Introduction

The World Alzheimer’s Report predicts that nearly 131.5 million people globally will suffer from dementia by 20501. Cognitive dysfunction is a hallmark characteristics of dementia, including Alzheimer’s disease ( AD ). This includes symptoms such as mild memory impairment, inattention and others. These symptoms typically emerge gradually in older adults, and the risk increases with age. The number of people with cognitive impairment and dementia is rapidly increasing at a rate of 300,000 per year2Recent epidemiological data suggest that the prevalence of mild cognitive impairment among the elderly in China is 20%3. Cognitive function is related to many factors, such as demographic characteristics, physical function, psychological problems, lifestyle and so on. The decline of cognitive function will affect the daily life of the elderly from various aspects, seriously affecting the quality of life and well-being of the elderly, and the resulting physical, psychological and social problems may lead to further decline in cognitive function, forming a vicious circle.

Linking depression to cognitive function

In neurology, studies have shown that depression has a negative impact on cognitive function, and this effect is more serious in the elderly4. Some longitudinal studies have shown that the elderly with depression have a higher risk of cognitive impairment5. Cross-sectional analyses reveal that specific cognitive functions, like memory and language, are impacted by depression in the later years of the elderly6. The characteristics of severe depression include changes in emotional state and impairment of cognitive function, while emotion is associated with cognitive dysfunction, both of which play a key role in the development of depression7. There is also evidence that there is cognitive impairment during the first episode of depression, and individuals with multiple episodes show greater cognitive impairment than individuals with single episodes8. Studying the factors that predict the cognitive performance of patients with depression can provide ideas for exploring how depression leads to cognitive impairment9. Therefore, the following hypothesis holds:

H1:Depression has a significant predictive effect on cognitive function in the elderly.

The mediating role of social activities.

Studies have shown that there is a cross-sectional association between social activities and cognitive function in the elderly population10. Better social connection is associated with higher well-being, better health outcomes and lower mortality, and is an aspect of successful aging11. The level of social isolation is low, that is, the elderly with higher participation in social activities and larger social networks have better cognitive function12. Participation in social activities such as leisure activities, cultural activities, and sports activities is associated with higher levels of cognitive function, but such as religious activities may be inversely related to cognitive function13. A regression model was constructed based on cross-sectional data, and the results showed that there was a negative correlation between social activity and depression14. This association is different between urban and rural elderly in China, and there is a significant correlation between specific social activities and depression15. A part of the moderated mediation model constructed by Weibo Ma et al. showed that social relations related to social activities can play a regulatory role between depression and cognitive function16. Social frailty can be defined as the lack of important social resources, social activities and self-management ability to meet basic social needs. Social frailty is associated with depression and cognitive function17. Therefore, we propose this hypothesis:

H2: Social activities play a mediating role in the relationship between depression and cognitive function in the elderly.

The mediating role of activities of daily living

Activities of Daily Living (ADL) are divided into two categories: Basic Activities of Daily Living (BADL) and Instrumental Activities of Daily Living (IADL).Cohort studies have shown that incorporating the degree of IADL damage into the structure of mild cognitive impairment can significantly improve the prediction of dementia18. The evaluation of BADL and IADL is helpful for the diagnosis of neurocognitive dysfunction in the elderly19,20. A number of studies have suggested that the relationship between cognitive function and ADL may be two-way21,22. Depression has an impact on the ADL of the elderly, and some studies suggest that depression is the third largest contributor to disability23. In a relatively short period of time, severe depression can also have a negative impact on ADL, and may be stronger than other traditional risk factors24. Through Meta-analysis, Green SL and other scholars believe that depression can significantly affect ADL and can be used as a predictor of ADL25. The results of Palacios-Navarro G et al. showed that ADL-based task can be used to detect cognitive impairment. At the same time, the task can also be used to promote cognitive function and prevent dementia26. The mediation model developed by C Peng et al. demonstrates that maintaining or improving activities of daily living, coupled with reducing depression, can lead to better cognitive function performance27. Therefore, the following assumptions are made:

H3: Activity of daily living plays a mediating role between depression and cognitive function in the elderly.

