The main purpose of this study was to investigate the effects and sex differences of physical frailty on 2-year disability outcomes among community-dwelling older adults. Even after controlling for potentially confounding factors, physical frailty independently contributed to the incidence of disability in mobility and IADLs in both men and women. This result is consistent with the few previous studies in other countries that have investigated the incidence of disability in relation to physical frailty [11–15]. The impact of physical frailty on incident mobility disability was the strongest among the three disabilities investigated in both men and women in this study, while the impact of frailty on ADL disability in men was not measured due to the limited sample size. Furthermore, the impact of physical frailty on incident mobility and IADL disability was stronger and affected more items in women than in men (Table 3).
As shown in Table 2, women showed a higher prevalence of disability at baseline and incidence after two years than men. As shown in Table 3, the impact of both prefrailty and frailty on mobility disability was greater in women than in men. This result correlates with previous studies showing that the prevalence of mobility disability was higher in women than in men and increased exponentially with increasing age [12, 22, 23, 29]. In addition, women are more likely than men to progress from no disability to having a disability in climbing stairs and from intermittent to continuous disability . Although the mobility assessment was based on self-reported data, previous research has demonstrated that self-reported and performance-based mobility measures for both men and women have a high level of concordance [30, 31]
Difficulty in bathing had the highest prevalence at baseline (1.9%) and incidence after 2 years (1.2%) among the ADL disability items assessed in Table 2. This finding is consistent with a prior study, which found that bathing is the first activity in which both older Americans and Chinese have difficulty . The higher percentage of difficulty in bathing also corresponds with the results of the 2017 Living Profiles of Older People Survey in Korea, conducted in line with the Welfare of Older Persons Act, that included 10,299 participants aged over 65 years .
As indicated in Table 3, the effect of frailty on IADL disability was also larger in women than in men after adjustment, whereas prefrailty had no statistically significant effect on IADL disability in men. Both prefrailty and frailty had the greatest impact on the 'using transportation' item for both men (OR = 12.70; OR = 21.49) and women (OR = 9.87; OR = 28.50), as shown in Table 4. It is reasonable to suppose that the item ‘using transportation’ is linked to mobility and that mobility disability adds to the construct of frailty, or vice versa, in both men and women.
It is also remarkable that physical frailty or prefrailty increased disability in outdoor activity-related items of IADL (going out, using transportation, and shopping) in men, while physical frailty was associated with disability in household work-related items (performing household chores, doing laundry) in older women. This can be interpreted as having an impact on the activities that the individual has been involved in, based on traditional gender roles. In previous study, Korean men said that they could not handle laundry, cook meals, or perform household chores, since these roles were unfamiliar to them .
Interestingly, frail older women had more difficulty in managing money than men. It is possible that older women lacked previous experience with similar tasks [20, 34]. This can also be explained by the higher risk of cognitive impairment in women than in men. According to prior studies, there are a number of factors related to cognitive function that differ according to age and sex, especially regarding working status and social participation [35, 36]. Working, using cell phones, using public transportation, and visiting a bank have traditionally been dependent on rigid gender roles in Korea. Because of physical and social factors, older women are more likely to require assistance with ‘cognitive tasks’ .
The substantial correlation between frailty and disability by sex may explain the disparity in frailty prevalence and average age: women made up 56.8% of the frail group, and their average age was higher than that of men, indicating that they were older and more frail (Table 1). According to a prior study comparing IADL disability by sex, women exhibited a higher tendency to report disabilities, use assistance, and a higher degree of disability than men [16, 20, 22, 23, 37]. In contrast, Table 2 shows that that the prevalence and incidence of IADL disability were higher in men than in women, particularly for the 'doing household chores,' 'cooking,' and 'doing laundry' components of the IADL item. Based on the data in Tables 3 and 4, by investigating the effect of frailty, it is reasonable to believe that physical frailty has a greater impact on women and that frailty aggravates household-related IADL disability in older women, whereas frailty worsens outdoor activity-related IADL disability in older men.
The prevalence of ADL and IADL disabilities (2.1% and 6.2%, respectively) at baseline according to Table 1 appears to be far lower for community-dwelling older adults. The low prevalence of ADL and IADL disabilities in this study sample at baseline may be attributed to the sampling method, which generally included ambulatory community-dwelling older adults. In contrast, the prevalence of ADL and IADL from the 2017 Living Profiles of Older People Survey in Korea was higher (8.7% and 16.6% respectively) .
As shown in Supplementary Table S.1, the risk of developing ADL disability (OR = 10.26) outweighed the risk of having IADL disability (OR = 4.11) among frail participants after 2 years, whereas pre-frailty did not increase the risk of ADL disability in two years. Physical frailty was associated with increased mortality, similar to previous studies, whereas some studies showed conflicting results regarding mortality [11, 14]. Despite the different frailty measurements, the mortality results from this study were similar to those of a previous Korean study that utilized the frailty index to predict all-cause death in the Korean population based on age and sex .
Our study also has several limitations. First, there is no universal consensus on the physical phenotype as the reference standard for frailty, and the prevalence varies according to countries [10, 38]. Furthermore, the effect of sex differences on each ADL disability was not assessed in this study because of the small incidence of each item. In addition, the relatively low prevalence of each item in IADL may have influenced the results of this study. Finally, this study had a relatively short follow-up period (2 years) compared to other studies, which had a follow up of 4 to 11 years [11–13, 15].
Despite the limitations mentioned above, this study provides a better understanding of frailty that may help researchers and policymakers focus on frailty intervention programs in the Korean aging population, allowing them to provide more sophisticated interventions to prevent frailty. The increasing number of frail older adults is one of the biggest challenges facing health and social care. Frail older adults are vulnerable to developing disabilities, which lead to higher care needs and resource consumption. A higher prevalence of disability indicates a higher level of dependency, which ultimately leads to a higher need for support and a higher burden on family caregivers, the community, and the state. Therefore, efforts must be focused on managing frailty before it leads to an irreversible disability or other negative consequences. The authors found that the incidence of disability differed according to sex. To the best of our knowledge, this is the first study of its kind to investigate predicting the incidence of disability (mobility, ADL, and IADL) by sex difference using a nationwide sample collected by the KFACS team.