PLOS ONERESEARCH ARTICLE

Impact of subjective well-being on physicalfrailty in middle-aged and elderly Japanesewith high social isolation
Kai Tanabe ID1,2☯*, Yuki Sugawara3,4☯, Eiichi Sakurai3, Yoichi Motomura3,
Yukihiko Okada5,6, Akiko Tsukao7, Shinya Kuno1,2

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OPEN ACCESS
Citation: Tanabe K, Sugawara Y, Sakurai E,
Motomura Y, Okada Y, Tsukao A, et al. (2024)
Impact of subjective well-being on physical frailty
in middle-aged and elderly Japanese with highsocial isolation. PLoS ONE 19(2): e0297837.
https://doi.org/10.1371/journal.pone.0297837
Editor: Sayani Das, International Institute of HealthManagement Research - New Delhi, INDIA
Received: July 1, 2023
Accepted: January 13, 2024
Published: February 26, 2024
Peer Review History: PLOS recognizes thebenefits of transparency in the peer review
process; therefore, we enable the publication of
all of the content of peer review and author
responses alongside final, published articles. The
editorial history of this article is available here:
https://doi.org/10.1371/journal.pone.0297837
Copyright: © 2024 Tanabe et al. This is an open
access article distributed under the terms of theCreative Commons Attribution License, which
permits unrestricted use, distribution, andreproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: Data cannot beshared publicly because of the contract with the
Ministry of Health, Labor and Welfare (MHLW).
Data are available from the MHLW Grants system

1 Faculty of Health and Sport Sciences, University of Tsukuba, Tsukuba, Japan, 2 R&D Center for Smart
Wellness City Policies, University of Tsukuba, Tsukuba, Japan, 3 Artificial Intelligence Research Center,
National Institute of Advanced Industrial Science and Technology, Tokyo, Japan, 4 Degree Programs in
Systems and Information Engineering, Graduate School of Science and Technology, University of Tsukuba,
Tsukuba, Japan, 5 Faculty of Systems and Information Engineering, University of Tsukuba, Tsukuba, Japan,
6 Center for Artificial Intelligence Research, University of Tsukuba, Tsukuba, Japan, 7 Tsukuba Wellness
Research Co., Ltd., Chiba, Japan☯ These authors contributed equally to this work.
* tanabe.kai.gm@u.tsukuba.ac.jp

Abstract
Social isolation exacerbates physical frailty and is associated with subjective well-being.
Even those with high levels of social isolation may have different health statuses dependingon the type of isolation and their subjective well-being. However, the effect of subjective
well-being on the relationship between social isolation and physical frailty remains unclear.
This study examined whether the risk of physical frailty was the same for individuals withsocial isolation according to high and low subjective well-being. The study participants
included 1,953 middle-aged Japanese adults aged 45 years and older. Physical frailty wasassessed using a modified version of the Fried phenotype criteria. Probabilistic Latent
Semantic Analysis was used to classify participants according to social isolation indicators.
Subsequently, we focused on the groups with high social isolation and classified themaccording to whether their subjective well-being was high or low. Subjective well-being wasevaluated using the Shiawase and Ikigai scales, which are concepts used in Japan. Finally,
we used survival time analysis to examine the relationship between Shiawase or Ikigai andphysical frailty in groups with high social isolation. The participants were classified into fourgroups based on their social isolation status. The physical frailty rate of the high social isolation class was 37.0%, which was significantly higher than that of the other classes. Survival
time analysis revealed that among people with high social isolation, those with high Shiawase and Ikigai had a significantly lower risk of physical frailty than those with low Shiawase
and Ikigai. All individuals with high social isolation are not at a high risk of physical frailty.
The findings reveal that even those with high level of social isolation may have a lower riskof physical frailty if their subjective well-being is high. These results will contribute to promoting the prevention of frailty in middle-aged and older adults.

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PLOS ONE(contact via e-mail; mhlw-grants@niph.go.jp) for
researchers who meet the criteria for access toconfidential data. The data underlying the resultspresented in the study are available from MHLW
Grants system. The contact information for the
MHLW Grants system is listed below. MHLW
Grants system 2-3-6, Minami, Wako, Saitama 3510197, Japan e-mail: mhlw-grants@niph.go.jp URL:
https://mhlw-grants.niph.go.jp/ Tel: +81-4-84586210
Funding: This work was supported by a Health
Labor Sciences Research Grant, Research for
Building Evidence on the Results of Verification
Programs for Prevention and Health Promotion(22FB1002).The funders had no role in study
design, data collection and analysis, decision to
publish, or preparation of the manuscript.
Competing interests: The authors have declaredthat no competing interests exist.

The relationship between physical frailty and subjective well-being in socially isolated individuals

Introduction
Aging of the global population is the most important medical and sociodemographic issue thatneeds to be addressed not only in developed regions where the population is already aging but
also in developing regions where the population is aging soon [1]. Japan has the highest percentage of population aged 65 years and older and has been called the advanced aging country; thecountry’s actions and future are attracting global attention. In Japan, the number of households
headed by elderly people aged 65 years and older accounts for 49.4% of all households, of which
28.8% are one-person households; this percentage is increasing every year [2]. Population
decline and aging in rural and mountainous areas precede those in urban areas, and rural andmountainous areas face a lack of resources to support the social isolation of the elderly. Physicalfrailty is a health outcome that should be considered in the context of social isolation in older
adults. Physical frailty, the leading complaint of frailty, has been reported to increase the risk ofmortality and shorten healthy life expectancy, and has been shown to be associated with other
health outcomes such as gait dysfunction, falls, fractures, cognitive decline, hospitalization,
nursing home placement, and quality of life [3, 4]. However, physical frailty can be prevented
because it is reversible [3]. Prevention of physical frailty not only contributes to extending
healthy life expectancy and improving the quality of life of individuals but also helps solve social
issues by reducing social security costs, such as medical and long-term care costs [5].
Several studies have examined the relationship between physical frailty and social isolation inolder adults. Recently, Kojima et al. (2022) [6] reported the results of a systematic review andmeta-analysis that provided evidence that social isolation contributes to an increased risk of physical frailty in older adults. In addition, a 14-year longitudinal study of a British population showedthat social isolation was associated with an increased risk of developing physical frailty [7].
Social Isolation is a diverse concept [8, 9]. Recently, attention has been paid to this diversity
when considering support for social isolation. Previous studies have typologies for diverse
social isolation and have attempted to identify groups at higher risk of declining mental andphysical health status [10, 11]. Barnes et al. (2022) [12] examined the impact of the combination of social isolation and loneliness on wellbeing. They reported lower well-being in theorder of isolation only, loneliness only, and both compared to groups with neither social isolation nor loneliness. Smith and Victor (2019) [13] used latent class analysis to classify study participants based on three factors: social isolation, loneliness, and living alone. The results
showed that the participants could be categorized into six clusters, of which the group thatexperienced social isolation and loneliness had poorer physical and mental health. Similarly
for physical frailty, there is a need to consider the diversity of social isolation in the preventionof physical frailty. However, only a few studies have clarified this issue [14].
When examining the diversity of social isolation among the elderly in Japan, it is necessary
to consider Japan-specific subjective well-being. Kumano (2018) [15] suggested subjective
well-being in Japanese culture includes not only Shiawase (Happiness) as a Hedonic wellbeing, but also Ikigai (the joy and goal of living / a life worth living) as an Eudaimonic wellbeing. Several studies support this claim [16–18]. Shiawase’s emotions contain strong elementsof joy and comfort and are oriented toward the present. On the other hand, the feeling of Ikigai
has a strong element of devotion to one’s favorite activities and is oriented toward the future.
Previous studies have reported that Shiawase and Ikigai are associated with social isolation, but
only a few studies have examined their relationship to physical frailty [19–21].
Therefore, the research question in this study was as follows: Does subjective well-being
affect physical frailty in middle-aged and elderly Japanese with high social isolation? This
study examined the impact of subjective well-being on physical frailty in middle-aged andelderly Japanese individuals living in mountainous areas with high social isolation.

