日本温泉気候物理医学会雑誌

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Association between Tub Bathing Frequency and
Onset of Depression in Older Adults:
A Six-Year Cohort Study from the JAGES Project
Shinya HAYASAKA1)＊, Toshiyuki OJIMA2), Akio YAGI3), Katsunori KONDO3), 4)
Abstract
Background: The traditional Japanese style of soaking in a hot bath is a lifestylecustom for many citizens, no study had ever investigated the association between
bathing as a lifestyle practice and the onset of long-term depression. Through a largescale six-year longitudinal study, we aimed to determine whether tub bathing as a lifestyle custom plays a role in preventing the onset of long-term depression.
Methods: Of 11,882 individuals who responded to surveys conducted in 2010 and2016 as part of the Japan Gerontological Evaluation Study (the JAGES) project, weanalyzed 6,452 and 6,465 individuals for whom information was available regardingsummer bathing frequency and winter bathing frequency, respectively; all of these
individuals were independent, had a Geriatric Depression Scale (hereafter, “GDS”)
score of ≤4, and did not suffer from depression. The cohort study involved dividing
participants into a group of those who bathed 0-6 times a week and a group of those
who bathed ≥7 times a week and determining the percentages of individuals whodeveloped depression based on their GDS scores six years later. Multiple logistic
regression analysis was performed to determine odds ratios for the association between depression onset and tub bathing.
Results: For individuals who tub bathed ≥7 times a week, the odds ratios fordepression onset versus individuals who bathed 0-6 times a week in summer andwinter were 0.84 (95% confidence interval, 0.64-1.10) and 0.76 (95% confidence interval,
0.59-0.98), respectively. Tub bathing ≥7 times a week in winter significantly reduced
the risk of depression onset.
Conclusions: New-onset depression was shown to be infrequent in older adults whobathe in a tub frequently. Tub bathing was suggested to potentially contribute to theprevention of depression in the elderly.
Key words: bathing, depression, older adults, cohort study, JAGESI

Introduction
In older adults, depression is known not only to di-minish vital functions but also to be a risk factor for
dementia and cardiovascular disease1),2). These resultsof depression not only reduce quality of life but also

(Received May 25, 2023; Accepted June 26, 2023)
doi: 10.11390/onki.2359
1) Faculty of Human Life Sciences, Tokyo City University
1-28-1 Tamazutsumi, Setagaya-ku, Tokyo 158-8557, Japan＊
Corresponding author (S. HAYASAKA): Tel: +81-3-5707-0104, Fax: +81-3-5707-1217
2) Department of Community Health and Preventive Medicine, Hamamatsu University School of Medicine
3) Center for Preventive Medical Sciences, Chiba University4) Department of Gerontological Evaluation, Center for Gerontology and Social Science, Research Institiute, National
Center for Geriatrics and Gerontology

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2increase outcomes such as the need for long-term care

3. Objective variable

as well as death3). Therefore, preventing depression in

Participants with an overall GDS score of ≥5 on theolder adults is a major public health concern.

secondary survey were defined as having developed

Japanese-style bathing, which involves soaking one’s

depressive tendencies (hereafter, “depression”). In theentire body thoroughly in hot water in a bathtub, is a

event of missing responses on the GDS, a total score of

traditional Japanese lifestyle practice that many Japa-≥5 on questions without missing responses was de-nese people continue to enjoy

4),5). However, research

has not yet sufficiently determined how bathing as afined as depression.
4. Explanatory variables

lifestyle practice affects health. Although previous

On the primary survey, participants were asked

studies have reported that frequent tub bathing isabout their weekly tub bathing frequency in summer

associated with high self-rated health6) and that older

and winter. For summer and winter, respectively,

adults who tub bathe frequently are less likely toparticipants were divided into two groups: those who

7)subsequently require long-term care insurance , theresponded 0-6 times a week (low-frequency group)

underlying mechanism has not been determined. In a

and those who responded ≥7 times a week (high-cross-sectional study, tub bathing frequency was

frequency group). These groups were used as thereported to be negatively associated with depression8).

explanatory variables.

A small-scale interventional study has also reported

5. Covariatesthat hyperthermic baths improve depression9).

The following participant background factors in the

As part of the Japan Gerontological Evaluation

primary survey were treated as covariates: age, sex,

Study (JAGES)10), the present large-scale six-year longi-

undergoing disease treatment (yes/no), alcohol con-tudinal study was conducted to assess the association

sumption (yes/previously/no), smoking (never/quit ≥5

between tub bathing frequency and subsequent onset

years ago/quit ≤4 years ago/current smoker), maritalof depression in older adults.

status, years of education, and equivalent income. Re-

II

sponses for marital status consisted of “married”,

Material and Methods“divorced/widowed”, and “single/other”. Equivalent

1. Participants

income was divided into three levels: 1) <JPY 2

The initial pool of participants consisted of 11,882

million/year, 2) JPY 2 million to <4 million/year, 3) ≥community-dwelling adults aged ≥65 years living in 14

JPY 4 million/year. Years of education was divided

municipalities in seven prefectures throughout Japan

into five levels: 1) <6 years, 2) 6-9 years, 3) 10-12

who had not been certified as requiring long-term careyears, 4) ≥13 years, and 5) other (uncountable, etc.).

