Patterns and risk factors of cognitive decline among community-dwelling Korean older adults: The Korean Longitudinal Study of Aging (2006–2016)

Background Dementia prevalence is increasing worldwide, and thus, the global impact of cognitive impairment and dementia has become a priority public health issue. This study aimed to assess the patterns of and risk factors for cognitive decline over time in community-dwelling Korean adults aged ≥ 65 years. Older adults with normal cognitive function at baseline in 2006 were evaluated, and changes in their cognitive function were followed over time. The data were obtained from the 2006–2016 Korean Longitudinal Study of Aging. Cognitive function in 2016 was classied based on the Korean-Mini-Mental State Examination score as normal, mild dementia, and moderate and severe dementia. The t-test, ANOVA, and linear mixed models were used to analyse the patterns and risk factors f cognitive decline over time. Of the 1,262 participants examined, 752 maintained normal cognitive function, 267 older adults showed a change from normal cognitive function to mild dementia, and 243 older adults showed a decline from normal cognitive function to moderate and severe dementia between 2006 and 2016. There were signicant differences in cognitive function over time among the three groups, and these were inuenced by age, gender, education level, religion, activities of daily living, instrumental activities of daily living, regular exercise, number of meetings with friends, and depression. The differences have gradually increased over 10 years.


Participants
This study analysed data from KLosA participants aged ≥ 65 years with normal cognitive function in 2006 who completed the survey for 10 years until the sixth investigation in 2016. Cognitive function was measured using the Korean-Mini-Mental State Examination (K-MMSE) score (22) in the KLoSA survey. The participants were classi ed into one of three groups according to their K-MMSE score in the 2016 survey: normal cognition (K-MMSE score ≥ 24), mild dementia (K-MMSE score [20][21][22][23], and moderate and severe dementia (K-MMSE score ≤ 19). This was a prospective cohort study in which participants without cognitive impairment were excluded at the initial stage. All participants included had the ability to understand the information from the researchers and to click the start button voluntarily; this was considered as an informed consent to participate in the survey (23). When a participant did not have the decisional capacity to participate in the survey owing to cognitive decline, a legal representative participated in the survey on the participant's behalf.

Cognitive function
Cognitive function was measured using the K-MMSE. Brie y, the K-MMSE consists of six domains of time and place orientation, registration, attention and calculation, recall, language, and visual construction. The maximum total K-MMSE score is 30 points, and mild dementia and moderate and severe dementia are de ned as scores of ≤ 23 and ≤ 19, respectively (22).

Demographic characteristics
The demographic factors included age, gender, education level, marital status, religion, and total household income in 2006. Age was regarded as a continuous variable, and educational level was classi ed into illiteracy, elementary school, middle school, high school, and college or higher. Income was de ned as the total household income in the previous year and was considered a continuous variable. Having a spouse and religion was measured with two responses "Yes" and "No".

Health status and behaviours
Health status and behaviours were assessed using the Korean activities of daily living (K-ADL), Korean instrumental activities of daily living (K-IADL), regular exercise, and depression scales. The K-ADL scale comprises seven items: dressing, washing face and hands, bathing, eating, transfer, toileting, and continence (24). The K-IADL scale comprises 10 items: shopping, mode of transportation, ability to handle nances, housekeeping, preparing food, ability to use a telephone, responsibility for own medication, recent memory, hobbies, watching television, and xing the house (25). A higher score in both ADL and IADL scales indicates higher dependency. ADL and IADL were analysed as continuous variables. Regular exercise was measured with two responses "Yes" and "No". Depression was measured using the 10-item short-form of the Center for Epidemiological Studies Depression (CES-D10) scale modi ed Korean version (26), which is divided into two parts: positive and negative experiences. A CES-D10 scale score higher than 4 points indicates depression (26).

Social interactions
Social interaction was measured according to the number of meetings with friends (one question) and the number of participations in regular meetings (six question) to measure the social network. Answers related to the number of meetings with friends were measured on a 9-point scale ranging from 0 (not in contact) to 9 (almost every day). For the analysis, the number of meeting with friends was classi ed into fewer than six times a year, fewer than four times a month, and almost every day. Regular meetings were categorised into six activities: religious meetings, social meetings, leisure/culture/sports meetings, alumni/birthplace meetings, volunteer service, and political party/civic society organisations/interest groups. For analysis, the participations in regular meetings was based on the number of activities (range, 0 to 6).