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PLOS ONEThe relationship between physical frailty and subjective well-being in socially isolated individuals

Methods
ParticipantsThis study was carried out in part by a Health Labour Sciences Research Program, Researchfor Building Evidence on the Results of Verification Programs for Prevention and Health Promotion. A portion of the cross-sectional study data was used. The data obtained for this project
were a combination of survey data and data from the medical and long-term care insurance
receipts of survey respondents [22]. The participants were middle-aged older adults aged 45
years or older living in a small Japanese city (population of approximately 40, 000 people)
located in a mountainous area. These participants were also enrolled in either the National
Health Insurance or Late-Stage Medical Insurance for the Elderly.
Participant recruitment and questionnaire surveys were conducted between October 15
and November 19, 2021. Medical and long-term care receipt data were collected on December
24, 2021, and combined with questionnaire data. The survey was conducted in a way that all
citizens aged 45 years and older were surveyed. Questionnaires were sent to 11,303 individuals
and data were collected from 4,045 participants. The data that could be combined with the
respondents’ medical and long-term care insurance receipts were 3,706. The total number ofparticipants in the analysis was 1,953. Exclusion criteria for the analysis were missing questionnaire items assessing physical frailty and social isolation and having been certified as needing
long-term care between 2011 and 2019. To assess the risk of physical frailty, individuals certified as requiring nursing care were excluded from the analysis. In addition, to eliminate theinfluence of COVID-19 as much as possible, participants were asked to recall two time points:
2019 (before the spread of COVID-19) and 2011. The variables recalled were frequency of conversation, participation in social activities, status of life activities, physical fitness, social capitalin the community, and subjective well-being (Shiawase (happiness) and Ikigai (the joy andgoal of living/a life worth living)). The questionnaire data included demographic variables, frequency of conversations, participation in social activities, status of life activities, physical fitness, social capital in the community, and subjective well-being. The medical and long-termcare insurance receipt data included the date of birth, sex, and whether the patient was certified
as needing long-term care.

MeasuresPhysical frailty. In this study, we assessed physical frailty using Fried’s criteria [23].
Frailty was defined as the meeting of three or more of the following five evaluation criteria:
shrinkage, exhaustion, low activity, slowness, or weakness. The frailty assessment index haschanged frequently in previous studies, and it has been suggested that all details regarding themeasurement of the frailty phenotype criteria should be reported to aid in the interpretation of
results [24]. Therefore, in this study, we modified and applied Fried’s assessment criteria(Table 1). Questionnaire data were collected in 2021 as a reminder of the 2019 conditions, anddata from 2019 were used to assess physical frailty.
Social isolation. Social isolation is an objective concept that refers to a state of limitedsocial connection or assistance [25]. Previous studies have shown that social isolation is associated with increased mortality, cardiovascular diseases, and reduced functional status [26–28].
Questions have been used to measure social isolation, such as marital status; household composition; frequency of contact with friends, family, and children; and participation in social
activities [7, 27, 29]. However, there are no uniform indicators of social isolation, because theitems used to define social isolation vary across studies [30]. Therefore, in this study, we
assessed social isolation using 22 questions, including job availability, frequency of phone and

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PLOS ONEThe relationship between physical frailty and subjective well-being in socially isolated individuals

Table 1. Frailty assessment criteria.
Evaluation
criteria

Fried et. al. (2001) [23]Modified

Shrinkage

Baseline: >10 lbs lost unintentionally in prioryear

Weight loss of at least 5% from 2011 to 2019

Exhausion

Self-reported exhaustion, identified by twoquestions from the CES–D scale

The total score of 21 points or more on a 6-point scale for “I felt cheerful and happy,” “I felt calmand relaxed,” “I was motivated and active,” “I rested well and woke up feeling good,” and “There
were many things in my daily life that interested me.”Low activity

Kcals/week: lowest 20% males: <383 Kcals/week No to both “Did you engage in light sweaty exercise for at least 30 minutes a day for at least 2 days
females: <270 Kcals/week
a week for at least 1 year?” and “Did you engage in walking or equivalent physical activity for atleast 1 hour a day in your daily life?”

Slowness

Walking time/15 feet: slowest 20% (by gender,height)No to “Do you think you walk faster than your peers of approximately the same age?”

Weakness

Grip strength: lowest 20% (by gender, bodymass index)

No to one of the following questions: “Are you able to put on socks, pants, or a skirt while
standing without support?” or “Are you able to lift a 10 kg bag of rice?”https://doi.org/10.1371/journal.pone.0297837.t001

SNS conversations, frequency of going out less frequently, willingness to contribute to thecommunity, and community cooperation trust, in addition to questions measuring social isolation used in previous studies. The questions assessing social isolation are listed in Table 2.
Table 2. Social isolation assessment criteria.
Question type

Variables

Discretization

Score

Demographics

Living alone

yes or no

yes: 1, no: 0

Marital status

having a spouse, bereavement, divorce,
unmarried, others

having a spouse: 1, theothers: 0

Frequency of conversation with family

everyday or not

everyday: 0
not everyday: 1

Frequency of conversation with non-family

three point scale

high: 1,
middle: 2,
low: 3
yes: 0
no: 1

Frequency of conversation

Frequency of conversation with phone or SNS
Social activity

Participation of social activity

yes or no

Participation of sports activity

yes or no

Participation of health and medical volunteer

yes or no

Participation of volunteer at elementary and juniorhigh schools

yes or no

Participation of other volunteer

yes or no

Participation of hobby activity

yes or no

Participation of neighborhood association, residents’associations etc.

