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 significantly 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 identified 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 significant 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 significantly 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 fitness benefits 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 findings 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 influenced 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 final 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.