INTRODUCTION
The global elderly population is growing rapidly as life expectancy increases due to rapid developments in medical technology and improvements in living standards over the past decades [1]. According to Statistics Korea, the elderly population aged 65 or older is expected to account for 34.3% of the total population in Korea in 2040, and is expected to reach 47.7% in 2072, approaching half of the total population [2]. This aging phenomenon is bringing about various social and economic challenges, and notably, the prevalence of chronic diseases is increasing significantly, emerging as a major public health issue [3].
Chronic diseases are difficult to cure and last for a long time due to the nature of the disease. As people get older, the number of chronic diseases accumulates, appearing in the form of multimorbidity [4]. Multimorbidity is defined as the simultaneous occurrence of two or more chronic conditions in an individual [4]. Patients with complex chronic diseases experience various physiological problems due to interactions between diseases, and side effects may increase during the treatment process, requiring complex medical management [5]. In previous studies, multimorbidity has been reported to be associated with functional limitations [6], greater use of medical resources [7], and higher mortality [8]. In particular, older adults with multimorbidity experience more problems related to functional disability, which becomes a major obstacle to main-taining Activities of Daily Living (ADL) [9]. Older adults with ADL disabilities have limited ability to live independently, leading to decreased self-esteem and social iso-lation [10]. Especially in patients with multimorbidity, ADL disability increases the risk of depression and can ultimately reduce the quality of life [11], making the main-tenance of ADL a very important issue for older adults with multimorbidity.
The strategy for coping with ADL disability in older adults is to maintain ADL through rapid recovery and active prevention of functional impairment. To effectively maintain ADL, it is essential for older adults with multimorbidity to accurately understand their own health conditions, and to properly integrate medical advice into their daily routines [12]. In particular, patients with multimorbidity must perform various self-management activities, such as continuously updating information and docu-menting their condition, to manage a variety of health problems more effectively than patients with a single chronic disease [13]. In order to effectively carry out such self-management, health literacy is essential [14]. Health literacy is the complex ability to access health information, obtain necessary information, understand the information, determine whether the information is appropriate, and use it for health management, disease prevention, and medical services use [14]. Low health literacy can cause difficulties in accessing, understanding, evaluating, and applying written and oral information provided by healthcare professionals. These difficulties can lead patients to make incorrect healthcare choices and ultimately worsen their health status [15]. Meanwhile, it has been reported that a high level of health literacy enhances self-efficacy in managing chronic diseases, leading to improvements in both the physical and mental health of older adults [16]. This suggests that health literacy extends beyond mere disease management and also positively influences ADL.
Previous studies have demonstrated that health literacy positively affects health outcomes in older adults [15,16]. However, these studies are generally focused on a single chronic disease, and research on older adults with multimorbidity, where disease management is more intricate and the importance of health literacy becomes increasingly critical, remains limited. In addition, this study aims to provide more practical results by investigating how health literacy affects not only simple disease management but also independence in daily living. Moreover, as life experiences, physical development, and health conditions vary with age, these factors may contribute to age-related differences in ADL disability characteristics, high-lighting the need to analyze them separately by age group. Therefore, this study aims to clarify the relationship between health literacy and ADL disability in older adults with multimorbidity by age group, which will contribute to confirming the usefulness of health literacy as an intervention method to promote ADL in older adults with multimorbidity.
METHODS
1. Study Design
This study is a cross-sectional study to determine the relationship between health literacy and disability in ADL in older adults with multimorbidity.
2. Participants and Data Collection
This study utilized data from the KHPS in 2021 collected by the Korean Institute for Health and Social Affairs and the National Health Insurance Corporation. The KHPS is classified as a nationwide survey, as it utilizes a two-stage stratified cluster sampling method, with probability proportional to size, based on the 2016 registration census, which encompasses the entire population of Korea. In the first stage, the sample enumeration district clusters were extracted, and in the second stage, the sample households were selected from these clustered districts. The 2021 annual data includes responses from 5,878 households and 12,874 household members. The survey was conducted using the Computer Assisted Personal Interviewing (CAPI) method, which is an in-person interview method in which a researcher visits and asks respondents questions while looking at a questionnaire on a computer.
In this study, 2,153 household members who were adults over 65 years of age, responded to all 16 questionnaire items regarding health literacy and one question regarding ADL disability, and having two or more chronic diseases were selected for the final analysis.