The roles of social activities and activities of daily living ability

Through the above literature analysis, it can be seen that social activities and activities of daily living play an important role between depression and cognitive function. According to the research results, the intervention group participating in social activities can maintain the level of ADL compared with other groups28. A number of studies have shown that social activities and leisure activities can play a positive role in maintaining the ADL of the elderly29. Compared with exercise alone, the incidence of functional disability in the elderly who exercise with others is lower30. ADL disability is associated with social participation, and social participation is also affected by ADL31. Qiuhong Li et al. constructed a mediating model of social interaction, lifestyle and depression between cognitive function and IADL, and all three played a complete mediating role32. The model contains a variety of variables covering biological, psychological and social aspects. The model and research results are consistent with the ' biological-psychological-social ' medical model proposed in the theoretical system of social medicine. Accordingly, the following assumptions are made:

H4: Social activities and activities of daily living play a chain mediating role between depression and cognitive function in the elderly.

Based on the four hypotheses mentioned above, this study constructs a chain mediation hypothesis model as shown in Fig. 1.

Fig. 1
figure 1

The chain mediation model of depression and cognitive function in the elderly.

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The purpose of this study is to explore the possible path of depression affecting cognitive function in the elderly population, and to propose interventions to protect the cognitive function of the elderly. Our research has made the following contributions. First of all, we proposed and verified a chain mediating model of depression affecting cognitive function, and considered that social activities and activities of daily living can be used as intervention factors. Secondly, most of the previous studies focused on the impact of physical status on psychological status, and we innovatively shifted the perspective. This study attempts to improve the psychological level of the elderly, in order to achieve the improvement of the physical condition of the elderly.

Research methods

Sample

The data used in this study are from the China Health and Retirement Longitudinal Study ( CHARLS ), which was released in September 2020. This study represents a large-scale interdisciplinary survey project, overseen by the National Development Institute of Peking University and conducted by the China Social Survey Institute. The purpose is to provide a more scientific basis for effectively alleviating the aging problem and improving the relevant policies of the elderly population by collecting high-quality data of individuals and families of the middle-aged and elderly people in China. CHARLS covers 150 counties and 450 communities ( villages ) in 28 provinces ( autonomous regions and municipalities directly under the central government ) across the country. The sample comprises 19,000 respondents across 12,400 households. In this study, a sample of individuals aged ≥ 60 years was selected for analysis, and the missing and illogical data of important fields were excluded. Finally, 7547 subjects were included in the study.

Measurements

Depression self-rating scale ( CES-D ) : the scale contains 10 symptom items, each item using 4-level Likert scale score, 0 = very little or no ; 1 = not much ( 1 ~ 2 d ) ; 2 = sometimes or half of the time ( 3 ~ 4 d ) ; 3 = most of the time ( 5 ~ 7 d ). The total score was between 0 and 30 points. The higher the score, the more serious the depression. The Cronbach’s alpha reliability score was 0.79.

Activities of daily living scale ( ADL ) : including physical self-care ability ( BADL ) and instrumental activities of daily living ( IADL ) in two parts. Physical self-care ability, as adapted from the CHARLS questionnaire, encompasses six functions: bathing, dressing, eating, getting up, toileting, and controlling urination and defecation. IADL includes six abilities: doing housework, cooking, shopping, calling, taking medicine and managing money. Assignment is based on the response to each ability: 1 = no difficulty; 2 = Difficult but can still be completed; 3 = have difficulties, need help; 4 = unable to complete. The total score was between 0 and 48 points. The higher the score, the worse the daily activity ability. The Cronbach’s alpha coefficient of internal consistency was 0.82.

Social activities: extracted from the CHARLS questionnaire ' Have you conducted the following social activities in the past month? The options for this item include ten common types of social activities. Each participation in a social activity score 1 points. The total score is between 0 and 10 points. The higher the score, the more types of social activities.

