Based on data from the 2018 cross-sectional survey of the CLHLS, this study explored the relationship between chronic disease, multimorbidity, and impairment of ADL and IADL among individuals aged 65–105 years. The overall incidences of ADLs and IADLs in the older adults’ population were 17.1% and 60.1%, respectively, with a gradual increase in impairment with age. It should be noted that the ADL scale used in this study represents basic living ability, whereas the IADL scale reflects the independent living ability and social function of the older adults, which refers to a higher quality of life. Thus, IADL damage is more apparent in older individuals.

According to a meta-analysis of 56 studies in China, the prevalence of ADL disability was 20.5% (95%CI: 17.7–23.3%); older individuals (80 + years) had a significantly higher ADL disability rate than younger elders (30.0%, 95% CI:26.2–33.9%) [8]. A study by South Korean researchers found that in 2020, ADL and IADL limitations were 6.14% and 15.49%, respectively. In the 85 + age group, 7.37% of individuals had severe disabilities for ADLs and 12.06% for IADLs. A high rate of mortality was also observed among people who were undereducated, underweight, inactive, depressed, and suffering from three or more chronic diseases [9]. A study of 49,732 older adults in Japan showed that participation in cultural leisure activities may contribute to improved IADL scores. In addition, the benefits that may be gained from social networks may boost social engagement and enhance health [10]. Impairment of ADL and IADL prevalence rates were not compared because they are assessed differently in each country. However, the prevalence of ADL disabilities increased with age, while IADL disabilities tend to occur earlier in life.

Data from 23 Chinese provinces (municipalities) were used in this study. Chronic diseases with a high prevalence in the older population included hypertension (44.1%), heart disease (17.6%), arthritis (11.0%), diabetes (10.8%), CVD (10.7%), and respiratory system disease (10.0%). This finding is consistent with the results of the recent Survey of Health, Aging, and Retirement in Europe, Swedish VAL data, and a meta-analysis of eight Asian studies [11,12,13]. Recently, chronic disease patterns in China have gradually evolved into those associated with cardiovascular, cerebrovascular, and metabolic diseases.

The findings of the study indicate that chronic diseases are prevalent in 66.3% of the older adults, and multimorbidity is prevalent in 33.7% of the older adults. As shown in Fig. 2, the prevalence of diabetes gradually decreased from the age of 65 years, indicating that the effects of diabetes on life expectancy began at least at that age or earlier. Many older individuals suffer from hypertension, heart disease, CVD, gastrointestinal ulcers, and arthritis, which first gradually increase as they grow older, peak in the 75–84 years age group, and then gradually decline with age. Figure 2 also illustrates the peak prevalence of chronic diseases and multimorbidities in the 75–84 years age group, followed by a gradual decline with age. Several scholars have studied the same database and found that the 75–79 years group is the key age group in which the quality of life of the older adults can change from good to poor, and that 42.58% of the older adults over the age of 100 years are able to take care of themselves [14]. Notably, the 2015 data from the China Health and Retirement Longitudinal Study also presented similar findings. The prevalence of multimorbidities peaked between the ages of 70–80 years and then gradually decreased in the following decades [15]. The results of a cross-sectional study conducted in Taiwan in 2013 revealed a peak prevalence of multimorbidity in the 80–89 years age group, followed by a gradual decline thereafter [16]. This phenomenon appears to be the result of a survivor bias. Theoretically, chronic diseases are more prevalent with aging, but they significantly affect the life expectancy of the older adults, and only healthy older individuals can survive till older age. This corresponded to an average life expectancy of 77 years in China in 2018. With age, the prevalence of dementia, and audiovisual impairment increases; however, they do not have a significant impact on life expectancy, as these disorders may be compensated for by family or institutional support. Other studies have shown that the incidence of chronic diseases gradually increases with age in countries with better medical conditions, such as Germany [17,18,19]. Some of this may be attributed to the relatively good medical conditions and complete chronic disease registration system in these countries. Differences in the types of multimorbidities, age stratification, awareness rate of chronic diseases, and even some older people who have limited mobility and cannot complete the survey result in different outcomes. Despite the heterogeneous results of various studies, the prevalence of chronic diseases and multimorbidity is expected to increase with economic development and population aging.

Analysis of this association revealed that hypertension, diabetes, CVD, and heart disease were the most common patterns of multimorbidity. In line with other studies on multimorbidity patterns [11, 13], two or three multimorbidities with the strongest associations were metabolic-related disorders, followed by degenerative conditions. The study was moderately comparable, although there were differences in the samples and chronic diseases. Several factors may contribute to the association between these multimorbidities, including the underlying aetiology, exposure to risk factors before onset, and the involvement of common intermediate metabolites in pathogenesis [20].