3. Measures
1) Health literacy
The independent variable of this study was health literacy, which was measured using the European Health Literacy Survey Questionnaire - short version (HLS-EU-Q16). This scale consists of 16 questions that focus on three do-mains of health literacy: healthcare (questions 1 to 7), disease prevention (questions 8 to 12), and health promotion (questions 13 to 16). In the HLS-EU-Q16, the healthcare domain consists of seven items: (1) finding information on treatments for illnesses one is concerned about, (2) finding medical professionals (e.g., specialists) who can help when sick, (3) understanding the content of explanations given by doctors during consultations, (4) understanding the instructions provided by doctors or pharmacists regarding prescribed medications, (5) assessing the need for additional consultations after receiving care, (6) using the information provided by doctors to make decisions about one's illness treatment, and (7) following the health management and medication instructions provided by doctors and pharmacists. The disease prevention domain includes five items: (8) finding information on managing mental health issues such as stress or depression, (9) understanding warning signs related to risky behaviors like smoking, lack of exercise, or excessive drinking, (10) understanding the importance of health screenings, (11) evaluating the credibility of health information provided by the media, and (12) using media information to make decisions regarding personal health management. The health promotion domain comprises four items: (13) identifying physical activities beneficial for mental health, (14) understanding health-related advice given by family or friends, (15) comprehending health promotion information provided by the media, and (16) assessing the relationship between daily habits and personal health. Participants responded to each question on a four-point Likert scale: “ very difficult,” “ fairly difficult,” “ fairly easy,” “ very easy,” or “ I don't know.” The responses “ fairly easy” and “ very easy” were dichotomized to a value of 1, while “ fairly difficult” and “ very difficult” were dichotomized to a value of 0. The response “ I don't know” was treated as a missing item. The respective scores were then summed (ranging from a minimum of 0 to a maximum of 16 points) and categorized according to the HLS-EU-Q16 criteria as follows: 0∼8 as an ‘ inadequate’, 9∼12 as a ‘ marginal’, and 13∼16 as an ‘ adequate’ level of health literacy [17]. The HLS-EU-Q16, vali-dated in diverse cultural and linguistic settings [18,19], was translated into Korean following the translation and back-translation method, achieving a Cronbach's ⍺ of .86 [20].
2) Disability in ADL
The dependent variable of this study is ADL disability. ADL is divided into basic ADL, which refers to essential abilities required for survival, and instrumental ADL, which refers to abilities required to live independently in the community [21]. Among these, the Korean Health Panel Survey measured instrumental ADL, and unlike previous studies that used objective scales, it used a single question, “ Which item best describes your current status regarding daily activities, including work, study, house-work, family, or leisure activities?” and asked to select one of the options: “ I have no difficulty performing daily activities,” “ I have some difficulty performing daily activities,” or “ I cannot perform daily activities.” In this study, the response “ I have no difficulty performing daily activities” was categorized as ‘ no,’ while the response “ I have some difficulty performing daily activities” or “ I cannot perform daily activities” was categorized as ‘ yes.’
3) Covariates
Potential confounders of ADL were identified and adjusted for in the regression models. Based on a literature review, covariates included demographic characteristics such as age, gender, education level, and presence of spouse, as well as health-related characteristics such as depressive symptoms, body mass index (BMI), and number of chronic diseases [22-24].
Among the demographic characteristics, age was classified as ‘ young-old’ for 65∼74 years old, and ‘ old-old’ for 75 years old or older [25]. Education level was classified as ‘ elementary school graduate or less’, ‘ middle school graduate’, ‘ high school graduate or higher’ based on the response to the education level. The presence of a spouse was classified as ‘ yes’ if the individual was married and living with their spouse (including common-law mar-riages) and ‘ no’ if they were separated, widowed, divor-ced, or had never been married [23].
Among health-related characteristics, depressive symptoms were categorized as ‘ yes’ or ‘ no’ based on the response to the question, ‘ Have you ever experienced excessive anxiety or worry that interfered with your daily life for more than two consecutive weeks in the past year?’ BMI was estimated by dividing body weight in kilograms by the square of height in meters (kg/m2). This study classified BMI as ‘ underweight’ below 18.5 kg/m2, ‘ normal weight’ between 18.5 and 25 kg/m2, ‘ overweight’ between 25 and 30 kg/m2, and ‘ obesity’ above 30 kg/m2, according to the criteria of the World Health Organization [26]. The number of chronic diseases was categorized as ‘2’, ‘3’, ‘4’, and ‘5 or more’ based on the sum of the cases in which the patient answered ‘ yes’ to the question of whether or not they currently had any of the 31 chronic diseases listed in the questionnaire and were diagnosed by a doctor.
4. Data Analysis
This study presented the subject's demographic and health-related characteristics as frequency, percentage, mean, and standard deviation. A x2 test was conducted to determine the difference in the ratio of ADL disability according to the subject's demographic and health-related characteristics. In addition, multiple logistic regression analysis was performed to determine the risk of disability in ADL according to the subject's health literacy level by age group, and the results were presented as an odds ratio (OR) and a 95% confidence interval (CI). Since this study focused on a specific group of patients with multiple chronic diseases within the 2021 KHPS annual data, no weighting was applied. The collected data were used with SPSS version 23.0 (IBM Corp, Armonk, NY), and the level of statistical significance was set at p<.05.
