the evidence summarized in Table 2 supports an integrated view of aging in which functional ability, physical activity, social resources, and mental health are deeply interconnected.

ADL and social networks are closely interconnected domains that jointly influence functional independence and psychosocial well-being in older adults. Functional capacity not only determines an individual’s ability to perform basic and instrumental tasks but also shapes opportunities for social participation and engagement (Na and Streim, 2017). As functional limitations increase, older adults often experience reduced mobility, restricted access to community activities, and diminished frequency of social interactions, which may lead to progressive social isolation (Nicholson, 2012). Empirical evidence consistently demonstrates that impairments in ADL and IADL are associated with smaller social networks and lower levels of social participation (Tomioka et al., 2016). Older adults with greater dependence on ADL are less likely to engage in social activities outside the home and often rely on a limited number of close contacts, such as family caregivers, rather than broader peer networks. Difficulties in IADL domains have been shown to significantly constrain opportunities for maintaining social ties and participating in community life (Turcotte et al., 2015). Conversely, robust social networks appear to play a protective role in preserving functional ability (Kelly et al., 2017). Social engagement and frequent interpersonal interactions provide both emotional encouragement and practical assistance, which may facilitate continued involvement in daily activities and promote adaptive coping with functional decline. Older adults with larger and more diverse social networks are more likely to receive instrumental support for ADL and IADL tasks, potentially delaying the onset of functional dependence and institutionalization (Oh et al., 2019). The relationship between ADL and social networks is increasingly recognized as bidirectional. While functional decline can restrict social participation and reduce network size, weakened social networks may also accelerate functional deterioration by limiting access to assistance, reducing motivation for activity, and increasing sedentary behavior (Li and Zhang, 2015). Social isolation has been associated with poorer physical performance, reduced physical activity levels, and greater risk of disability (Philip et al., 2020), suggesting that social networks may serve as both a determinant and an outcome of functional status in later life. These findings highlight the importance of considering ADL and social networks as interdependent components of healthy aging. Integrated interventions that simultaneously target functional capacity and social engagement, such as community-based physical activity programs, group exercise, and social engagement activities, may be particularly effective in maintaining independence and improving quality of life in older adults (Yen and Lin, 2018). Understanding the dynamic interplay between ADL and social networks is therefore essential for the development of comprehensive strategies aimed at promoting functional resilience and social connectedness in aging populations. A review of recent data on the relationship between ADL and social networks is shown in Table 3.

Early foundational evidence by Cornwell and Waite (2009) established a clear distinction between structural social disconnectedness and perceived isolation, showing that both dimensions independently contribute to poorer physical and mental health outcomes in older adults. This work provided an important conceptual framework, suggesting that objective reductions in social networks and subjective feelings of isolation each exert unique influences on health, including functional and psychological domains. More recent cross-national and longitudinal research has expanded this understanding by situating functional limitations within broader social and temporal contexts. As shown in Table 3, Kekäläinen et al. (2023) demonstrated that difficulties in ADL and IADL are highly prevalent across countries and consistently increase with age, highlighting functional decline as a universal feature of aging, albeit with contextual variation. Complementing this, Lunansky et al. (2025), using 30-year longitudinal data, showed that functional decline follows a temporal sequence, often emerging earlier than overt disability or clinical deterioration. These findings suggest that subtle changes in functional capacity may serve as early indicators of vulnerability. The role of social networks as a modifying factor is further underscored by longitudinal evidence. Manabe et al. (2025) reported that smaller or weaker social networks significantly increased the incidence of both frailty and depressive symptoms over a 2-year period, indicating that social relationships are not merely correlates but active determinants of functional and mental health trajectories. Similarly, Pengpid et al. (2025) found that among older adults living alone, social and health-related factors were key determinants of depressive symptoms and life satisfaction, emphasizing the heightened vulnerability of socially isolated populations. Several studies also illuminate specific pathways linking physical impairment, social isolation, and functional decline. Xin et al. (2024) demonstrated that social isolation and cognitive impairment mediated the association between sensory impairments and functional limitations, providing empirical support for a mechanistic model in which physical deficits reduce social engagement, which in turn accelerates functional deterioration. Narayanan et al. (2025), through a narrative review, further reinforced the global impact of physical disability, showing consistent associations between ADL impairment and reduced quality of life, thereby situating functional decline within a broader biopsychosocial context.