yes or no

Participation of learning and culture activity

yes or no

Participation of local event

yes or no

Participation of activity to convey special skills toothersyes or no

yes: 0
no: 1

Participation of other activity

yes or no

Eating with someone once a day

yes or no

yes: 0
no: 1

Frequency of going out decreasing

yes or no

yes: 1
no: 0

Employed

yes or no

yes: 0
no: 1

Community cooperating trust

Community cooperating trust

three point scale

Willingness of contribution to thecommunity

Willingness of contribution to the community

three point scale

high: 1,
middle: 2,
low: 3

Living conditions

https://doi.org/10.1371/journal.pone.0297837.t002

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PLOS ONEThe relationship between physical frailty and subjective well-being in socially isolated individuals

We measured the frequency of conversation on a 5-point scale from “daily” to “rarely/never”
in response to the question, “How often did you talk on the phone or social networking sites?”
It was discretized into three levels: “every day, 5–6 days a week,” “3–4 days a week,” and “1–2days a week, hardly ever/never.” We measured willingness to contribute to the community on
a 5-point scale from “applicable” to “not applicable” to the question “I have always wanted tocontribute to the community.” It was discretized into three levels: “Yes/somewhat,” “Neither
agree nor disagree,” and “Not really/not really.” We measured community cooperating trust
on a 5-point scale from “very much so” to “not at all” in response to the questions “There arepeople in this community that I can cooperate, consult, and rely on” and “I think people inthis community are generally trustworthy.” We defined a total score of 9 or higher as high incommunity cooperating trust, 4–8 as normal in community cooperating trust, and 3 or lower
as low in community cooperating trust.
Subjective well-being. Although the concept of well-being has been widely discussed, its
meaning has not been defined as it varies across cultures. Kumano et al. (2018) [15] argue thatin Japan, subjective well-being is expressed as both Shiawase, which is Hedonic well-being,and Ikigai, which is Eudaimonic well-being. The former is translated into English as “happiness” and the latter as “the joy and goal of living” or “a life worth living.” These two well-being
dimensions have been reported to be associated with health outcomes among Japanese people
and investigated through national surveys [31–33].
In this study, Shiawase and Ikigai were evaluated based on the previous survey scales [15,31, 34]. Shiawase was evaluated based on the question, “To what extent did you feel Shiawase?”
It uses an 11-point scale ranging from 0 to 10, with higher scores indicating greater happiness.
Scale scores of 0 to 4 were classified as low Shiawase, 5 to 6 as normal Shiawase, and 7 to 10 ashigh Shiawase.
Ikigai was evaluated based on the question, “To what extent did you feel Ikigai?” This scale
was measured on a 5-point scale from “I feel well” to “I don’t feel well at all.” Scale scores of 1
to 2 were classified as high Ikigai, 3 as normal Ikigai, and 4 to 5 as low Ikigai.

Analysis flow
We used probabilistic latent semantic analysis (PLSA) and survival time analysis to examine
the associations between physical frailty, social isolation, and subjective well-being. First, we
extracted potential clusters using the PLSA with questions on social isolation, as listed inTable 2. PLSA was proposed by Hofmann as a document classification method [35]. Thismethod assumes that words wj in sentence di are generated by the latent variable zk. Likelihood
maximization with the EM algorithm, with the latent variable z2Z = {z1,. . .,zk} to accompany
the co-occurring data. The simultaneous probability of sentence di and word wj is expressed in
Eq (1) using the latent variable zk.
XPðdi ; wj Þ ¼
Pðdi jzk ÞPðwj jzk ÞPðzk Þ
ð1Þ
k

The EM algorithm is then used to calculate PðdjzÞPðwjzÞPðzÞ, which maximizes the following log-likelihood function L as shown in Eq (2). In addition, n(i,j) is the number of times
word wj co-occurs with document di.
XX
L¼
nði; jÞlogPðdi ; wj Þ
ð2Þ
i

j

There is an initial value dependence because the likelihood calculation for PLSA uses the
EM algorithm. Therefore, the number of iterations was set to 1,000, the initial value was given

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PLOS ONEThe relationship between physical frailty and subjective well-being in socially isolated individuals

three times, and the number of latent classes was increased from 1 to 10 to search for the bestmodel. The number of latent classes was determined based on the Akaike’s Information Criterion (AIC). The features of each cluster obtained by the PLSA were analyzed by constructing a
Bayesian network model and applying a probabilistic inference algorithm based on loopy belief
propagation. In this model, we used probabilistic inference to determine the direction of thecontribution of question choices based on the positivity or negativity of the probability of
belonging, and identified the characteristics of each cluster. An analytical method combining
PLSA and Bayesian networks was developed by the National Institute of Advanced Industrial
Science and Technology (AIST) in Japan and has been used in many empirical studies [36]. Ide
et al. (2017) [37] used a questionnaire and ZIP code data of elderly people in 24 municipalities
in Japan and extracted potential regional characteristics as segments. Furthermore, a Bayesian
network was constructed using the questionnaire data and variables of the obtained segments,
and probabilistic inferences were made. Consequently, this study reveals the potential regional
characteristics and factors contributing to regional disparities. Kawai et al. (2022) [22] applied
PLSA to questionnaire data, such as those used in this study, and classified people according totheir health literacy. They also used a Bayesian network to construct a prediction model forphysical frailty in each health literacy category. Consequently, this study enabled the construction of models with high predictive accuracy. Extracting potentially common meaningful classes
using PLSA allows useful model building in subsequent analyses. In this study, we applied thismethod to 1,953 questionnaires and classified people into several clusters according to theirsocial isolation, with sentence di as the respondent of the questionnaire and word wj as the stateof whether they selected each questionnaire item. We then analyzed the characteristics of eachcluster. We analyzed the differences in sex, age group proportions, and physical frailty rates
between clusters obtained by PLSA using the Z-test, with p-value adjustment using the Bonferroni method. We also analyzed differences in mean age between clusters using a one-way analysis of variance (ANOVA), as normality was confirmed using the Shapiro-Wilk test.
Next, we focused on clusters with high social isolation among the obtained clusters and analyzed the association between subjective well-being and physical frailty using survival time
analysis. Survival time analysis is an analytical method that focuses on the relationship between
events and the time until a specific event occurs, such as time to death, disease onset, and time
to recovery. In this study, we used the Kaplan-Meier method, which represents the evolutionof event rates with respect to the observation period, among survival time analysis methods.
The observation period was defined based on the age of each individual and the occurrence ofan event was defined as physical frailty. The log-rank test was used to determine statistical significance. Shiawase and Ikigai were used to assess subjective well-being. In the analysis of Shiawase, a total of 1,950 data were used, excluding three cases in which questions assessing
Shiawase were missing. In the analysis of Ikigai, a total of 1,953 data were used because there
were no missing questions that assessed Ikigai. We used PLASMA and Bayonet, an intellectual
property software from the National Institute of Advanced Industrial Science and Technology,
to run the PLSA and Bayesian network [38, 39]. We used IBM SPSS Statistics (Version
28.0.1.0) for the other data analysis. Statistical significance was set at p < 0.05.