5. Ethical Consideration
This study was conducted with the approval of the Institutional Review Board (IRB No. 2024-ICCU-IRB-11) of the researcher's affiliated university for the secondary analysis of the data from the KHPS. The data of the KHPS are collected with a unique number to ensure the anonym-ity and confidentiality of personal information.
RESULTS
1. Sociodemographic and Health-related Char-acteristics
The average age of the subjects was 74.83 years old. There were more women (63.2%) than men (36.8%), and those with elementary school education or lower account-ed for the largest group at 54.8%. A total of 64.6% of the subjects had a spouse, and 11.4% had depressive symptoms. The average BMI of the subjects was 23.98 kg/m2, with 61.2% being normal weight and 31.5% being overweight. The average number of chronic diseases the subjects had was 2.84, with 47.0% having two chronic diseases, the largest group, and 32.4% having three diseases, the next largest group. The subjects’ health literacy was 7.5 out of 16 on average, with 61.9% being inadequate, 22.3% being marginal, and 15.8% being adequate. Among the subjects, 31.4% had ADL disability (Table 1).
Table 1.
Sociodemographic and Health-related Characteristics of the Participants (N=2,153)
2. Differences in ADL Disability according to Characteristics of the Subjects
There were significant differences in the ADL disability rate according to the subjects’ age (p<.001), gender (p< .001), education level (p<.001), presence of spouse (p<.001), depressive symptoms (p<.001), BMI (p=.019), number of chronic diseases (p<.001), and health literacy level (p< .001). Compared with the group without ADL disabilities, the group with ADL disability had a higher proportion of those aged 75 years or older, women, those with low education levels, those without spouses, those with depressive symptoms, those with abnormal weight, those with a high number of chronic diseases, and those with low health literacy (Table 2).
Table 2.
ADL Disability by Sociodemographic and Health-related Characteristics (N=2,153)
3. Association between Health Literacy Level and ADL Disability by Age Group
For the young-old group (65∼74 years), compared with an adequate level of health literacy, the risk of ADL disability was 2.02 times (95% CI 1.20∼3.42) higher in those with a marginal level, and 2.53 times (95% CI 1.54∼4.16) higher in those with an inadequate level after adjusting for covariates. For the old-old group (75 years and older), the risk of ADL disability was 2.57 times (95% CI 1.59∼4.16) higher in those with an inadequate level of health literacy compared to those with an adequate level, when covariates were not adjusted. However, this association was no longer significant after adjusting for covariates (Table 3).
Table 3.
Association between the Health Literacy Level and Disability in ADL according to Age Group (N=2,153)
DISCUSSION
This study was conducted to identify the relationship between health literacy and ADL disability in older adults with multimorbidity, and to confirm the usefulness of health literacy as an intervention method for improving ADL in older adults with multimorbidity. The main research results are discussed as follows.
The results of this study showed that 31.4% of the subjects had ADL disability. This was higher than the 23.8% reported for European older adults [24] and lower than the 45.8% reported for German older adults [22]. The ADL disability rates are reported differently depending on the study, which is thought to be due to differences in the study subjects, measurement scales, and sociocultural environments.
In this study, the health literacy score was 7.50 points on average, which corresponds to the level of an inadequate. As for the classification according to the level of health literacy, 61.9% of the subjects were at an inadequate level and 22.3% were at a marginal level, resulting in 84.2% of the total having limited health literacy. This showed a clear difference when compared to the results of a previous study that used the same health literacy scale as this study, and was much higher level than the 47% of German older adults diagnosed with breast cancer [27]. This suggests that the subjects of this study are an important group in need of improved health literacy, and this difference is thought to be influenced by the number of chronic diseases they have. These results were similar to those of a study of older adults living in domestic communities [28], which reported that the difficulty in understanding hospi-tal use information was 2.36 times higher for individuals with two chronic diseases and 2.91 times higher for those with three or more chronic diseases than for those without chronic diseases. This indicates that the level of health literacy is not limited to simply the accessibility of information, but is deeply connected to the complexity of diseases and their management ability. Older adults with multimorbidity may have lower health literacy than those without chronic diseases because the treatment is complex and various information is provided for each disease [29]. It is very important for older adults with multimorbidity to understand the interactions between different diseases and make appropriate decisions based on them. Older adults with multimorbidity need to be careful because if they misunderstand or confuse different drug information or treatment instructions, this can not only reduce the effectiveness of treatment, but also increase side effects or even cause new health problems such as ADL disability [9].
