1 Introduction

Extension of healthy life expectancy is an important topic in Japanese healthy aging policy, which has already achieved extended life expectancy. In 2013, there was about a 9-year gap between life expectancy and healthy life expectancy among men, and a 13-year gap of them among women. Although the definition of healthy life expectancy is diverse, one definition is measured by a period that has not seen the onset of functional disability in public long-term care certification, or that has independence in instrumental activities of daily living (IADL; a higher-living functional capacity of older adults). About 18% of older adults were certified for long-term care/support needs, and the number was about 5.9 million people (as of 2014).

Many studies indicate that the ability to conduct IADL, the onset of functional disability, and death are intimately related to not only lifestyles and health behaviors, but also to socioeconomic situations. While several systematic reviews focusing on the daily functions of older adults exist [1,2,3], overseas findings are not necessarily applicable to Japan because of genetic, environmental, and cultural factors. There is thus a need to review findings on older adults in Japan. Longitudinal studies are needed when doing so because there is a bidirectional relationship between health and other variables (for example, the unhealthier one is, the harder it is to acquire income; the poorer one is, the easier it is to become unhealthy). As opposed to cross-sectional studies, which are based on synchronic data, longitudinal studies trace individuals. Furthermore, considering that there are few cases in which individuals incur functional disabilities or die when tracked for only a few years, it is preferable to conduct large-scale baseline surveys to acquire more robust data.

The number of studies based on large-scale epidemiological surveys in Japan has increased in recent years. The Japan Epidemiological Association website introduces 16 studies, each of which track over 10,000 people for longer than 5 years (as of April 2018). A search for articles based on longitudinal studies discussing the health and life functions of older adults in JapanFootnote 1 revealed 53 studies in Japanese and 212 in English. These were narrowed down to those covering 10,000 or more people, review papers and articles that discussed specific diseases and medical treatments were excluded, and hand-searching was added to give a set of 48 articles. This set was reduced to 34 articles after excluding those covering depressive symptoms, falls, and dementia outcomes, which are discussed in another chapter. This chapter provides an overview of the findings of these articles with large-scale longitudinal research on Japanese older adults that discussed factors relating to their IADL, functional disability levels, and mortality.

2 Overview of Large-Scale Longitudinal Studies

The outcomes covered by the articles were as follows: death (20 articles), functional disability (16 articles), and IADL (2 articles). The primary explanatory variables were classified as follows: mental health and psychological well-being [4, 5], health behaviors (e.g., smoking and obesity) [6,7,8,9,10,11], oral function/dental status [12,13,14], socioeconomic status (SES) [15,16,17,18,19,20,21,22,23], social participation [24,25,26,27,28,29] , social network and social support [30,31,32,33,34,35,36], and others [37] (Table 15.1).

Table 15.1 Main results of associated factors with functional disability or mortality
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The Japan Gerontological Evaluation Study, from which approximately 80% of the articles are derived, is a research project that aims to create a scientific foundation for preventative policies that seek to create a healthy aging and long-living society (principal investigator: Katsunori Kondo, Professor, Chiba University). Since 2003, the self-administered postal survey has worked with municipalities to survey (through a questionnaire) tens of thousands of older individuals without certified functional disabilities. The response rate was 60–70% (there were approximately 13,000 responses in 2003, 40,000 in 2006, and over 100,000 from 2010 onward, see Fig. 15.1). The survey also included prospective cohort data that enabled understanding of the subsequent outcomes (e.g., death and functional disability level) based on information regarding public long-term care insurance records. At the same time, the survey is also notable for simultaneously collecting panel data that individually compares cross-sectional data at certain intervals. Since 2010, the survey has expanded to cover more areas and has changed in name from AGES to JAGES (see website for details: https://www.jages.net/). In the 14 years from 2004 to 2017, the JAGES project has published 364 articles and books (267 in Japanese, 97 in English), including 55 papers based on longitudinal studies (including sub-projects).

Fig. 15.1
figure 1

Overview of longitudinal survey in JAGES (AGES )

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3 Mental Health, Health Behavior, and Oral Functions

While subjective health indicators have been criticized as unreliable, large-scale Japanese longitudinal studies have found them to be important indicators for predicting future objective health. For example, regardless of age, chronic conditions, or disabilities at the baseline survey, both older men and women who stated that their health was poor had an approximately 1.7 higher risk of unmature death compared to those who answered that their health was good [4]. Furthermore, individuals with depression were found to have a 1.26 higher risk of functional disability and a 1.33 times higher mortality risk [5].

Regarding health behaviors, older male smokers were found to have 1.41 times the risk of death due to cardiovascular disease compared to nonsmokers. For women, the risk was 1.69 times higher [6]. In addition, smokers in the pre-1920 birth cohort had 1.47 times the risk of death [7]. Regarding body mass index (BMI), some large-scale cohort studies have consistently shown that Japanese older adults have approximately 1.6–2.6 times the risk of subsequent death if obese or underweight [8, 10]. Regarding food, Japanese dietary habits may reduce subsequent functional disability risk (HR = 0.77; 95% CI: 0.68–0.88) [11].