Mini-Mental State Examination ( MMSE ) : including orientation, memory, attention and computing power, memory ability, language ability five aspects. According to the results of each answer, the score is assigned. The correct answer to 1 question is 1 point, and the wrong answer is 0 point. The total score was between 0 and 30 points. Lower scores indicate greater cognitive impairment. The Cronbach’s alpha reliability score was 0.85.

Statistical analysis

SPSS 26.0 software was used to analyze the data. The descriptive statistics of the variables and the correlation between the variables were tested by Pearson correlation test. Regression analysis and mediating effect analysis were performed using Model 6 in the Process 4.1 program compiled by Hayes. The mediating effect was tested by repeated sampling 5000 times through the Bootstrap method.

Results

Descriptive statistics and correlation analysis between variables

Table 1 shows the descriptive statistical results of the demographic characteristics of the participants. The independent variables, dependent variables and two mediating variables involved in this study, and the correlation between each two variables is the premise of the mediating effect test. Therefore, it is necessary to first analyze the Pearson correlation between depression score variables, recent social activity variables, daily activity ability variables and cognitive function score variables. The results are as shown in Table 2. The depression score of the elderly is significantly positively correlated with the daily activity ability score. The recent social activity is significantly positively correlated with the cognitive function score. The depression score is significantly negatively correlated with the recent social activity and cognitive function score. The daily activity ability score is significantly negatively correlated with the recent social activity and cognitive function score.

Table 1 Characteristics of the participants.
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Table 2 Descriptive statistics and correlation analysis of each variable.
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Chained mediation analysis

In line with the research hypothesis, the SPSS Process macro, Model 6, as developed by Hayes, was utilized to examine the mediating role of recent social activities and daily activity ability between depression and cognitive function, employing 5000 Bootstrap resamples for testing. The advantage of the Bootstrap method is that it does not require the variables to satisfy the normal distribution and is more sensitive to the results33. Gender, age, marital status, and drinking status were used as control variables, depression scores were included as independent variables, recent social activities and daily activity ability scores were used as mediating variables, and cognitive function scores were used as dependent variables for regression analysis. The results are shown in Table 3; Fig. 2. The depression score has a positive predictive effect on the daily activity ability score, and has a negative predictive effect on the recent social activity and cognitive function score. Recent social activities have a positive predictive effect on cognitive function scores and a negative predictive effect on daily activity scores. Daily activity ability score has a negative predictive effect on cognitive function score.

Table 3 Model regression analysis.
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Fig. 2
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The chain mediation model and the β of each path.

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The results of mediating effect analysis are shown in Table 4, and the total effect of depression score on cognitive function score is −0.1232. Among them, the direct effect was-0.0454, accounting for 36.85% of the total effect, and the 95% confidence interval did not contain 0, indicating that the effect was significant; the indirect total effect is −0.0778, accounting for 63.15% of the total effect, and the 95% confidence interval does not include 0, indicating that the effect is significant. There are three paths in the indirect effect: the individual mediating role of recent social activities, the individual mediating role of daily activity ability, and the chain mediating role of recent social activities → daily activity ability. The 95% confidence intervals of the three paths do not include 0, indicating that the indirect effects are significant. The mediating effect of recent social activities and daily activity ability accounted for 15.99% and 44.64% of the total effect, respectively. The chain mediating effect of recent social activities → daily activity ability accounted for 2.44% of the total effect.