Statistical analysis
Descriptive statistics, such as frequency (%) and averages, standard deviation (SD), and median (interquartile range), of demographic characteristics, health status and health behaviour, and social interaction were used in the rst step of the analysis. The changes and the differences in the K-MMSE score between the three groups were analysed using mean and SD, via analysis of variance. A linear mixed model was used to analyse the patterns and risk factors of cognitive decline; these were identi ed by con rming signi cance in the three groups over time from 2006 and 2016. All statistical analyses were conducted using the PASW SPSS WIN 24.0 programme (SPSS Inc., Chicago, IL), and p ≤ 0.05 was considered statistically signi cant.

Participant characteristics
A total of 1,262 older adults were included in the study. Of these, 243 declined from normal cognitive function to moderate and severe dementia, 267 changed from normal cognitive function to mild dementia, and 752 maintained normal cognitive function between 2006 and 2016. Table 1 shows the differences in demographic characteristics, health status and health behaviour, and social interaction among the three groups in 2006. In the overall population, the average age was 69.89 (SD = 4.25) years; 52% were female; 40.5% were elementary school graduates; and 74.4% had a spouse. Further, 32.8% had depression. There were signi cant differences in age, gender, education level, spouse, income, depression, regular exercise, and number of participations in regular meetings among the three groups.  19.3% greater in the moderate and severe dementia group and 6.6% greater in the mild dementia group than that in the normal group. The proportion of elementary school graduates were in the moderate and severe dementia group, 12.3% higher, and in the mild dementia group, 4.8% were more than the normal group. For those with a spouse, 65.4% were in the moderate and severe dementia group, 72.7% in the mild dementia group, and 77.9% in the normal group. In the normal group, the average income was 1132.18 (SD = 2016.14) (10,000 South Korean won), and the number of regular meeting was 1.37 (SD = 0.69), the highest among the three groups. Regular exercise was performed by 65% of patients in the moderate and severe dementia group, 61.8% in the mild dementia, and 54.1% in the normal group.
severe dementia group, 26.50 (SD = 1.91); mild dementia group, 26.71 (SD = 1.93); normal group, 27.60 (SD = 1.89). The K-MMSE score in 2016 signi cantly differed between the three groups (F = 2437.30, p < .001) (   Risk factors of cognitive functional decline Table 4 presents the results of linear mixed models used to con rm the effects of risk factors on K-MMSE score. The variance in ation factor values for the multicollinearity con rmed the nal model analysis was 1.03 to 1.53, indicating that there was no multicollinearity. The K-MMSE score over time was  The risk factors for cognitive decline in the three groups were age, gender, education level, religion, regular exercise, depression score, K-ADL, K-IADL, and number of meetings with friends. The K-MMSE score decreased as the age increased (β=−0.044, p = .001). In education level, the K-MMSE score was signi cantly lower in the illiteracy (β=−1.954, p < .001), elementary school graduate (β =−0.967, p < .001), middle school graduate (β=−0.981, p = .002), and high school graduate (β=−0.582, p = .005) groups than that in the college or higher group. The K-MMSE score was also lower when the participants had a religion group compared with when they had no religion (β = 0.368, p < .001). Furthermore, as the K-ADL score (β=− 0.057, p < .001) and K-IADL score (β=−0.239, p < .001) increased, the K-MMSE score was signi cantly lower. For the number of meetings with friends, the K-MMSE score was signi cantly lower for those who met with their friends less than 6 times a year than in those who met with them almost daily (β=− 0.050, p < .001). Male (β = 0.591, p < .001), regular exercise (β = 0.326, p < .001), and no depression (β = 0.486, p < .001) were signi cantly associated with a higher K-MMSE score.

