Older adults, as a distinct demographic, are particularly susceptible to psychological issues due to changes in their physical health, living conditions, and social roles. In recent years, the aging population and the rise of empty nesters have contributed to a notable increase in the incidence of geriatric depression. A meta-analysis revealed that the prevalence of depressive symptoms among older adults in China was approximately 22.6% between 2000 and 2009, rising to 25.5% between 2010 and 2019 among the general older population (16). The ability to perform activities of daily living, a critical indicator of physical health, is strongly associated with the onset of depression in older adults (17). Functional abilities are commonly assessed using the ADLs and IADLs scales. IADLs assess more complex tasks required for independent living, such as shopping, using the telephone, preparing meals, housekeeping, managing medications, traveling independently, and handling finances (18). In some cases, IADLs also include walking long distances, caring for children, navigating the neighborhood, and climbing stairs (19). A decline in functional capacity may lead to feelings of uncertainty and hopelessness about maintaining independence (20).

Extensive research has established a strong relationship between functional abilities and depression (8, 20, 21). In particular, functional impairments are associated with a higher risk of depression in older men compared to women (4, 9). Kiyoshige et al. (22) found that the impact of IADL decline on depressive symptoms varies significantly across age groups, with the strongest effects observed among individuals in their 70s. Furthermore, Zhao et al. (23) demonstrated in a comparative study that rural older adult parents experience functional impairments at an earlier age than their urban counterparts. Numerous studies reinforce the connection between declining functional abilities and heightened levels of depression in older adults (23, 24).

Caring for grandchildren can significantly influence the health of older adults (15). However, research findings on the relationship between caregiving and the functional health of aged caregivers vary. Studies on this subject can be categorized into three main perspectives. The first is the “positive effect” perspective, which posits that caregiving has a beneficial impact on older adults’ functional health (25). For instance, Xu (11), using 2011–2013 CHARLS data, found that caregiving reduced hypertension among grandparents, with the health benefits being more pronounced for urban grandparents compared to rural ones. The second category, “negative effects,” argues that caregiving worsens the physical health of aged caregivers and diminishes their functional ability to perform daily tasks (26). Several studies have reported that caregiving negatively affects older adults’ physical health (27, 28). Lastly, the “no effect” perspective asserts that caregiving has no significant impact on the functional health of aged caregivers. For example, Oshio (29), using data from the 14th National Survey of Japan, concluded that caregiving neither benefits nor harms grandparents’ health.

There is no academic consensus on the relationship between caregiving for grandchildren and the mental health of older adults. The first perspective, “positive effects,” suggests that caregiving benefits the mental health of older adults (30, 31). Studies have shown that grandchildren can provide emotional support, reducing loneliness and enhancing mental well-being (25, 32). Improved mental health may also contribute to better physical health outcomes for older adults (33, 34). Burn and Szoeke (35) further argue that caregiving can physically engage older adults, slowing cognitive decline. Additionally, caregiving offers opportunities for older adults to receive financial and emotional support from their children, thereby increasing life satisfaction. The second perspective, “negative effects,” asserts that caregiving can adversely affect the mental health of older adults (28, 36). For instance, Komonpaisarn and Loichinger (37), using survey data from Thai older adults, found that caregiving negatively impacts psychological well-being. Finally, the “no effect” perspective suggests that caregiving has no significant influence on the mental health of older adults (14). Some studies even report that caregiving behaviors have no effect on the overall health or life satisfaction of aged caregivers (29, 38). While limited theoretical models have conceptualized caregiving as a moderator, empirical studies have suggested that the context of caregiving can shape health outcomes (13, 39), justifying further exploration.

This study proposes a conceptual framework (see Figure 1) where caregiving moderates the relationship between functional abilities and depression. The model is grounded in empirical research but also responds to the current lack of theoretical elaboration on this interaction in Chinese rural contexts.

Based on this framework, the following hypotheses are proposed:

This study utilizes data from the 2018 China Health and Retirement Longitudinal Study (CHARLS), a nationally representative longitudinal survey designed to reflect the conditions of residents in mainland China aged 45 and older, without an upper age limit. CHARLS aims to provide comprehensive insights into the health, economic, and social conditions of China’s aging population. To ensure representative sampling, the baseline survey covered 150 counties/districts and 450 villages/urban communities across the country, ultimately involving 17,708 individuals from 10,257 households (40). The focus of this study is on rural older adults aged 65 and above who participate in grandchild caregiving. After filtering for invalid responses, a total of 4,366 valid samples were obtained for analysis, ensuring a robust dataset for examining the interplay between caregiving, functional ability, and mental health outcomes among this population. All survey instruments were administered in Mandarin Chinese. The CES-D-10 and IADL/ADL items used in CHARLS have been validated in Chinese populations and demonstrated acceptable reliability (Cronbach’s alpha > 0.75) in previous studies (4, 41).

