SS encompasses assistance from individuals, groups, and communities, serving as a crucial external resource that contributes to positive experiences, stress reduction, and overall wellbeing throughout an individual's lifespan. As outlined by Xiao, SS comprises three facets: objective support, subjective support, and support utilization. Objective support involves material assistance, the existence of social networks, and participation in group relationships. Subjective support pertains to the emotional experiences and satisfaction of individuals feeling respected, supported, and understood in society. Support utilization refers to the extent to which individuals make use of SS (9).

Drawing from the regulation model of lifelong development theory and the adaptive function of control strategies, SA can be understood as an individual's capacity to access long-term self-coordination. Individuals can optimize lifelong development management by enhancing resource inputs and employing compensatory strategies to regulate lifelong development successfully (10). SS, as a vital external resource, plays a positive role in facilitating long-term access to resources or potentials, counteracting intrinsic aging processes, and thereby promoting SA. Conversely, a lack of SS resources hinders the regulation of lifelong development, negatively impacting the SA process.

Empirical studies support the significant impact of SS on SA (11). For instance, a survey involving 210 sexual minority men over 50 highlighted high levels of SS as crucial protective factors for SA. Among older sexual minority men, maintaining social contacts, reducing loneliness, and improving the quality of life were identified as determinants of SA (12). A longitudinal study of 230 adults aged 36–65 (134 with HIV, 96 without HIV) revealed that a substantial increase in SS correlated with enhanced SA. High SS was found to improve SA among individuals with HIV and older adults, even in the face of declining health status (13). These findings underscore SS as a significant predictor of SA, forming the basis for Hypothesis 1.

Hypothesis 1: There is a significant positive relationship between SS and SA.

The concept of Meaning in Life (MIL) was initially introduced by psychiatrist Prof. Frankl, denoting an individual's perception of the purpose and value of their existence. MIL encompasses a sense of life's value and fulfillment, comprising both the pursuit and experience of meaning (14). Frankl contends that the quest for meaning is an intrinsic motivator for life, helping individuals endure “any” pain positively and optimistically. Conversely, a lack of MIL may lead to feelings of boredom, emptiness, and adversely impact an individual's physical and mental health (14).

SA is not solely influenced by SS but is also directly connected to MIL. The mid-range nursing theory of SA posits that as individuals age, somatic functioning gradually changes. Those who can experience spiritual connectivity and perceive meaning and purpose in life are better equipped to adapt to these somatic changes, thereby achieving SA (4). Yu's study of 339 community-dwelling older adults in Chongqing, China, demonstrated an association between SA, positive mental health, and higher levels of MIL. Older adults with elevated mental health and MIL levels were more likely to exhibit optimistic attitudes and better adaptability to life, constituting important protective factors for promoting SA (15). Similarly, Huiping's study of 417 retired teachers in Kunming revealed a significant positive correlation between MIL and SA, suggesting that a greater sense of meaning in life increases the likelihood of achieving SA (16).

Moreover, there exists a close correlation between SS and MIL. The main effect model of SS posits that SS has a positive gain effect on individual development. Increased SS perception leads to positive life intentions, heightened positive emotions, greater enthusiasm for life, and an active pursuit of the meaning and value of life, fostering a sense of belonging, fulfillment, and significance (17). Conversely, decreased SS perception is associated with negative emotions, such as loneliness and despondency, and a reduced likelihood of experiencing positive aspects of MIL. Empirical studies affirm the impact of SS on MIL, indicating that good SS is fundamental for enhancing MIL and serves as a motivational source for exploring the meaning of life (18). Other studies suggest that SS influences older adults' MIL, with positive social relationships promoting MIL development (19, 20). Lin's study of 215 older adults revealed a positive correlation between SS and MIL (21). Martela's research further confirmed that SS independently influences MIL (22). Thus, both theoretical and empirical studies affirm the correlation between perceived SS and MIL. Higher SS levels contribute to increased MIL, potentially mediating the relationship between SS and SA. Based on this, Hypothesis 2 is proposed.

Hypothesis 2: MIL mediates the relationship between SS and SA.

MIL is a psychosocial variable that has the potential to contribute to improved SA and positively predict SA outcomes. However, it is observed that not all older adults in nursing homes with the same MIL exhibit the same level of SA. This variability could be attributed to various factors, including physiological and psychological aspects of older adults. MIL may be moderated by other factors, particularly in the case of older adults with poor health. The significance of the positive effect of MIL on SA remains a subject of debate.