Results
Comparing the physical frailty rates by social isolation cluster
Of the 1,953 participants included in the analysis, 996 (51.0%) were male and the mean age
was 69.8 years. The averages and standard deviations for age, Shiawase score, Ikigai score,
physical frailty score, and social isolation score are shown in Table 3. Social isolation scores inthis study were calculated by summing the scores of the social isolation questionnaire items

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PLOS ONEThe relationship between physical frailty and subjective well-being in socially isolated individuals

Table 3. Descriptive statistics of the analyzed data.
Question typesn

Average (standard deviation)

Age (years)

1,953

69.8 (8.4)

Shiawase score

1,936

7.35 (2.23)

Ikigai score

1,942

1.95 (0.98)

Physical frailty score

1,953

1.44 (1.20)

Social isolation score

1,953

18.85 (3.32)

n

rate

Question types

Variables

Employment (n = 1,953)

Employed

679

34.8%

Education status (n = 1,924)

Elementary and junior high school graduates

421

21.9%

High school graduate

1,092

56.8%

Specialized or vocational school graduate

164

8.5%

Junior college or technical college graduate

87

4.5%

University graduate

153

8.0%

Master’s graduate

7

0.4%

https://doi.org/10.1371/journal.pone.0297837.t003(Table 2), in which 1 point was allocated for each of the following: living alone; not having a
spouse; frequency of conversation with family members is not daily; not participating in any of
the social activities, respectively; not eating with someone once a day; frequency of going out
decreasing; not employed. In addition, the frequency of conversation with non-family and thefrequency of conversation with phone or SNS, community cooperating trust, willing of contribution to community was divided into three-point scales: high, normal, and low, and assigned
a score of 1, 2, and 3, respectively. In addition, the number and percentage of people in theanalyzed data for employment status and education status are shown in Table 3.
The results of the PLSA using questions on social isolation showed that the AIC score was
the lowest when K = 4. We adopted four classes 4, C1–C4. The details of the AIC scores areprovided in S1 Fig. The number of persons belonging to each cluster, as well as their gender,
age category, and mean age are shown in Table 4. C1 included 155 participants (63.9% male),C2 701 participants (52.6% male), C3 851 participants (47.4% male), and C4 246 participants(50.8% male). Sex and age group percentages are significantly different at the 5% level between
groups with different symbols. The mean age for all age groups was 69.5 years for C1, 68.6
years for C2, 70.6 years for C3, and 70.4 years for C4. One-way analysis of variance confirmed
a significant difference in mean age among the four groups C1–C4 (p<0.01).
Next, we constructed a Bayesian network model using the variables of the four clusters andthe social isolation question to confirm the characteristics of the four clusters obtained using
the PLSA. Fig 1 shows the nodes around C1 cluster in the constructed Bayesian network. The
Table 4. Percentage of sex and age group and average age by cluster.

#clusternPercentage of sex (Male)#

Percentage of age #under 65

over 65, under 75

over 75

all

C1

155

63.9%a

19.3%a,b,c

52.9%a

27.7%a,b

69.5 (8.34)

C2

701

52.6%a,b

26.0%c

50.4%a

23.7%b

68.6 (8.52)

C3

851

b

47.4%

b

15.9%

a

54.4%

29.7%a70.6 (7.50)

C4

246

50.8%a,b

25.6%a,c

38.2%b

36.2%a70.4 (10.57)

all

1,953

51.0%

21.0%

50.8%

28.2%

69.8 (8.42)

Average age (years) (standard deviation)

Groups with different symbols (a-c) indicate significant differences at the 5% level.

https://doi.org/10.1371/journal.pone.0297837.t004

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PLOS ONEThe relationship between physical frailty and subjective well-being in socially isolated individuals

Fig 1. Bayesian network about C1.
https://doi.org/10.1371/journal.pone.0297837.g001

nodes around C2–C4 are shown in S2–S4 Figs. The nodes with arrows pointing directly toeach cluster are choices for questions in which the probability of belonging to each clusterchanges. In this model, we used probabilistic inference to determine the direction of the contribution of question choices based on the positivity or negativity of the probability of belonging, and identified the characteristics of each cluster. The results of the probabilistic inferenceof the relevant questions for C1–C4 are shown in S1 Table. We used this probability inference
to determine that C1 is characterized by “participates in sports and hobby-related activities”
and “has a high sense of community contribution,” C2 is characterized by “frequent conversations outside the family and on the phone and SNS” and “does not have a high sense of community contribution,” C3 is characterized by “normal sense of community contribution” and“does not participate in social activities,” and C4 is characterized by “does not participate insocial activities” and “infrequent conversations outside the family, on the phone, and on SNS”
and “low awareness of community contribution.” These differences in characteristics suggestthat C1 is a cluster with low social isolation and C4 is a cluster with high social isolation.
Table 5 shows the results of the comparison of physical frailty rates and the number of people with physical frailty for each of the four clusters. We observed that C4, with high social isolation, had a significantly higher rate of physical frailty than the other three clusters did.

Comparing the risk of physical frailty by the difference of subjective wellbeing among people with high social isolation
In this chapter, we focus on C4, with high social isolation, and describe the results of the
Kaplan-Meier method to determine the relationship between subjective well-being and

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PLOS ONEThe relationship between physical frailty and subjective well-being in socially isolated individuals

Table 5. Physical frailty rates by cluster.
Physical frailty rates #cluster (n)(number of persons)under 65#aOver 65, under 75

over 75

a

a

all

C1

10.0%

7.3%

18.6%

11.0%a(n = 155)(n = 3)(n = 6)(n = 8)(n = 17)C2

11.5%a

10.8%a

22.3%a

13.7%a(n = 701)(n = 21)(n = 38)(n = 37)(n = 96)C3

17.8%a

16.2%a

31.2%a,b

20.9%b(n = 851)(n = 24)(n = 75)(n = 79)(n = 178)

C4

38.1%b

28.7%b

44.9%b

37.0%c(n = 246)(n = 24)(n = 27)(n = 40)(n = 91)

all

17.6%

14.7%

29.8%

19.6%

(n = 1,953)(n = 72)(n = 146)(n = 164)(n = 382)

Groups with different symbols (a-c) indicate significant differences at the 5% level.