Discussion
Dementia is an important global health issue owing to the increasing elderly population. Given that cognitive decline is irreversible, preventive strategies to reduce the risk and delay the onset of dementia through early management of risk factors are important. Cognitive decline is distressing not only to the patient but also to their family and the country as a whole owing to the profound health and economic burden associated with it. Therefore, the importance of prevention and early detection of dementia has been emphasised, and attention to mild dementia as well as moderate and severe dementia is increasing.
In this study, 1,262 community-dwelling older adults were categorised according to their cognitive function as having moderate and severe dementia, mild dementia, or normal cognitive function and followed using changes in the K-MMSE score for 10 years. Of these, 243 older adults progressed to moderate and severe dementia, 267 older adults developed mild dementia, and 752 older adults maintained normal cognitive function. The K-MMSE score signi cantly differed over time among the moderate and severe dementia, the mild dementia, and the normal cognition groups. In the moderate and severe dementia group, the K-MMSE score was rapidly decreased after 6 years, while the mild dementia group showed a steady decrease in MMSE score over time. These results suggest that the rate of cognitive decline over time may be different in adults with normal cognitive function, and thus it is necessary to prevent the deterioration of cognitive function by conducting regular screening and managing risk factors.
The results of the current study are consistent with those of many previous studies that had identi ed factors of cognitive function decline. Cognitive decline was associated with age (16,27,28), being female (29), and level of education (30,31). Given that the incidence of dementia increases with age, early management of the risk factors for dementia should be reinforced for early detection and prevention in old adults (32). In a systematic literature review study published from 1985 to 2010, 51 of 88 studies (58%) reported signi cant negative effects of lower education on the risk for dementia (33). Furthermore, religion appears to have a preventive role in cognitive decline by providing mental stability and peace associated with participation in religious activities (34).
Health status and health behavioural factors, physical function (ADL, I-ADL), regular exercise, and depression were signi cantly different according to cognitive function. Several studies have reported that physical function is related to cognitive decline (35,36); subjects with mild cognitive and IADL impairment are more likely to develop dementia than subjects with mild cognitive impairment but unrestricted IADL (36). Older adults with good physical function have high cognitive function (37).
The 6-month walking programme for nursing home residents with Alzheimer's disease has shown to improve ADL scores and stabilise the progressive cognitive dysfunctions and improvement (38). Physical exercise enhances and maintains the general health, quality of life, and physical tness bene ts of the older population. Regular exercise has recently been studied as a predictor of various physical health conditions, and interest in the relationship between physical health and cognitive function has been recently increasing. Studies have shown that regular exercise in older adults may delay the progression of dementia (39). Older adult females who are physically active have been reported to have low risk of cognitive decline (18). Walking lowers the risk of dementia, and thus older adults who exercise regularly are expected to have lower risk of cognitive decline (40). Exercise improves cognitive function and survival by stimulating nerve growth and nerve function and facilitating adequate blood, oxygen, and nutrient supply to the brain (41,42). There is a need to better characterise walking behaviours and patterns related to cognitive age, including metabolism equivalents, that can be easily adopted by older adults individuals. Health care providers should also promote an active lifestyle for older adults.
Depression is known as the most common mental health disease in old age and strongly affects cognitive function. Consistent with other studies (16,19,35,43), we also found that depression is associated with cognitive function decline, indicating that prevention and early detection and treatment of depression in older adults can prevent cognitive decline.
Further, social interaction, as evaluated according to the number of meetings with friends, was associated with a decrease in cognitive function. Particularly, older adults who met with friends less than six times a year developed decreased cognitive function compared with those who met with friends almost every day. These results suggested that continued participation in social activities was associated with decreased risk of cognitive function decline. This is consistent with ndings on the associations between social participation and cognitive function (44,45). A previous longitudinal study of community-dwelling adults and older adults suggested that maintaining many social activities may help prevent and delay cognitive function (44). A Taiwanese cohort study also showed that participating in more social activities reduces the risk of cognitive impairment (46). Therefore, it is necessary for community-dwelling older adults to actively implement programmes that enable the elderly in the community to continue social interaction.
This study had some limitations. First, while the pattern of K-MMSE was evaluated in a large cohort of 1262 older adults between 2006 and 2016, we only included only community-dwelling older adults and not those living in nursing homes. Second, we may not have fully controlled all potential risks or confounding factors in this study; we could not include possible factors related to physical and mental health status except depression. Lastly, the results might be in uenced by survival effects because we excluded censored participants such as those who died, admitted to a nursing home and those moved to a critical care unit due to severe dementia before the nal survey in 2016. However, despite these limitations, we believe that our study remains valuable in that it had a relatively longitudinal follow-up and used a representative elderly sample. In addition, by identifying risk factors that affect the K-MMSE score, the results can be utilised to improve care of the older adults in the future.

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Ageing-related cognitive function decline is associated with high medical costs and family burden, thus making it an important issue today owing to the increasing ageing population. We found that demographic characteristics, health status, health behavioural factors, and social interaction in uenced the risk for cognitive decline. Given that mild dementia can progress to moderate and severe dementia, timely and appropriate strategies according to the stage of cognitive impairment are needed to reduce personal, family, and social burdens. Improving physical function through regular exercise, increasing social activity, and managing depression via early detection and treatment according to the cognitive function status are recommended.