In this study, the risk of ADL disability in older adults under 75 years with multimorbidity was 2.02 times higher when health literacy was at a marginal level and 2.53 times higher when health literacy was at an inadequate level, even after controlling for covariates. These results were similar to those from a previous study [30] targeting cardiac rehabilitation patients, which found that when health literacy was inadequate, the level of functional independence was significantly lower. This suggests that health literacy is significantly associated with ADL disability in older adults under 75 years with multimorbidity, beyond simply understanding and applying information. A previous study that revealed the mechanism between health literacy and health outcomes in older adults with chronic diseases found that individuals with higher health literacy had stronger self-efficacy for chronic disease management, which led to health promotion behaviors and better health outcomes [16]. Therefore, healthcare professionals should help older adults acquire appropriate health literacy to enhance their self-efficacy for chronic disease management to engage in self-management behaviors and achieve better health outcomes.
Patients with low health literacy may have difficulty communicating with healthcare providers, which may lead to ineffective delivery of medical information [31]. In particular, patients may miss or misunderstand important information if the explanations provided by the healthcare providers are too complex or not patient-centered. This can result in patients not receiving appropriate treatment or not fully understanding their treatment plan, which can increase the risk of healthcare failure and ultimately affect their daily lives. Therefore, strategies to improve health literacy are important, which can improve communication between patients and healthcare providers and support patients to become more actively involved in their own healthcare. Health literacy can be improved through patient education and changes in health information delivery methods [32]. According to a previous study that conducted a program to improve health literacy for hyper-tensive patients, the experimental group that provided information tailored to the patient's health literacy level and considered the patient's preferred learning style in the method of providing information had significantly higher health literacy than the control group that only received general discharge education [32]. Additionally, remote tel-erehabilitation [33] and short message service [34] were ef-fective in increasing health literacy. Thus, healthcare providers caring for older adults need to be aware of the health literacy issues of older adults, and provide information in a language and manner that older adults can easily understand, while also periodically checking whether older adults have sufficiently understood the information. This approach can enhance health literacy among older adults with multimorbidity, enabling them to manage their health more effectively.
The significance of this study is that by analyzing the relationship between the health literacy and ADL disability of older adults with multimorbidity, it provides evidence that educational programs and policies aimed at improving health literacy can be an important means of improving ADL and the quality of life. Additionally, while previous studies have mainly targeted patients with a single chronic disease, this study provides practical results showing the relationship between health literacy and independence in daily life in older adults with multimorbidity under the age of 75 for whom health literacy plays an important role. However, this study has the following limitations: First, this study is a secondary data analysis that utilized KHPS data. Therefore, the researcher could not di-rectly intervene in the data collection process, and thus there may be bias due to the limitations of the tools used to measure health literacy and ADL disability or the sub-jective perceptions of the respondents. Additionally, factors influencing ADL disability may not have been sufficiently controlled.
Second, this study adopted a cross-sectional research design, making it difficult to clearly identify causal relationships. Long-term follow-up studies are needed to further clarify whether low health literacy leads to ADL disability or whether ADL disability lowers health literacy levels. Third, the health literacy scale used in this study mainly focuses on evaluating the understanding of written and oral information, so digital health literacy or other forms of information accessibility and comprehension may not be sufficiently reflected. Future studies should aim to supplement these limitations and conduct a more in-depth analysis.
Lastly, the participants of this study were older adults aged 65 and above, which may have limitations in the survey process, especially in measuring health literacy. Al-though the KHPS is conducted with the assistance of trained interviewers, health literacy tests are closely related to visual cues and require the ability to follow oral directions [35]. Therefore, severe impairments in vision and hearing can be classified as limitations in literacy. Therefore, in future studies, it is necessary to increase the accuracy of health literacy measurement by using objective screening tools to evaluate the vision and hearing of older adults.
CONCLUSION
This study was conducted to identify the relationship between health literacy and ADL disability in older adults with multimorbidity and to confirm the usefulness of health literacy as an intervention to improve ADL in this population. The results confirmed that lower levels of health literacy are associated with a higher risk of ADL disability, especially under the age of 75. This suggests that health literacy is an important factor associated with ADL in older adults with multimorbidity, going beyond the mere ability to understand and apply information. Based on these findings, we would like to make the following suggestions for future research: First, to overcome the limitations of the cross-sectional design of this study, it is necessary to conduct a long-term follow-up study to more clearly identify the causal relationship between health literacy and ADL disability. Second, as the use of digital technology increases in modern medical environments, the importance of digital health literacy is also rising. Therefore, we propose a study to evaluate the information accessibility and comprehension of older adults with multimorbidity, including digital health literacy. Third, to improve the health literacy of older adults with multimorbidity, we propose a study to develop learning mod-ules on drug interactions and essential lifestyle adjust-ments according to the combination of frequent chronic diseases, and to provide customized programs according to the individual's health literacy level and verify the effectiveness.
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