https://doi.org/10.1371/journal.pone.0297837.t005

physical frailty in this population. We used the questionnaire items on Shiawase and Ikigai as
indicators of subjective well-being. Among the C4 with high social isolation, the percentage of
those with physical frailty by the level of Shiawase was 24.3% in the group with a high level of
Shiawase, 38.3% in that with a normal level of Shiawase, and 59.3% in that with a low level of
Shiawase. Similarly, among the C4 with high social isolation, the number percentage of thosewith physical frailty according to the level of Ikigai was 23.5% in the group with a high level of
Ikigai, 40.6% in that with a normal level of Ikigai, and 60.3% in that with a low level of Ikigai.
Fig 2 shows the results of the Kaplan-Meier analysis of the rate of physical frailty in C4 withhigh social isolation according to the level of Shiawase. The risk of physical frailty increased as
the level of well-being decreased (Fig 2). Table 6 shows the average age of survival by the levelof Shiawase. We found significant differences in all comparisons between the two groups: thehigh and low level of Shiawase (p<0.01), the high and normal level of Shiawase (p<0.01), andthe normal and low level of Shiawase (p<0.05). At the age of 75, the group with a high level of
Shiawase had a significantly lower risk of physical frailty by 47.6% than that with a low level of
Shiawase.
Fig 3 shows the results of the Kaplan-Meier analysis of the rate of physical frailty in C4 withhigh social isolation by the level of Ikigai. The risk of physical frailty increased as the level of
Ikigai decreased (Fig 3). Table 7 shows the average age of survival according to the level of Ikigai. We found significant differences between the group with the high and low level of Ikigai(p<0.01) and between the group with the normal and low level of Ikigai (p<0.01). At age 75,
the group with a high level of Ikigai had a significantly lower risk of physical frailty (42.7%)
than that with a low level of Ikigai.

Discussion
Relationship between social isolation and physical frailty
Participants with a higher degree of social isolation had a significantly higher rate of physicalfrailty than those with a lower degree of social isolation. This result was similar by age group(< 65 y., 65–74 y., > = 75 y.). The results of the present study are similar to those of several
previous studies and support the hypothesis that social isolation increases the risk of physicalfrailty [6].

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PLOS ONEThe relationship between physical frailty and subjective well-being in socially isolated individuals

Fig 2. Kaplan-Meier analysis according to degree of Shiawase.
https://doi.org/10.1371/journal.pone.0297837.g002

Effects of Shiawase and Ikigai on physical frailty in people with high degreeof social isolation
Shiawase and Ikigai in middle-aged and elderly people may affect the rate of physical frailty[21]. In the present study, we examined whether Shiawase and Ikigai influenced the rate ofphysical frailty in people with high social isolation. The results showed that Shiawase and Ikigai
were factors that differentially affected the rate of physical frailty in people with high social isolation. In other words, even if the degree of social isolation is high, those who live with Shiawase or Ikigai may have a lower risk of physical frailty.
Although few previous studies have examined the relationship between Shiawase and Ikigai
and physical frailty, several studies have reported that these factors explain low rates of frailty
Table 6. The average age of survival by the level of Shiawase.
The level of Shiawase

Estimated valueStandard error

Lower limit

Upper limithigh83.6431.069

81.54785.739

middle78.9391.603

75.79682.081low

72.964

1.922

69.19876.730all

79.9770.964

78.08781.86795% confidence interval

https://doi.org/10.1371/journal.pone.0297837.t006

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PLOS ONEThe relationship between physical frailty and subjective well-being in socially isolated individuals

Fig 3. Kaplan-Meier analysis according to degree of Ikigai.
https://doi.org/10.1371/journal.pone.0297837.g003[21]. The present study is the first to show that Shiawase and Ikigai affect the rate of physicalfrailty in people with high social isolation.

Why do people with Shiawase and Ikigai have lower rates of physical frailty
even among those with high levels of social isolation?
Relatively strong evidence suggests that social isolation increases the risk of physical frailty [6].
However, in the present study, even among those with high social isolation, those with highlevels of Shiawase and Ikigai maintained a low risk of physical frailty. Intentional or selfimposed social isolation might have contributed to these results.
Social isolation among older adults is diverse and includes both accidental and intentional
isolation. Intentional social isolation is believed to lead to a reduction in social networks andTable 7. The average age of survival by the level of Ikigai.
The level of Ikigai

Estimated valueStandard error

95% confidence interval
Lower limit

Upper limithigh82.9601.080

80.84285.077

middle80.4091.644

77.18783.631low

72.3681.788

68.86275.873all

80.051

0.961

78.16781.934https://doi.org/10.1371/journal.pone.0297837.t007

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PLOS ONEThe relationship between physical frailty and subjective well-being in socially isolated individuals

personal social interactions as a result of prioritizing emotional satisfaction and spending
more time with those who are closer to them in old age [40, 41]. Tanaka et al. (2020) [41]reported that those who were socially isolated were characterized by less interaction with thecommunity and poor physical function, although they had no history of current or prior illness. They were also highly educated, did not tend to suffer economically, and were in situations where they could deliberately choose to live in social isolation.
Toyoshima and Kusumi (2022) [42] examined the relationship between preference for solitude (which in this study refers to time spent alone) and subjective well-being and reported
that enjoyment of solitude and productivity while being solitary were associated with themaintenance of subjective well-being among the elderly. In our study, the number of individuals with high levels of social isolation who were intentionally socially isolated or enjoyed solitude was unclear; however, it could be inferred that there was a certain number of them.
Because among the participants with high social isolation, the percentages of those with highShiawase and Ikigai scores were 42.4% (n = 103) and 48.4% (n = 119), respectively. These people may be intentionally socially isolated or may enjoy solitude. Even if they are socially isolated, those who are aware of their social isolation and lead a lifestyle that facilitates happinessor live with joy and the goal of living may be less likely to fall into the frailty cycle proposed byFried et al. (2001) [23]. Although only a few studies have tested this hypothesis, the results ofthe present study support it.
Previous studies have shown that physical activity reduces the risk of frailty the mostamong physical, cultural, and community [43]. Nagai et al. (2018) [44] also found that if 30
minutes of sedentary activity time could be replaced with low-and even medium-to-highintensity physical activity, frailty status could be improved by 16% and 42%, respectively. Furthermore, sedentary activity styles centered on TV viewing were associated with lower physicalfunction than other sedentary activity styles (less sedentary activity, work and computer usecentered, and leisure activities other than TV viewing) [45]. Increasing the amount of physicalactivity performed alone and reducing the amount of time spent on passive sedentary activities
may reduce the risk of physical frailty even if the degree of social isolation is high. This study
did not examine the extent to which the participants preferred to be alone. In the future, it isnecessary to investigate the extent to which people prefer to spend time alone as well as theirsocial isolation status, and to investigate what kind of activities make them feel Shiawase or
Ikigai.
Finally, the promotion of physical frailty prevention in middle- and older-aged adults withhigh levels of social isolation is discussed. A survey of the Japanese elderly population revealed
that awareness of the term “frailty” was low, at about 20%, and that the more people needed totake measures against frailty, the lower their level of awareness [46]. Understanding and preventing physical frailty are important for maintaining and improving the quality of life of individuals, regardless of whether they are socially isolated or prefer to spend time alone.
Therefore, it is necessary to provide information and preventive programs for frailty amongolder adults in social isolation. Delivering such information to socially isolated people andencouraging them to take preventive actions are subjects for future research.