Frailty is characterized as a complex clinical state involving acute changes in the health status of the organism, diminished ability to maintain homeostasis, reduced resistance to disease and stress due to age-related declines in reserve capacity, and deterioration in the functioning of multiple organ systems (23). The theory of active aging underscores that health is fundamental to active aging, and higher degrees of frailty and poorer health in older adults are less conducive to SA. The SOC model of SA emphasizes that despite experiencing resource losses during aging, individuals also gain opportunities and positive energy. The key to SA lies in balancing the negative energy lost with the positive energy gained. In the context of aging, frailty, and declining physical functioning, “gains” such as MIL and family care can play protective roles, guiding older adults toward successful aging (8).

A Swedish study with 618 older adults confirmed a significant negative correlation between frailty and MIL (24), indicating that higher levels of meaning are associated with better health. Conversely, more debilitated older adults tend to perceive themselves as more useless, resulting in lower MIL. Gordon's research suggests that frailty is a crucial factor influencing SA, and interventions to alleviate frailty in older adults can contribute to SA (25). Additionally, Huiping explored the path of MIL on SA using psychological capital as a mediator and age as a moderator among retired teachers (16), yet no study has investigated the correlation between MIL and frailty as a moderator variable. The moderating effect of frailty on MIL and SA warrants further investigation. In summary, a high level of frailty may weaken the protective effect of MIL on SA, while a low level of frailty may enhance the impact of MIL on SA. Frailty is proposed to play a negatively moderating role between MIL and SA among older adults in nursing homes, forming the basis for Hypothesis 3.

Hypothesis 3: Frailty negatively moderates between MIL and SA.

Presently, there is a lack of comprehensive studies examining the interplay between SS, MIL, frailty, and SA. This study formulates research hypotheses based on the main effects model of SS and the SOC model of SA, with the objective of exploring a complex moderated mediation model elucidating the relationship between SS and SA in older adults residing in nursing homes (see Figure 1). The study aims to achieve the following objectives: (a) ascertain whether SS significantly predicts SA; (b) assess whether MIL mediates the relationship between SS and SA; and (c) examine whether frailty moderates the relationship between MIL and SA.

The convenience sampling method was employed to select older adults residing in six nursing homes in Sichuan Province from March 2023 to August 2023 as participants for the study. Inclusion criteria encompassed individuals who were (1) aged 60 years or older, (2) residing in the nursing homes for at least 12 months, and (3) providing informed consent and willingly participating in the survey. Exclusion criteria involved the presence of cognitive impairment, consciousness disorder, language communication disorder, or any other conditions hindering cooperation.

A total of 600 questionnaires were distributed, and 541 were collected after excluding invalid submissions, resulting in a validity rate of 90.2%. The participant demographics were as follows: 212 males (39.2%) and 329 females (60.8%); the average age was 75.07 ± 67.03 years, with an age range of 60–92 years. Regarding education level, there were 98 individuals classified as illiterate (18.1%), 246 as elementary school graduates (45.5%), 96 as junior high school graduates (17.7%), 64 as senior high school graduates (11.8%), and 37 with specialization or higher education (6.8%). Marital status distribution consisted of 372 married individuals (68.8%), 153 widowed individuals (28.3%), and 16 divorced individuals (3.0%). In terms of the number of children, 10 individuals (1.8%) had none, 118 (21.8%) had one, 185 (34.2%) had two, and 228 (42.1%) had three or more. Regarding per capita monthly household income, 131 individuals (24.2%) had < 2,000 RMB, 234 (43.3%) had 2,000–5,000 RMB, and 176 (32.5%) had more than 5,000 RMB. The distribution of chronic diseases included 7 individuals (1.3%) with none, 109 (20.1%) with one, 201 (37.2%) with two, and 224 (41.4%) with three or more chronic diseases.

This study received approval from the Ethics Committee of Deyang People's Hospital. Prior to the survey, the research objectives, significance, and instructions for completing the questionnaire were thoroughly explained to the participants. Informed consent was obtained before distributing the anonymous questionnaire, and participants were assured that their responses would be kept confidential. Throughout the survey, three investigators, all trained uniformly, conducted one-on-one surveys with older adults in nursing homes who met the inclusion and exclusion criteria.