Table 4 Analysis of model mediating effect.
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Discussion

The results of this study show that depression can significantly predict cognitive impairment in the elderly population in China, which is consistent with the research results of Wei J et al.6, that is, the higher the depression score, the more serious the depression, the lower the cognitive score, the more serious the cognitive impairment. Therefore, this study assumes that H1 holds. At the same time, HE Yan and other scholars’ research shows that the decline of cognitive level will also lead to the aggravation of depression, and other factors will play a mediating role in this process34. Elderly individuals experiencing memory decline, a component of cognitive, may encounter difficulties in their daily life and work, and then gradually produce psychological pressure, even lead to anxiety or depression, and this negative psychology will aggravate the damage of their cognition. This situation shows from a perspective how depression and cognitive impairment interact with each other. There are many similar ways of interaction between the two, which cannot be fully listed. Most scholars’ research results show that there is a significant correlation between depression and cognitive impairment, and the aggravation of depression leads to the decline of cognitive level, which leads to the further aggravation of depression. The treatment of depression may alleviate cognitive impairment resulting from depression. Therefore, this study takes depression as an independent variable to find a possible way to protect or improve the cognitive level of the elderly.

This study found that in the elderly population in China, recent social activities play a significant mediating role in the process of depression affecting cognition, that is, the more severe the depression, the less recent social activities, and the less recent social activities, the worse the cognitive level, which proves that the research hypothesis H2 is established. In the correlation between depression and social participation, Holtfreter K and other scholars believe that social participation is significantly negatively correlated with depression14, which is consistent with the results of this study. Different from this study, the scholars’ main concern is the impact of depression on social participation. This may depend on the focus of their research. One of the typical manifestations of depression is depression and loss of interest or pleasure. The more severe the depression, the more difficult it is to obtain happiness and satisfaction from social activities, so the time originally used for social interaction is placed on self-tanglement. In the theoretical system of social medicine, Engel put forward the ' biological-psychological-social ' medical model. According to this medical model, an individual’s psychological issues can lead to difficulties in their social activities; the problems at the level of social activities will also affect their physiological conditions, which is consistent with the results of the impact of social activities on cognition in this study, and the results of Fu C et al.10.

In the results of this study, the mediating effect path of depression affecting cognition by affecting daily activity ability accounted for the highest proportion of indirect effects, accounting for 44.64%. The ratio of mediating effect and direct effect of this path is 121.15% (−0.0550/−0.0454). The more severe the depression, the worse the daily activity ability, and the worse the cognitive level, which confirms the hypothesis H3 of this study. YUAN Mei and other scholars used depression as an intermediary variable between ADL and cognition to explore the mediating effect of depression. The correlation between ADL and depression and the correlation between ADL and cognition were consistent with this study35. C Peng et al. also reached a consistent conclusion when exploring the mediating effect of ADL27. The research of these two scholars focused on the impact of daily activity ability on depression, while this study focused more on the decline of daily activity ability caused by depression. The difference in focus leads to the difference in the direction of influence between these studies and this study, which can also reflect the uniqueness of this study. It is worth noting that Wang J and other scholars believe that the decline of cognitive level precedes the deterioration of ADL function through their model36. However, its research does not indicate whether the protection of ADL can prevent cognitive decline, while other scholars have proved this point28. In addition, the longitudinal study of Sun Q and other scholars showed that in a certain year, the self-care ability of daily living of the elderly as a risk factor for cognition did not show significance, which may be due to the small sample size or the urban population22. In the theory of health management, psychosomatic diseases are summarized, and it is believed that biological or physical factors are the basis of the pathogenesis of some diseases, while psychological factors play a ' fuse ' role. In the relationship between physical problems and psychological problems in the theory of psychosomatic diseases, psychological problems can lead to physical diseases, which theoretically explains the process of depression affecting daily activity ability in this study. The influencing factors of cognitive impairment and depression are different, mainly mental disorders caused by objective factors including external environment, somatic nerves, genetics and other objective factors. There is also a relationship between physical problems and psychological problems to produce organic and symptomatic mental disorders. On the surface of this relationship, the ability of daily activities in this study has a theoretical basis for the process of affecting cognitive level.