Limitations
This study had several limitations. The study design was cross-sectional; therefore, causal relationships between the variables could not be identified. In addition, all questionnaire responses
were self-reported. This may have generated misclassification bias. To eliminate the influenceof COVID-19 as much as possible, the participants were asked to recall two time points: 2019
(before the spread of COVID-19) and 2011. This may have resulted in inaccurate results. In

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PLOS ONEThe relationship between physical frailty and subjective well-being in socially isolated individuals

Table 8. The difference of the analyzed and excluded data.
n

Percentage of sex (Male)#
under 65

over 65, under 75

over 75

allanalyzed data

1,953

51.0%a

21.0%a

50.8%a

28.2%a69.8 (8.42)

19.6% aexcluded data

1,753

42.7%b

9.6%b

34.7%bb

55.7%b74.7 (8.34)

29.4% b#

Percentage of age #Average age (years) (standard deviation)

Physical frailty rates #Groups with different symbols (a, b) indicate significant differences at the 5% level.

https://doi.org/10.1371/journal.pone.0297837.t008

addition, missing data were removed from 3, 706 data points collected in this study, resulting
in a final sample of 1,953 participants. Because of the removal of missing data, only those who
responded to all items used in the analysis were extracted. In addition, information on individuals who had never been certified as needing long-term care between 2011 and 2019 was
extracted. The sociodemographic characteristics and health statuses of the included andexcluded participants are shown in Table 8. In the analyzed data (n = 1,953) and the excluded
data (n = 1,753), the percentage of males differed by 8.3 percentage point, the average age was
4.9 years, and the rate of physical frailty was 9.8 percentage point. Among the excluded data,physical frailty was assessed in 988 samples. These differences suggest that this study analyzed
a relatively healthy sample of the total data. The survey items did not include feelings of loneliness or a preference for solitude. Investigating these items would have allowed for more multidimensional considerations. Finally, the study area is a small city in the mountainous region ofJapan. Therefore, similar results may not be obtained for small, urban, or large cities.
This study has several limitations. The study design was cross-sectional, so causal relationships between variables could not be identified. In addition, all responses to the questionnaire
were self-reported. This may have generated a misclassification bias. Furthermore, to eliminate
the influence of COVID-19 as much as possible, the participants were asked to recall two time
points: 2019 (before the spread of COVID-19) and 2011. This may have resulted in inaccurate
responses. In addition, missing data were removed from the 3,706 data collected in this study,
resulting in a final sample of 1,953 analyzed. As a result of the removal of missing data, only
those who responded to all items used in the analysis were extracted. In addition, people whohave never been certified as needing long-term care from 2011 to 2019 were extracted. The
sociodemographic characteristics and health status of the analyzed and excluded data areshown in Table 8. In the analyzed data (n = 1,953) and the excluded data (n = 1,753), the percentage of males differs by 8.3 percentage point, the average age by 4.9 years, and the rate ofphysical frailty by 9.8 percentage point. Of the excluded data, physical frailty was assessable in988 samples. These differences suggest that this study is an analysis of a relatively healthy sample of the total data. And the survey items did not include feelings of loneliness and preference
for solitude. Investigation of these items would have allowed for more multidimensional consideration. Finally, the study area was a small city in a mountainous region of Japan. Therefore,
the same results may not be obtained in other small cities, urban areas, or large cities.

Conclusions
The research question for this study was,” Does subjective well-being affect physical frailty in
middle-aged and elderly Japanese with high social isolation?.” The results showed that subjective well-being affected physical frailty in middle-aged and elderly Japanese with high social
isolation.
In conclusion, our results suggest that subjective well-being, such as Shiawase and Ikigai,
may influence physical frailty risk among middle-aged and elderly Japanese individuals withhigh levels of social isolation living in mountainous areas. The strength of this study is that it

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PLOS ONEThe relationship between physical frailty and subjective well-being in socially isolated individuals

suggests that living with a feeling of happiness and purpose may reduce the risk of physicalfrailty, even among individuals with high levels of social isolation. This study provides important insights for the prevention of frailty among socially isolated individuals. On the other
hand, a shortcoming of this study is that the reasons why socially isolated individuals are
happy and what kind of purpose they have in life were not investigated. These aspects require
further investigation. The results of this study can serve as a reference for other countries withan aging population. The participants in this study live in a typical regional city with an aging
and declining population; the average income of its citizens is about $18,000 (converted to147.78 yen/dollar), which is relatively low among all Japanese cities [47]. In the future, many
countries will face aging populations. In these countries, it is expected that the aging population, social isolation, and frail population will increase in rural areas compared with urban
areas, resulting in widening regional disparities. The findings of this study suggest that physicalfrailty must be prevented in rural cities with aging populations, considering both the diversityof social isolation and subjective well-being of individuals.

Supporting information
S1 Fig. AIC scores in clustering of social isolation by PLSA.
(TIF)
S2 Fig. Bayesian network about C2.
(TIF)
S3 Fig. Bayesian network about C3.
(TIF)
S4 Fig. Bayesian network about C4.
(TIF)
S1 Table. Characteristics of each cluster by probabilistic inference.
(TIF)

Author Contributions
Conceptualization: Kai Tanabe, Yuki Sugawara, Yukihiko Okada.
Data curation: Yuki Sugawara, Eiichi Sakurai.
Formal analysis: Yuki Sugawara, Eiichi Sakurai, Yoichi Motomura, Yukihiko Okada.
Funding acquisition: Shinya Kuno.
Investigation: Kai Tanabe, Akiko Tsukao.
Methodology: Yuki Sugawara, Eiichi Sakurai, Yoichi Motomura, Yukihiko Okada, Akiko
Tsukao.
Project administration: Kai Tanabe.
Resources: Shinya Kuno.
Supervision: Kai Tanabe, Shinya Kuno.
Validation: Yuki Sugawara, Eiichi Sakurai, Yoichi Motomura, Yukihiko Okada.
Writing – original draft: Kai Tanabe, Yuki Sugawara.

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PLOS ONEThe relationship between physical frailty and subjective well-being in socially isolated individuals

Writing – review & editing: Kai Tanabe, Yuki Sugawara, Eiichi Sakurai, Yoichi Motomura,
Yukihiko Okada, Akiko Tsukao, Shinya Kuno.

References
1.

Rudnicka E, Napierała P, Podfigurna A, Męczekalski B, Smolarczyk R, Grymowicz M. The World HealthOrganization (WHO) approach to healthy ageing. Maturitas. 2020; 139: 6–11. https://doi.org/10.1016/j.
maturitas.2020.05.018 PMID: 32747042

2.

Cabinet Secretariat. Annual Report on the Ageing Society. 2021 July [Cited 2023 June 21]. Available
from: https://www8.cao.go.jp/kourei/english/annualreport/index-wh.html

3.

Makizako H, Shimada H, Doi T, Tsutsumimoto K, Suzuki T. Impact of physical frailty on disability incommunity-dwelling older adults: a prospective cohort study. BMJ Open. 2015; 5(9): 1–9. https://doi.
org/10.1136/bmjopen-2015-008462 PMID: 26338685

4.