Oral health indicators such as bite strength and number of teeth are closely related to subsequent healthy aging. For example, it has been reported that people without teeth were more likely to experience a significant IADL decline compared to people with 20 or more remaining teeth; this influence was nearly equal to having a history of stroke [13]. The risk of death for older adults who brushed their teeth two or more times per day, visited the dentist at least once per year, or used dentures was reported as being 0.54 times those who performed none of these activities [12]. Furthermore, men aged 85 years or older with 20 or more remaining teeth had an estimated lifespan of 92 days longer that those who did not, while women with the same characteristics lived an average of 70 days longer [14].

Although it analyzed fewer than 10,000 subjects, another study reported that those with 19 or fewer remaining teeth had 1.21 times the risk of functional disability onset [38] and 1.83 times the risk of death because of cardiovascular or respiratory disease [39]. In addition, young-old persons aged 65–74 with fewer than 20 remaining teeth were 1.78 times more likely to become housebound [40].

4 Differences in Health Risks Due to SES

It is a solid fact that poverty and social exclusion are one of the social determinants of health [41]. Research on Japanese older adults has found that low SES could be a health risk, and these trends are remarkable among men [41]. Older men with less education or income were found to have 1.6–2.0 times the risk of unmature death [17]. The same trend was also found in objective income data from levy cost in public long-term care insurance [16]. It has been reported that income that is low in either absolute terms or when compared to others creates psychosocial stress and becomes a risk for functional disability [15], overall death [18], and cardiovascular disease death [20]. Furthermore, research indicates that economic poverty in both old age and childhood increases the overall risk of death during old age [21] in addition to causing declines in IADL [22]. In other words, the influence of SES on health extends throughout the course of people’s lives, suggesting the effectiveness of countermeasures that begin at a young age. It is also clear that the destruction of one’s house or job loss resulting from natural disaster (e.g., the Great East Japan Earthquake) is connected to worsening IADL [42].

5 Differences in Health Risks Due to Social Participation and Social Networks

Social participation is an important element of successful aging. According to activity theory in social gerontology, older individuals who actively participate in society maintain a sense of happiness due to steady social relationships and increased opportunities to receive positive feedback from others. One would therefore expect that social participation protects older adult health; this has in fact been confirmed by some large-scale cohort studies. People who hold positions and participate in social organizations (e.g., residents’ associations) have significantly lower subsequent risks of functional disability [29] and death [28], and there is a lower risk of death and functional disability among older individuals who participate in sports or physical activities [25,26,27]. In addition, an intervention study in Taketoyo Town, Aichi Prefecture, indicated that those who participated in community salon activities had more positive health self-assessments 2 years later than those who did not [43]. Their risk of functional disability was 51% lower [44], while their risk of functional disability involving dementia was approximately 30% lower [45].

Large-scale longitudinal studies have also found that having a poor social network (i.e., social isolation) is also a health risk. It has been reported that isolated older individuals had 1.3–1.8 times the risk of functional disability [32]. Reports have also indicated that contact with others less than once per week was a functional onset disability risk and that less than once per month was a mortality risk [33]. In addition, eating alone while living with someone else increased the risk of death for both men and women [34]. It has also been found that those who belonged to an infrequently meeting sports club had lower functional disability risks than those who frequently played sports alone [23]. This suggests that connecting with others is an important element for health. Studies focusing on social support (i.e., the functional side of social networks) have found that men and women without social support had 1.25 and 1.08 times the risk of functional disability, respectively [36]. While individuals in single-person households were more likely to have functional disabilities, social support could possibly reduce these risks by 25% [35].

Although individual-level social participation and social relationships are important, community-level social participation (social capital) is also associated with health among older adults. There have been efforts to develop a health-related community-level social capital instrument (comprised of three factors: civic participation, social cohesion, and reciprocity) [46]. For example, it has been found that even after adjusting for individual attributes, older women living in communities in which there is a 1% higher prevalence of the view that people cannot generally be trusted were 1.68 times more likely to receive functional disability certification [31]. Such studies suggest the importance of developing age-friendly community/cities in addition to individual interventions in healthcare and social welfare.

6 Summary

The proper handling of population aging is a global health problem that must be addressed for sustainable development. While a diverse set of issues must be addressed, one of the most important is healthy aging. In 2017, the World Health Organization released “10 Priorities for a Decade of Action on Healthy Ageing” [47], which presents a collection of high-quality data regarding healthy aging as a priority and points out the need for analytical reviews of current data sources to identify where gaps exist in measuring healthy aging over the life course.

As demonstrated in this review, large-scale studies of the older adults in Japan have found that, in addition to lifestyle, health behaviors, and medical history, the following items were also related to the healthy aging: mental health and psychological well-being, oral function/dental status, the up-stream factors of social participation, social networks/social support, educational attainment, income, income disparities, SES throughout one’s life course, loss of living environment, and community-level social capital including civic participation, social cohesion, and reciprocity.

“Healthy Japan 21 (The Second Term)” [48], which established the basic policy for health promotion in Japan, discusses improving the quality of social environments as a way to increase healthy aging and decrease health disparities. The results of this review also suggest that along with approaches that focus on high-risk individuals, it is important to implement social policies that increase social capital, reduce socioeconomic disparities, construct environments that promote social participation and physical activity, and offer education to individuals from a young age (thus enabling anyone to develop their abilities without relying on household economic resources). However, with the exception of some, the articles considered in this review are observation studies, and there is a need to elucidate the mechanisms behind the discussed factors in addition to conducting intervention studies.