Depression and social networks are intricately linked determinants of mental health and well-being in older adults (Nyqvist et al., 2013). Social relationships provide emotional support, instrumental assistance, and a sense of belonging, all of which are essential for psychological resilience in later life (Thoits, 2011). Conversely, disruptions in social networks—such as reduced contact with family, friends, or community members—have been consistently associated with elevated depressive symptoms among aging populations (Stoeckel and Litwin, 2016). A substantial body of evidence indicates that older adults with smaller or less diverse social networks are at increased risk of depression. Social isolation and loneliness, which often arise from life transitions such as retirement, bereavement, or declining health, have been identified as strong predictors of depressive symptoms (Segel-Karpas et al., 2018). In contrast, frequent social interactions and participation in meaningful social activities are associated with lower levels of depression and greater emotional well-being. Both the structural characteristics of social networks (e.g., network size and frequency of contact) and the functional quality of social support (e.g., perceived adequacy and satisfaction) appear to play critical roles in shaping mental health outcomes.

The relationship between depression and social networks is increasingly understood as bidirectional. On one hand, weakened social networks may contribute to the onset and persistence of depression by limiting emotional support and increasing feelings of loneliness and helplessness. On the other hand, depressive symptoms can lead to social withdrawal, reduced motivation for social engagement, and deterioration of existing relationships, thereby further shrinking social networks (van den Brink et al., 2018). This reciprocal process can create a self-perpetuating cycle of social disengagement and worsening mental health. Moreover, social networks may function as both mediators and moderators in the development of depression among older adults. Strong social ties can buffer the negative psychological impact of stressors such as functional decline, chronic illness, and life events, whereas poor social support may amplify vulnerability to depression (Bisschop et al., 2004). Interventions that strengthen social connectedness, such as community-based programs, group activities, and peer-support programs, have been shown to be effective in reducing depressive symptoms and improving quality of life in later life. Collectively, these findings underscore the importance of addressing social networks as a core component of depression prevention and management strategies for older adults. Integrating social engagement initiatives with mental health and functional interventions may offer a more comprehensive and effective approach to promoting psychological well-being and healthy aging (Seah et al., 2019). A review of recent data on the relationship between depression and social networks is shown in Table 4.

Table 4 collectively underscores the central role of social engagement, perceived social support, and loneliness in shaping depression and overall well-being across aging and vulnerable adult populations. Although study populations and methodologies varied, a consistent pattern emerged in which diminished social connections were associated with poorer mental health outcomes. Several studies demonstrated that lower levels of social engagement and weaker social ties were strongly linked to increased depressive symptoms and loneliness. De Main et al. (2025) showed that individuals with reduced social participation experienced higher levels of depression, loneliness, and stigma, highlighting the psychological consequences of social disengagement in populations living with chronic conditions. Similarly, Islam et al. (2025), using 16-year longitudinal data, provided robust evidence that persistent loneliness predicts sustained and worsening depressive symptom trajectories over time, emphasizing the long-term mental health consequences of social isolation. Longitudinal and life-course perspectives further revealed the protective role of stable social relationships. Lin (2025) demonstrated that enduring social partnerships across adulthood were associated with better health outcomes in later life, suggesting that social connectedness accumulated over time contributes to resilience against psychological and physical decline. These findings align with evidence from Wu et al. (2025), who reported that stronger social support networks were associated with improved health and well-being among community-dwelling older adults. Several studies also clarified potential mechanisms linking social disconnection to adverse outcomes. Quach and Burr (2021) identified depression as a mediating factor between social isolation, social disconnectedness, and fall risk, illustrating how psychological distress may translate social vulnerability into tangible health consequences. In a similar vein, Pietrzak et al. (2025) found that higher perceived social support was associated with better mental health among healthcare workers, reinforcing the importance of subjective perceptions of support alongside objective social ties. Contextual and structural influences on social isolation were particularly evident in vulnerable populations. Tippens et al. (2025) highlighted how systemic exclusion and urban marginalization contributed to profound social isolation and loneliness among older refugees, demonstrating that social disconnection in later life is often shaped by broader societal and environmental factors rather than individual choice alone. Meanwhile, Shibahashi et al. (2025) emphasized the dominant role of physical capability in self-rated health, suggesting that physical function may interact with social and psychological factors to influence overall well-being in older adults. These findings reinforce the necessity of integrated assessment and intervention strategies that address not only mental health symptoms but also social engagement, perceived support, and structural barriers to social participation when aiming to promote healthy aging and psychological resilience.