Hoogendijk EO, Afilalo J, Ensrud KE, Kowal P, Onder G, Fried LP. Frailty: implications for clinical practice and public health. Lancet. 2019; 394(10206): 1365–1375. https://doi.org/10.1016/S0140-6736(19)31786-6 PMID: 31609228

5.

Yamada M, Arai H, Sonoda T, Aoyama T. Community-based exercise program is cost-effective by preventing care and disability in Japanese frail older adults. J Am Med Dir Assoc. 2012; 13(6): 507–511.
https://doi.org/10.1016/j.jamda.2012.04.001 PMID: 22572554

6.

Kojima G, Aoyama R, Tanabe M. Associations Between Social Isolation and Physical Frailty in Older
Adults: A Systematic Review and Meta-Analysis. J Am Med Dir Assoc. 2022; 23(11): 3–6. https://doi.
org/10.1016/j.jamda.2022.06.012 PMID: 35810790

7.

Davies K, Maharani A, Chandola T, Todd C, Pendleton N. The longitudinal relationship between loneliness, social isolation, and frailty in older adults in England: a prospective analysis. Lancet Healthy
Longev. 2021; 2(2): 70–77. https://doi.org/10.1016/S2666-7568(20)30038-6 PMID: 36098160

8.

Finlay JM, Kobayashi LC. Social isolation and loneliness in later life: a parallel convergent mixed-methods case study of older adults and their residential contexts in the Minneapolis metropolitan area, USA.
Soc Sci Med. 2018; 208: 25–33. https://doi.org/10.1016/j.socscimed.2018.05.010 PMID: 29758475

9.

Hawkley LC, Cacioppo JT. Loneliness matters: a theoretical and empirical review of consequences andmechanisms. Ann Behav Med. 2010; 40(2): 218–227. https://doi.org/10.1007/s12160-010-9210-8
PMID: 20652462

10.

Menec VH, Newall NE, Mackenzie CS, Shooshtari S, Nowicki S. Examining social isolation and loneliness in combination in relation to social support and psychological distress using Canadian Longitudinal
Study of Aging (CLSA) data. PLoS One. 2020; 15(3): 1–20. https://doi.org/10.1371/journal.pone.
0230673 PMID: 32203553

11.

Hsu HC. Typologies of loneliness, isolation and living alone are associated with psychological wellbeing among older adults in Taipei: a cross-sectional study. Int J Environ Res Public Health. 2020; 17(24): 1–14. https://doi.org/10.3390/ijerph17249181 PMID: 33302603

12.

Barnes TL, MacLeod S, Tkatch R, Ahuja M, Albright L, Schaeffer JA, et al. Cumulative effect of loneliness and social isolation on health outcomes among older adults. Aging Ment Health. 2022; 26(7):1327–1334. https://doi.org/10.1080/13607863.2021.1940096 PMID: 34215167

13.

Smith KJ, Victor C. Typologies of loneliness, living alone and social isolation, and their associations withphysical and mental health. Ageing & Society. 2019; 39(8): 1709–1730. https://doi.org/10.1017/
S0144686X18000132

14.

Xie B, Ma C. Effect of social participation on the development of physical frailty: Do type, frequency anddiversity matter? Maturitas. 2021; 151: 48–54. https://doi.org/10.1016/j.maturitas.2021.06.015 PMID:
34446279

15.

Kumano M. On the concept of well-being in Japan: Feeling Shiawase as hedonic well-being and feeling
Ikigai as eudaimonic well-being. Appl Res Qual Life. 2018; 13: 419–433. https://doi.org/10.1007/
s11482-017-9532-9

16.

Bass SA. An overview of work, retirement, and pensions in Japan. J Aging Soc Policy. 1996; 8(2–3):
57–78. https://doi.org/10.1300/J031v08n02_05 PMID: 10183244

17.

Mathews G. Finding and keeping a purpose in life: well-being and ikigai in Japan and elsewhere. In:
Mathews G, Izquierdo C, editors. Pursuits of happiness: well-being in anthropological perspective. New
York: Berghahn Books; 2009. pp. 167–185.

18.

Ryff C, Boylan JM, Coe CL, Karasawa M, Kawakami N, Kitayama S, et al. Adult development in Japanand the United States: comparing theories and findings about growth, maturity, and well-being. In: Jensen LA, editor. The Oxford handbook of human development and culture: an interdisciplinary perspective. New York: Oxford University Press; 2015. pp. 666–679.

PLOS ONE | https://doi.org/10.1371/journal.pone.0297837 February 26, 2024

15 / 17

PLOS ONEThe relationship between physical frailty and subjective well-being in socially isolated individuals

19.

Qi X, Zhang W, Wang K, Pei Y, Wu B. Social isolation and psychological well-being among older Chinese Americans: does resilience mediate the association? Int. J. Geriatr. Psychiatry. 2022; 37(8): 1–9.
https://doi.org/10.1002/gps.5791 PMID: 35866312

20.

Nakagomi A, Tsuji T, Saito M, Ide K, Kondo K, Shiba K. Social isolation and subsequent health andwell-being in older adults: a longitudinal outcome-wide analysis. Soc Sci Med. 2023; 327: 1–10. https://doi.org/10.1016/j.socscimed.2023.115937 PMID: 37167920

21.

Tsujishita S, Nagamatsu M, Sanada K. Overlap of physical, cognitive, and social frailty affects Ikigai incommunity-dwelling Japanese older adults. Healthcare. 2022; 10(11): 1–11. https://doi.org/10.3390/
healthcare10112216 PMID: 36360557

22.

Kawai Y, Sakurai E, Sugawara Y, Okada Y, Tanabe K, Tsukao A, et al. Predictive model of frailty onset
using Bayesian network. 2022 IEEE International Conference on Big Data (Big Data). 2022: 2017–2023. https://doi.org/10.1109/BigData55660.2022.10020951

23.

Fried LP, Tangen CM, Walston J, Newman AB, Hirsch C, Gottdiener J, et al. Frailty in older adults: evidence for a phenotype. J Gerontol: Series A. 2001; 56(3): 146–157. https://doi.org/10.1093/gerona/56.
3.m146 PMID: 11253156

24.

Theou O, Cann L, Blodgett J, Wallace LM.K, Brothers TD, Rockwood K. Modifications to the frailty phenotype criteria: systematic review of the current literature and investigation of 262 frailty phenotypes in
the Survey of Health, Ageing, and Retirement in Europe. Ageing Res Rev. 2015; 21: 78–94. https://doi.
org/10.1016/j.arr.2015.04.001 PMID: 25846451

25.

The Office for Policy on Loneliness and Isolation, Cabinet Secretariat. Kodoku・koritsu taisaku no jyutenn keikaku. 2022 Dec 26[Cited 2023 June 5]. Available from: https://www.cas.go.jp/jp/seisaku/juten_
keikaku/r04/jutenkeikaku.pdf

26.

