Sample and Data Collection
Data for this study was obtained from the China Health and Retirement Longitudinal Study (CHARLS). CHARLS is a long-term tracking survey project jointly initiated by the Institute of Sociology of the Chinese Academy of Social Sciences and the National School of Development at Peking University. The project commenced in 2010 and employed a multi-stage random sampling method with stratified processing of county-level units nationwide, aiming to understand the trends and changes in China's socio-economic development through a longitudinal study of Chinese households. The survey subjects are mainland Chinese residents, and the survey covers various aspects including basic demographic information, family background, health status, economic income, education level, and labor force participation. The survey is conducted through face-to-face interviews, with a follow-up survey conducted every two years. CHARLS involves a total of 10,257 households and 17,708 individuals. The research has been approved by the Peking University Institutional Review Board (IRB 00001052-11015), and informed consent has been obtained from each participant. It represents a high-quality micro dataset of Chinese households and individuals aged 45 and above.
The data for this study was obtained from the 4th wave of the national sample of the CHARLS in 2018. A total of 3,782 subjects were ultimately included based on the following criteria: (1) Participants had multiple chronic diseases, defined as any combination of 14 specific chronic conditions, including but not limited to hypertension, dyslipidemia, diabetes, cancer, chronic lung disease, liver disease, heart disease, stroke, kidney disease, gastrointestinal diseases, emotional and mental health issues, memory-related diseases, arthritis or rheumatic diseases, and asthma; (2) Subjects with memory-related diseases (such as Alzheimer's disease, brain atrophy, Parkinson's disease) and physical disabilities were excluded; (3) No missing data on sociodemographic characteristics, socioeconomic status, and health-related variables; (4) No missing information on the scores from the Center for Epidemiologic Studies Depression Scale, Mini-Mental State Examination, and Activities of Daily Living assessment scale.
Measures
Social Participation
To assess the level of social participation, we extracted information from the CHARLS database on the social activities in which the respondents participated in the past month. The selected social participation activities included: (1) visiting friends or relatives; (2) playing mahjong, chess, or cards, or attending community activities; (3) providing help to non-cohabitating relatives, friends, or neighbors; (4) dancing, exercising, or practicing qigong; (5) participating in club or organization activities; (6) engaging in volunteer or charity work; (7) caring for non-cohabitating patients or disabled individuals; (8) attending school or training courses; (9) trading stocks (funds and other financial securities); (10) using the internet; (11) other social activities. Each activity was assigned one point, resulting in a total score ranging from 0 to 11. A higher score indicated a higher level of social participation.
Depressive Symptoms
A simplified version of the Center for Epidemiologic Studies Depression Scale (CESD) was used to measure the depressive symptoms of patients with multiple chronic conditions. The scale comprises 10 items: (1) I felt bothered by things that usually don't bother me; (2) I had trouble keeping my mind on what I was doing; (3) I felt depressed; (4) I felt that everything I did was an effort; (5) I felt hopeful about the future; (6) I felt fearful; (7) My sleep was restless; (8) I was happy; (9) I felt lonely; (10) I could not get going. Respondents were required to self-assess the frequency of their feelings and behaviors over the past week and assign scores of 0, 1, 2, or 3 based on "rarely or none of the time (<1 day)", "some or a little of the time (1-2 days)", "occasionally or a moderate amount of time (3-4 days)", and "most or all of the time (5-7 days)". Notably, items 5 and 8 are positively framed and thus require reverse scoring. The total score ranges from 0 to 30, with higher scores indicating more severe depressive symptoms. A score of ≥10 is considered indicative of depressive symptoms[36].
Cognitive function
The Mini-Mental State Examination (MMSE) was used to assess the cognitive function of patients with multiple chronic conditions, encompassing dimensions such as orientation, memory, attention, calculation, language comprehension, and expression. Orientation primarily evaluated the respondents' temporal and spatial orientation, with a total score of 10. Memory assessment involved immediate and delayed recall of three phrases (ball, flag, and tree), with a total score of 6. Attention and calculation required participants to perform simple calculations (e.g., subtracting 7 from 100, repeated five times), with a total score of 5. Language abilities were assessed through naming, repetition, reading, three-stage command comprehension (e.g., "Please pick up the paper with your right hand, fold it in half with both hands, and place it on your left thigh"), writing, and construction, with a total score of 9. Each correct answer scored one point, resulting in a total score ranging from 0 to 30, with higher scores indicating better cognitive function.
Activities of Daily Living
The Activities of Daily Living scale (ADLs) was used to comprehensively evaluate the respondents' ability to perform daily living activities. The measurement of daily living activities was divided into basic activities of daily living (BADL) and instrumental activities of daily living (IADL). BADL included toileting, eating, dressing, controlling bowel and bladder, getting in and out of bed, and bathing. IADL included shopping, using the telephone, cooking, housekeeping, taking medication, and managing finances. Respondents were asked to choose "no difficulty", "some difficulty but still able to complete", "difficulty and need assistance", or "unable to complete" based on their situation, with scores of 1, 2, 3, or 4 assigned, respectively. The total score ranged from 12 to 48, with lower scores indicating higher levels of daily living activity.
Covariates
Based on the existing research, we included several potential influencing factors as control variables in the relationship between social participation and depressive symptoms among patients with multiple chronic conditions. These factors encompass sociodemographic characteristics, socioeconomic status, and health-related variables. Sociodemographic characteristics consist of age (years), gender (1=male, 2=female), and marital status (1=married/cohabiting, 2=married/living separately, 3=divorced, 4=widowed, 5=unmarried). Socioeconomic status includes educational level (1=no education, 2=primary school or below, 3=middle school, 4=high school and above), residential area (0=rural, 1=urban), employment status (0=unemployed, 1=employed), and geographical distribution (1=eastern region, 2=central region, 3=western region). Geographical distribution is based on the theory of regional economic development in China, dividing provinces into eastern, central, and western regions. Health-related variables comprise self-rated health, sleep duration (hours), life satisfaction, and physical exercise (0=no, 1=yes). Physical exercise refers to a daily exercise duration of ≥30 minutes.
Statistical Analysis
The data for this study were analyzed using SPSS 25.0 for statistical analysis. Initially, a descriptive statistical analysis of the variables was conducted, using (M±SD) to describe the metric data, and using percentages or proportions to describe the count data. Subsequently, the correlation between the research variables was examined. Finally, a chained mediation analysis was performed using Model 6 in the PROCESS 4.0 macro. After controlling for covariates, the level of social participation was designated as the independent variable (X), depressive symptoms as the outcome variable (Y), and cognitive function (M1) and activities of daily living (M2) as the mediating variables. The mediation effects were tested and confidence intervals were estimated using the Bootstrap method with 5,000 resamples, and the significance of direct, indirect, and total effects was determined based on the confidence intervals (whether they included 0 or not).