For participants facing challenges in filling out the questionnaires due to low literacy levels or advanced age, the investigators addressed these concerns on the spot. The investigators assisted by going through the questionnaire item by item, ensuring truthful responses based on the answers provided by the patients. This approach aimed to accommodate participants with difficulties in completing the questionnaire, promoting accurate and comprehensive data collection.

The Social Support Rate Scale (SSRS), developed by Xiao Shuiyuan, was utilized to assess the level of SS in individuals (9). The scale comprises 10 items that measure three dimensions: objective support, subjective support, and SS utilization. Items 1–4 and 8–10 are single-choice questions, with response choices assigned scores ranging from 1 to 4. Item 5 is divided into A, B, C, and D, offering a total of 4 choices, each assigned scores from 1 to 4, representing “none” to “full support”, respectively. For items 6 and 7, a score of 0 is assigned if the response is “no source”, while if the response is “the following sources”, points are awarded based on the number of sources. The total score on the SSRS is the sum of the scores from all 10 items, ranging from 12 to 66. A higher total score indicates a higher level of perceived SS. In the present study, the internal consistency of the scale was assessed using Cronbach's α coefficient, which was found to be 0.86, indicating good reliability.

The Meaning in Life Questionnaire (MLQ), developed by Steger et al., was employed to evaluate an individual's MIL (26). The Chinese version of MLQ, revised by Liu et al. (27), comprises 9 questions organized into two dimensions: meaning experience and meaning seeking. Respondents rate their responses on a Likert 7-point scale, ranging from 1 (no meaning at all) to 7 (very much meaning), where higher scores indicate a higher level of meaning in an individual's life. In this study, the internal consistency of the revised MLQ was assessed using Cronbach's α coefficient, which yielded a value of 0.82. This indicates good reliability for the scale in measuring MIL among the study participants.

The assessment of SA in older adults was conducted using the Successful Aging Inventory (SAI), developed by Troutman (28). The scale comprises 20 entries distributed across 5 dimensions: inner context and meaning of existence, functional coping, transcending aging, sense of inheritance, and spirituality. Respondents provide ratings on a 5-point Likert scale, ranging from “never” to “always”. Total scores on the SAI range from 0 to 80, where a higher score reflects a higher level of Successful Aging. The Chinese version of the SAI, adapted by Cheng (29), was employed in this study, and its reliability was assessed using Cronbach's α coefficient, yielding a value of 0.81. This indicates good internal consistency for the SAI in evaluating SA among the study participants.

The Tilburg Frailty Indicator (TFI), developed by Gobbens et al. at the University of Tilburg, the Netherlands, in 2010, was utilized for the self-assessment of frailty in older adults (30). The scale comprises 15 entries organized into three dimensions: physical debility, psychological debility, and social debility. Participants use a dichotomous scoring method (0–1 points) for each entry, resulting in a total score ranging from 0 to 15. A score of ≥5 points is considered indicative of frailty, with higher scores indicating a more severe degree of debilitation. The Cronbach's alpha coefficient for the TFI is reported as 0.74. The Chinese version of the TFI, adapted by Xi Xing in 2013, was utilized in this study, with its Cronbach's alpha coefficient reported as 0.686 (31). In the present study, the internal consistency of the TFI was assessed using Cronbach's α coefficient, yielding a value of 0.64. While slightly lower than the reported values, this still suggests an acceptable level of reliability for the TFI in evaluating frailty among the study participants.

Statistical analyses utilized SPSS 26.0. Common method bias was assessed employing Harman's single-factor test within the exploratory factor analysis approach. P-P plots and Q-Q plots were employed to ascertain the departure of data from normal distribution. Descriptive analyses featured median (P25, P75), while Spearman correlation analyses examined the relationships among SS, MIL, frailty, and aging correlation. For mediation analysis, Hayes' PROCESS 4.1 macro program's Model 4 assessed the mediating effect of MIL on the SS to SA relationship. Model 14 examined frailty's moderating effect on the second half path of the mediation model (32). A simple slope approach was employed to analyze frailty's moderating role between MIL and SA. Significant differences in total SA scores among older adults in nursing homes were observed for age and marital status (Z = 8.935, p < 0.05; Z = 6.299, p < 0.05). Consequently, gender and age served as control variables. All tests utilized the bias-corrected percentile Bootstrap method, repeated 5,000 times, calculating 95% confidence intervals. Significance was determined if confidence intervals for the mediation effect, moderated mediation effect, and moderated effect did not include 0 (33). The test level was set at α = 0.05.