Ethical approval for all the CHARLS waves was granted from the Institutional Review Board at Peking University. The IRB approval number for the main household survey, including anthropometrics, is IRB00001052-11015; the IRB approval number for biomarker collection, was IRB00001052-11014. During the fieldwork, each respondent who agreed to participate in the survey was asked to sign two copies of the informed consent, and one copy was kept in the CHARLS office, which was also scanned and saved in PDF format. Four separate consents were obtained: one for the main fieldwork, one for the non-blood biomarkers and one for the taking of the blood samples, and another for storage of blood for future analyses.

In this study, respondents with missing data on age, residence, depression, or IADLs were excluded from the analyses to ensure data integrity and representativeness. The primary analysis focuses on the relationship between functional abilities, caregiving for grandchildren, and depression among older adults aged 65 and above in rural China. Participants without grandchildren or with missing depression data were excluded from the final analysis. This resulted in a total of 4,366 valid responses, as illustrated in Figure 2.

To test the study’s hypotheses, hierarchical regression analysis was employed. In the first step, control variables—including gender, age, marital status, education, personal income, smoking, and drinking—were entered into the model to account for their influence on depression. In the second step, the functional abilities variable was added to examine its direct impact on depression. Finally, in the third step, an interaction term between functional abilities and caregiving for grandchildren was included to assess the moderating effect of caregiving on the functional abilities-depression relationship. Specifically, the interaction term between functional abilities and caregiving for grandchildren was created by centering both variables and multiplying them to reduce multicollinearity, then entered in the final step of the hierarchical regression model. The significance of these interactions was evaluated using the coefficient and p-value of the interaction term. Specifically, all analyses were conducted using STATA 17.0.

The descriptive statistics for all participants are presented in Table 2. Notably, 67.8% of rural older adults reported providing care for their grandchildren, a percentage significantly higher than those who did not engage in caregiving. Additionally, 47.14% of the participants were male, and 72.35% of the respondents were married, indicating a majority of the sample had a spouse. In terms of education, 39.14% of respondents were uneducated, 48.79% had completed elementary school, and 12.07% had received secondary or higher education, highlighting the generally low educational attainment among rural older adults in China. Lastly, 47.05% of the respondents reported drinking, while 45.03% reported smoking, with relatively balanced distributions of smoking and drinking behaviors among the participants.

Table 3 presents the descriptive statistics and correlations among depression, functional abilities, and caring for grandchildren. Depression is positively correlated with functional abilities (r = −0.237, p < 0.001), suggesting that better functional abilities are associated with lower levels of depression. The relationship between depression and caring for grandchildren is weak and not statistically significant (r = −0.057), indicating little direct association. Additionally, functional abilities and caring for grandchildren are significantly correlated (r = 0.117, p < 0.001), implying that those who care for grandchildren tend to have higher functional abilities. A positive correlation was observed between functional abilities and caregiving (r = 0.117, p < 0.001), suggesting that those with better functional abilities were more likely to take care of grandchildren. While this correlation is statistically significant, the magnitude is relatively low, and collinearity diagnostics (VIF values) were within acceptable limits, indicating that multicollinearity is unlikely to bias the regression results. Nonetheless, this relationship should be interpreted with caution, and future research could apply structural equation modeling or other approaches to better disentangle these associations. The skewness and kurtosis values for all variables are within acceptable ranges, confirming that the distribution is appropriate for further analysis.

Table 4 presents the results of the full sample analysis on the effect of functional abilities on depression among older adults in rural China. Model 1 includes control variables and the moderating variable (caring for grandchildren), allowing us to observe the direct effect of grandchild caregiving on depression. In Model 2, functional abilities were added, and its effect on depression is highlighted. Model 3 incorporates the interaction term between functional abilities and caring for grandchildren to assess the moderating role of caring for grandchildren.

The results from Model 1 indicate the following for the control variables: (1) gender—women are more likely to experience depression compared to men; (2) age—depression levels in rural older adults decrease with age; (3) marital status—divorced, widowed, or never-married individuals are more prone to depression than those who are married; (4) education—higher education levels are negatively associated with depression, suggesting that the more educated the individual, the lower their depressive symptoms; (5) smoking and drinking—neither variable had a significant impact on depressive symptoms; (6) personal income—higher income is associated with lower levels of depression.

In Model 2, the addition of functional abilities reveals a negative correlation with depression, where an increase in functional abilities corresponds to a decrease in depression levels. Finally, Model 3 examines the moderating role of caregiving for grandchildren. The interaction between functional abilities and caregiving for grandchildren (β = −0.203, p < 0.001) is significant, demonstrating that caring for grandchildren moderates the effect of functional abilities on depression. As shown in Figure 2, depression decreases as functional abilities improves, and this effect is more pronounced among those who care for grandchildren compared to those who do not. This indicates that caregiving for grandchildren can moderate the relationship between caring for grandchildren and depressive symptoms in rural older adults.

To further examine the moderating role of caregiving, we conducted a stratified analysis by splitting the sample into high and low functional ability groups based on the median score. Among participants with higher functional ability, caregiving for grandchildren was significantly associated with lower depression levels (β = −0.137, p < 0.01). However, among those with lower functional ability, caregiving did not have a statistically significant effect on depression. This suggests that the mental health benefits of caregiving are more evident in older adults with greater functional independence (Figure 3).