Despite the conclusion of this study that smoking is a protective factor against IADL, since 1957 when the British Medical Research Council reported an increase in lung cancer related to smoking, an increasing amount of evidence has led to the conclusion that smoking is harmful to health. In addition to causing lung cancer, cardiovascular disease, and CVD, smoking contributes to a serious reduction in life expectancy and death rates [21,22,23,24]. The prevalence of dementia and hemiplegia increases gradually with age, making smoking extremely difficult for them. As defined now, smoking may not cover the full range and dose of tobacco exposure, and smoking cessation may underestimate smoking's health risk [21]. In 2022, some researchers published an article in Nature Genetics stating that the mutation rates of single nucleotide variations and small fragment insertions, and deletions were significantly higher in smokers than in the non-smoker population; however, these mutations tended to be stable after 23 packs of smoking, which was thought to be related to DNA loss repair enhancement [25]. Based on a comprehensive review of several studies [11, 26,27,28], we still believe that smoking is injurious to health.

In older patients, being obese is generally considered detrimental to health. According to the Finnish and British Biobank cohort studies, obese and overweight older patients have a much greater risk of developing multimorbidities [29]. A Korean study involving more than 11,000 adults found that those who were overweight or obese were twice as likely to develop hypertension, dyslipidaemia, diabetes, and osteoarthritis compared to those with a normal BMI [30]. Additionally, these diseases are highly correlated, with the presence of one disease increasing the risk of the other, and many of their associations are bidirectional. Whether obese individuals have a single chronic disease or a combination of chronic diseases [11, 28], obesity increases mortality in these populations. Obesity is related to impairments in ADLs and IADLs. As life expectancy increases, the proportion of people with a BMI between 18.5–24.0 kg/m2 increases with age, indicating that their quality of life may be maintained, and they can reach an older age group.

Living in rural areas and exercising regularly reduced the risk of ADL impairment. As rural life does not offer the same convenience as urban living and health care conditions are generally poor, rural residents must be able to exercise greater self-care skills to remain healthy. Thus, healthier older adults are more likely to survive because of the health selection effect. Moreover, while the level of medical care in cities is relatively high, which can prolong life, most people live with diseases without being able to significantly improving their level of living. Additionally, older adults who regularly exercise outdoors are more capable of self-care [11, 26, 28].

There is significant evidence that depression among the elderly in many countries is on the rise, and it is significantly associated with impairments in ADLs and IADLs [31, 32]. The negative impact of declining ADLs on older adults' mental health can potentially be mitigated by better social networks [33]; the presence of neighborhood resources in the community for older adults in China is an important protective factor for the mental health of older adults, and may reduce the effects of depressive conditions on declining ADLs. A fall in the elderly may result in a decline in physical, psychological, or social function, while a severe fall may result in impaired daily living abilities and even disability as a result of the fall [34, 35]. ADLs and IADLs were negatively affected by falls in older adults in this study, which examined whether falls occurred in the past year. Accordingly, it would be worthwhile to explore and analyze the risk factors and the protective factors for falls in older adults.

Consistent with previous studies, the number of chronic diseases significantly affected ADL and IADL. In this study, taking no chronic disease as a reference, the number of chronic diseases increased, and the OR increased accordingly, with a trend test result of p < 0.001. Age had a significant impact on ADL. In addition, the influence of age on the ability to perform daily activities was observed. Taking the lower age group as a reference, the OR value increased as the age group increases, and the trend test p < 0.001. Therefore, it is imperative that primary healthcare providers pay special attention to older adults and improve screening for multimorbidity and follow-up needs.

Although this study was a nationwide large-scale survey, it still had the following limitations. First, causal analysis could not be performed based on cross-sectional data due to inherent defects. Second, because all chronic diseases were self-reported, it was difficult to determine the awareness rate of the respondents. However, the survey indicates that 67.4% of the respondents underwent physical examinations annually; therefore, these results were deemed somewhat reliable. Third, owing to the large amount of missing data for some chronic diseases, scale data and variables, it was not possible to include all chronic diseases, ADL, IADL, and CESD-10 scores in this study. Furthermore, the low percentage of senior citizens in the population included in the analysis, and the even lower knowledge of chronic diseases in this group, contributed to the instability of the regression model.