Leigh-Hunt N, Bagguley D, Bash K, Turner V, Turnbull S, Valtorta N, et al. An overview of systematic
reviews on the public health consequences of social isolation and loneliness. Public Health. 2017; 152:
157–171. https://doi.org/10.1016/j.puhe.2017.07.035 PMID: 28915435

27.

Shankar A, McMunn A, Demakakos P, Hamer M, Steptoe A. Social isolation and loneliness: prospective associations with functional status in older adults. Health Psychol. 2017; 36(2): 179–187. https://doi.org/10.1037/hea0000437 PMID: 27786518

28.

Holt-Lunstad J, Smith TB, Baker M, Harris T, Stephenson D. Loneliness and social isolation as risk factors for mortality: a meta-analytic review. 2015; 10(2): 227–237. https://doi.org/10.1177/
1745691614568352 PMID: 25910392

29.

Liu H, Zhang M, Yang Q, Yu B. Gender differences in the influence of social isolation and loneliness on
depressive symptoms in college students: a longitudinal study. Soc Psychiatry Psychiatr Epidemiol.
2020; 55: 251–257. https://doi.org/10.1007/s00127-019-01726-6 PMID: 31115597

30.

Courtin E, Knapp M. Social isolation, loneliness and health in old age: a scoping review. Health andSocial Care in the community. 2015; 25(3): 799–812. https://doi.org/10.1111/hsc.12311 PMID:
26712585

31.

Cabinet Secretariat. Summary of the Results of the 2003 Survey on the Attitudes of the Elderly Toward
Participation in Local Communities. (Koureisha-no-chiikishakai-heno-sanka-ni-kansuru-ishikichosakekka-no-gaiyou) 2004 [Cited 2023 June 21]. Available from: https://www8.cao.go.jp/kourei/ishiki/h15_
sougou/gaiyou.html

32.

Shirai K, Iso H, Ohira T, Ikeda A, Noda H, Honjo K, et al. Perceived level of life enjoyment and risks ofcardiovascular disease incidence and mortality. The Japan Public Health Center-Based Study. 2009;
120(11): 956–963. https://doi.org/10.1161/CIRCULATIONAHA.108.834176

33.

Miyazaki J, Shirai K, Kimura T, Ikehara S, Tamakoshi A, Iso H. Purpose in life (Ikigai) and employment
status in relation to cardiovascular mortality: the Japan Collaborative Cohort Study. BMJ Open. 2022;
12(10): 1–11. http://dx.doi.org/10.1136/bmjopen-2021-059725

34.

Kabasawa K, Tanaka J, Ito Y, Yoshida K, Kitamura K, Tsugane S, et al. Associations of physical activity
in rural life with happiness and ikigai: a cross-sectional study. Humanit Soc Sci Commun. 2021; 8(46):
1–10. https://doi.org/10.1057/s41599-021-00723-y

35.

Hofmann T. Probabilistic latent semantic indexing. Proceedings of the 22nd annual international ACM
SIGIR conference on Research and development in information retrieval. 1999: 50–57. https://doi.org/
10.1145/312624.312649

36.

Motomura Y. Daikibo data modeling gijyutsu ni yoru service kougaku. In: National Institute of Advanced
Industrial Science and Technology (AIST), editor. Service Engineering in Society for Society: The Frontiers of Research for Creating Products, Things, and People. (Shakai no naka de shakai no tame noservice kougaku ~mono・koto・hito dukuri no tame no kennkyuu saizennsen) Canaria Communications; 2014. pp. 108–135.

37.

Ide A, Yamashita K, Motomura Y, Terano T. Analyzing regional characteristics of living activities of
elderly people from large survey data with probabilistic latent spatial semantic structure modeling. 2017

PLOS ONE | https://doi.org/10.1371/journal.pone.0297837 February 26, 2024

16 / 17

PLOS ONEThe relationship between physical frailty and subjective well-being in socially isolated individuals

IEEE International Conference on Big Data (Big Data). 2017: 3071–3077. https://doi.org/10.1109/
BigData.2017.8258280
38.

Motomura Y. Bayesian Network Software Bayonet. Journal of SICE. 2003; 42(8): 693–694. https://doi.
org/10.11499/sicejl1962.42.693

39.

Artificial Intelligence Research Center. PLASMA. [Cited 2023 June 8]. In: ACHIEVEMENTS [Internet].
Available from: https://www.airFc.aist.go.jp/achievements/ja/p-023.html

40.

Waldinger R. What Makes a Good Life? 2016 Jan 8[Cited 2023 June 7]. Available from: https://www.
dailygood.org/story/1196/what-makes-a-good-life-robert-waldinger/

41.

Tanaka M, Katsura T, Ishikawa S, Hoshino A, Shizawa M, Usui K. Characteristics of socially isolated
elderly people in a rural area according to a new classification system based on intentionality in social
choices. Journal of the Japanese Association of Rural Medicine. 2020; 68(6): 773–780. https://doi.org/
10.2185/jjrm.68.773

42.

Toyoshima A, Kusumi T. Examining the relationship between preference for solitude and subjective
well-being among Japanese older adults. Innova Aging. 2022; 6(1): 1–10. https://doi.org/10.1093/
geroni/igab054 PMID: 35146129

43.

Yoshizawa Y, Tanaka T, Takahashi K, Fujisaki M, Iijima K. The associations of frailty with regular participation in physical, cultural, and community social activities among independent elders in Japan. Jpn J
Public Health. 2019; 66(6): 306–316. https://doi.org/10.11236/jph.66.6_306 PMID: 31231100

44.

Nagai K, Tamaki K, Kusunoki H, Wada Y, Tsuji S, Ito M, et al. Isotemporal substitution of sedentary
time with physical activity and its associations with frailty status. Clinical Interven Aging. 2018; 25(13):1831–1836. https://doi.org/10.2147/CIA.S175666 PMID: 30288035

45.

Mitsutake S, Shibata A, Ishii K, Amagasa S, Kikuchi H, Fukushima N, et al. Clustering of domain-specific sedentary behaviors and their association with physical function among community-dwelling older
adults. J Phys Act Health. 2020; 17(7): 709–714. https://doi.org/10.1123/jpah.2019-0219 PMID:
32432441

46.

Seino S, Kitamura A, Tomine Y, Tanaka I, Nishi M, Nofuji Y, et al. Awareness of the term "frailty" and its
correlates among older adults living in a metropolitan area. Jpn J Public Health. 2020; 67(6): 399–412.
https://doi.org/10.11236/jph.67.6_399

47.

Ministry of Internal Affairs and Communications, Fiscal Year 2022 Municipal Taxation Status Report[Cited 2023 September 19]. Available from: https://www.soumu.go.jp/main_sosiki/jichi_zeisei/czaisei/
czaisei_seido/ichiran09_22.html

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