The exploratory factor analysis method in Harman's single-factor test was employed to validate the reliability and accuracy of the data. Findings revealed 16 factors with eigenroot values exceeding 1 in the unrotated condition. The 1st factor explained 15.78% of the variance, falling below the critical 40% criterion. This outcome indicates the absence of significant common method bias in the study's data (34).

Table 1 displays the median, quartiles, and correlations among study variables. SS in older adults from nursing homes exhibited positive correlations with MIL (r = 0.371, P < 0.01) and SA (r = 0.411, P < 0.01), and a negative correlation with frailty (r = −0.380, P < 0.01). MIL demonstrated positive correlations with SA (r = 0.538, P < 0.01) and negative correlations with frailty (r = −0.399, P < 0.01). Additionally, SA was negatively correlated with frailty (r = −0.382, P < 0.01).

To assess Hypothesis 2, Model 4 in the PROCESS 4.1 macro program was employed to examine the mediating role of MIL between SS and SA. As indicated in Table 2, with age and marital status controlled for, SS in older adults from nursing homes positively predicted SA (β = 0.400, P < 0.001) and positively predicted MIL (β = 0.339, P < 0.001). Additionally, MIL positively predicted SA in older adults from nursing homes (β = 0.374, P < 0.001). The direct effect of SS on SA was significant (β = 0.273, p < 0.001).

Using the bias-corrected Bootstrap method with 5,000 repetitive samples, the results revealed a significant indirect effect of SS on SA among older adults in nursing homes through MIL: b = 0.127, SE = 0.020, 95%CI = (0.089, 0.168). The direct effect of SS (0.273) and the mediating effect of MIL (0.127) accounted for 68.25 and 31.75% of the total effect, respectively. Therefore, the mediating effect of MIL on the relationship between SS and SA supports Hypothesis 2.

To examine Hypothesis 3, Model 14 within the PROCESS 4.1 macro program was utilized to assess the moderated mediation effect of frailty on MIL and SA. As presented in Table 3 and Figure 2, with age and marital status controlled for, MIL and frailty exhibited a significant interaction effect on SA among older adults in nursing homes (β = −0.114, p = 0.008). A bias-corrected Bootstrap method with 5,000 repetitions indicated that the 95% confidence intervals for MIL and the moderating cross-sectional items did not include 0 [95%CI = (−0.197, −0.030)]. Frailty demonstrated a moderating effect on the indirect effects of SS and SA in older adults in nursing homes, with a moderating index of −0.039, SE = 0.020, and 95%CI = (−0.082, −0.003). MIL significantly mediated the relationship between SS and SA in older adults in nursing homes when frailty was low (one standard deviation below the mean): ab = 0.143, 95%CI = (0.097, 0.196). Simultaneously, MIL significantly mediated the relationship between SS and SA in older adults in nursing homes when frailty was high (one standard deviation above the mean): ab = 0.066, 95%CI = (0.010, 0.125).

To enhance comprehension of the moderating impact of frailty on MIL and SA, analyses were performed using a simple slope test. As depicted in Figure 3, when frailty was at a low level (M – 1SD), MIL of older adults in nursing homes significantly positively predicted SA (βsimple = 0.422, P < 0.001). The positive predictive effect of MIL on SA was also significant (βsimple = 0.194, p = 0.004) when the level of frailty was high (M + 1SD), though the predictive effect was relatively diminished. Consequently, the predictive influence of SS on MIL tended to decrease as frailty increased, indicating that frailty played a moderating role in the effect of MIL on SA.