This study demonstrates that depression sequentially impacts the cognition of the elderly in China via a chained mediating pathway involving recent social activities and daily living abilities. Although this pathway contributes a relatively small proportion of the effect in this study, the results are statistically significant, thus validating Hypothesis H4. The main difference from the two separate intermediary paths mentioned above is that this path incorporates the correlation between social activities and daily living ability. These two are important indicators of the health status of the elderly. GENG Shuo-ji and colleagues contend that the daily activity abilities of the elderly are impaired to varying extents, which will lead to a significant reduction in social participation37. The correlation between the two is consistent with the results of this study. Depressed people will reduce communication with others and avoid social activities as much as possible to protect their psychological safety. The reduction of social activities also means that the training of language, sports and other related abilities has declined for the elderly. If the elderly ‘s various activities are not exercised for a long time, the related abilities will decline rapidly. In the end, the weakening of language ability and exercise ability will make it difficult for the elderly to obtain new information to exercise their thinking ability. The degradation of brain-related functions shows a decline in cognitive level. This study focuses more on the psychological impact on the body from the inside out, that is, the psychosomatic diseases mentioned above; at the same time, it also highlights the role of social factors on biological factors and psychological factors on biological factors in the ' biological-psychological-social ' medical model. This unconventional idea may be easily ignored by most scholars and people because of its low weight. With diminishing breakthroughs in medical technology, it becomes increasingly important to promote health from other perspectives. This study highlights this point and can also meet the definition of health by the World Health Organization, that is, physical, psychological and social aspects are in good condition.

The above research results and discussions can be seen that the impact of social activities and activities of daily living on cognition is obvious. Among them, the key to how to promote the social activities of the elderly is how to make the elderly get satisfaction and happiness from participating in social activities. The elderly will be willing to continue this social activity only if they can obtain happiness from it, and the acquisition of happiness will change due to various factors. It is a good way to take different measures to promote the social activities of the elderly in different regions. At the same time, listening to the ideas of the elderly to improve measures, giving full play to the subjective initiative of the elderly, and improving their sense of participation are also part of social activities.

In addition, for the elderly, it is more important and easier to maintain a good ability of daily activities than to improve. In this regard, it is necessary to minimize the risk factors that will lead to impaired physical activity in the elderly, the most important of which is to prevent falls in the elderly. Fall is one of the main causes of direct or indirect death of the elderly in China. Preventing falls has a protective effect on all the factors mentioned in this study. Secondly, it is gradually improved on the basis of maintaining the activity ability of the elderly. The physical function of the human body is a process of use and abolition. It is necessary to build relevant public sports equipment, implement relevant promotion policies, and strengthen the exercise of the elderly. Exercise itself can not only be improved, but also a process to maintain the ability of daily life activities.

This study also has some limitations. First of all, the data used in this study belong to the second-hand data nationwide, so the collection of statistical cycle is long, which leads to the lack of timeliness of the data. At the same time, this study counts the types of participation in social activities, rather than the frequency of participation. If the frequency of participation in social activities is analyzed, the results may be slightly different. Finally, this study is a cross-sectional analysis, without considering the long-term impact, future research can consider longitudinal study.

The increasingly serious situation of depression and cognition in the elderly population in China makes the psychological and mental status of the elderly receive the attention of all sectors of society. Exploring an effective way to alleviate these problems and improving the quality of life of the elderly is one of the important measures to reduce the burden of old-age care and achieve healthy aging. This study innovatively starts from the depression status, and proposes to first improve the depression status to break the vicious circle between depression and cognitive decline. At the same time, we proposed and verified two mediating variables in the process of depression affecting cognition, namely social activities and activities of daily living. We believe that in the process of protecting cognition by alleviating depression, the protective effect can be enhanced by increasing social activities and protecting activities of daily living.

Conclusion

The results of this study support the hypothesis that depression can indeed affect the cognitive function of the elderly. For older individuals, the more severe the depression, the worse the cognitive function. In this study, social activities and activities of daily living were used as the mediating variables in the process of depression affecting cognitive function, and a hypothesis model was constructed accordingly. Furthermore, this study substantiates the hypothesis through data analysis.