The findings of the present study revealed a significant negative correlation between SS and SA among older adults in nursing homes, corroborating Hypothesis 1 and aligning with previous research results. Past investigations have consistently demonstrated that robust social support significantly contributes to enhancing SA in older adults (12, 13). On one hand, older adults in nursing homes with higher levels of SS tend to seek external assistance when facing challenges, receiving diverse support sources. This abundance of resources equips them to effectively navigate daily life challenges, thereby reducing psychological stress, alleviating mental strain, mitigating negative emotions, enhancing social adaptability, and maintaining physical and mental wellbeing (7). Such positive adaptations facilitate better adjustment to the life of older adults, thereby promoting the attainment of successful aging. On the other hand, in accordance with the regulation model of Lifespan Development Theory and the adaptive function of control strategies, individuals necessitate adaptive self-regulation and optimization of Lifespan Development management to achieve successful aging by enhancing resource input and applying the varied effects of different control strategies (10). Conversely, individuals with lower levels of SS receive fewer support resources, impeding the process of successful aging and diminishing the regulation of lifelong development. This, in turn, adversely affects the level of successful aging among older adults in nursing homes. In essence, the lower the level of SS, the poorer the regulation of lifelong development, hindering the process of successful aging. Especially after the pandemic, 35.86% of the general population sometimes or often feel lonely (35). For older adults in nursing homes, the inability of their children to accompany them, reduced exposure to social interactions, and decreased social support contribute to heightened feelings of loneliness. This exacerbates the accumulation of loneliness, and older adults experiencing severe loneliness may not proactively seek help and support when facing problems and difficulties. This forms a vicious circle that further diminishes their level of SA.

The current study's outcomes lend support to Hypothesis 2, affirming that a sense of MIL acts as a mediating factor in the connection between SS and SA among older adults in nursing homes. This finding is consistent with prior research that identifies SS as a significant predictor of MIL (19, 20, 36, 37) and underscores MIL as a vital predictor of SA (15, 16).

In line with the main effects model of SS, which posits a generalized positive impact on emotional experience, physical and psychological wellbeing, irrespective of the current level of support, SS proves advantageous to an individual's psychological wellbeing (6). Concurrently, the maintenance model of MIL asserts that the structure and function of social relationships influence an individual's MIL. Understanding social support as a social resource aids individuals in realizing MIL, fostering goal achievement, a sense of belonging, identity, and overall enhancement of MIL (38). The perception of greater SS by older adults in nursing homes equips them to effectively confront environmental challenges, actively pursue and realize the significance of life, and promote a meaningful understanding of life, ultimately enhancing their MIL. Conversely, reduced SS perception leads to heightened negative experiences of loneliness, loss, and depression, limiting opportunities for MIL realization and diminishing positive MIL experiences.

Research underscores MIL as a pivotal source of an individual's mental health, psychological wellbeing, and sense of personal worth and existence (39). It plays a crucial role in stress regulation, mental health promotion, and positive coping with illness (40). According to Frankl's theory of MIL and Flood's mid-range nursing theory of SA (4, 14), MIL stands as the most critical component of an individual's spiritual dimension. Higher MIL levels in older adults in nursing homes facilitate the mobilization of positive emotions, alleviation of negative emotions like loneliness, anxiety, and depression, and the maintenance of adaptive psychology and behaviors, ultimately contributing to the achievement of successful aging. Conversely, lower MIL levels result in increased discomfort and helplessness in dealing with stressful events, negatively impacting the physical and mental health of individuals and hindering the pursuit of successful aging.

In summary, the study establishes that SS predicts MIL and SA, MIL predicts SA, and MIL serves as a mediating factor in the relationship between SS and SA among older adults in nursing homes.

The findings of the current study supported Hypothesis 3, revealing that frailty among institutionalized older adults moderated the latter part of the indirect effect of SS on SA, specifically moderating the relationship between a sense of MIL and SA. The correlation between MIL and SA was more pronounced among older adults in nursing homes with lower levels of frailty. Conversely, this correlation weakened among those with higher levels of frailty. Essentially, as the debilitation of older adults in nursing homes increased, the influence of MIL on SA diminished. The impact of MIL on SA was stronger in individuals with lower frailty levels, indicating that more debilitated older adults experienced a reduced effect of MIL on SA.

Moreover, in accordance with the theory of lifelong development and the SOC model of SA, aging leads to the loss of various resources, both physical and psychological, resulting in an escalating degree of frailty. Nonetheless, older adults can attain successful aging by effectively regulating lifelong development, strategically deploying and optimizing acquired resources such as MIL (8, 10). As frailty deepens, the frequency and amplitude of resource allocation widen in the process of resource selection, compensation, and optimization. This serves to compensate for the losses incurred due to frailty, making it more challenging for individuals to achieve successful aging through the acquisition of resources like MIL.

In summary, higher levels of frailty among older adults in nursing homes diminish the impact of MIL on successful aging.