and had provided valid responses to the JAGES 2010

6. Analysis methods

and 2016 surveys, the former of which included

Descriptive statistics were determined for eachquestions regarding bathing habits. Of these 11,882

background factor (covariate) at the time of the primary

individuals, those who met the following criteria weresurvey in the low- and high-frequency groups for bothincluded in the present study: 1) independent in activi-

summer and winter. All background factors except forties of daily living as of the 2010 survey, 2) Geriatricage were compared between bathing groups with the

11)score ≤4 as of the 2010

chi-square test, while age was compared with thesurvey, and 3) no missing data on questions regarding

independent-samples t-test. Next, following univariate

bathing frequency as of the 2010 survey. A total ofanalysis with the chi-square test by asymptotic signifi-

6,452 individuals (mean age (standard deviation, SD)

cance probability (two-sided), we performed multivari-

73.4 (5.8) years; 3,129 men and 3,323 women) and 6,465

ate analysis with multiple logistic regression analysis

individuals (73.4 (5.8) years; 3,126 men and 3,339 women)

to calculate the odds ratios (OR) and 95% confidence

were included in the analyses for summer bathing andintervals (95% CI) for the onset of depression. Age, sex,

winter bathing, respectively.

undergoing disease treatment, alcohol consumption,

Depression Scale (GDS)

2. Survey method

smoking, marital status, years of education, and equiv-

Panel data were prepared via linkage of the 2010

alent income were used as covariates. As for adjust-survey (hereafter, the primary survey) and the 2016

ment factors, age was input as a numerical value as is,survey (hereafter, the secondary survey) and used towhile all other factors were input as categorical varia-

examine new-onset depression.

bles. As for equivalent income, if there were any missing data for a participant’s adjustment factors, they

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were classified as the “missing data group” for analy-

demonstrating a statistically significantly reduced risk

sis. Differences of P <0.05 (two-sided) were considered

of depression onset in the high-frequency group.

statistically significant. Odds ratios and φ were calculated as effect sizes. All statistical analyses were performed with SPSS ver. 28 (IBM Corp., Armonk, NY).

IV

Discussion

The main result of this study is that older adults

7. Ethical considerations

who frequently bathe in a tub are less likely to develop

Ethical approval for the study was obtained fromnew-onset depression. In addition, comparisons of sum-the Ethics Committee at Chiba University (Approval

mer and winter showed that new-onset depression

number: 2493), the National Center for Geriatrics andwas less likely in older adults who bathed in a tub more

Gerontology (Approval number: 992), and Nihon Fuku-

frequently in winter. These results suggested thatshi University (Approval number: 10-05). All individuals

frequent tub bathing may play a role in preventing

enrolled in the baseline survey were informed that their

the onset of depression in older adults and that thisparticipation in the study was voluntary and that com-

effect is greater in winter than in summer.

pletion and return of the questionnaire indicated theirconsent to participate.

III

Results

There are two possible mechanisms underlying
these results. The first is regulation of the autonomic
nervous system by hyperthermic action. Tub bathing
increases both core and surface body temperatures12),

Table 1 shows the participant background factors

which improves autonomic balance13) and thereby may

for the summer and winter bathing frequency groups

contribute to preventing depression. The second isat the time of the primary survey. In the summer

improved quality of sleep associated with tub bathing.

high-frequency group, many participants were non-Tub bathing has been shown to improve the quality ofsmokers and married, while few were low income. In

sleep14), which may contribute to preventing depres-the winter high-frequency group, the mean age wassion. Seasonal depression in winter has been noted aslower than in the low-frequency group, many partici-

a seasonal affective disorder15). Prevalence rates ofpants were married, and few were low income.

1-9% have been reported and the causes are pre-Table 2 shows the percentages of new-onset depres-sumed to be reduced daylight hours and disrupted

sion as well as OR and 95% CI for the summer andcircadian rhythms15). Tub bathing in winter may there-

winter bathing frequency groups. In summer, the per-fore more effectively prevent depression.

centage of new-onset depression in participants analyzed

The results of this study are consistent with thosewas 12.9% in the low-frequency group versus 11.2% inof two previously-cited studies, one showing that tub

the high frequency group. In winter, the percentage ofbathing frequency is negatively associated with de-new-onset depression in participants analyzed waspression8), and another small-scale interventional study

13.9% in the low-frequency group versus 10.6% in theshowing that warm bathing improved depression9). In

high-frequency group; thus, the percentage of new-addition, a cohort study has shown that older adults

onset depression in winter was significantly lower in

who bathe in a tub frequently are less likely to be cer-

the high-frequency group. With the winter low-frequency

tified as needing long-term care7). Depression is a risk

group as a reference, the OR for new-onset depression

factor for new certification of needing long-term care3),in the high-frequency group was 0.74 (95% CI, 0.59-

a finding which, combined with the results of the pres-0.92), P = 0.007 and effect sizeφ = − 0.047.

ent study, suggests that tub bathing may prevent de-

Results for the multivariate analysis adjusted forpression and consequently prevent the need for long-covariates were as follows. Regarding summer bathingterm care. In homes with bathtubs, tub bathing may

frequency, with the low frequency group as a refer-be recommended to older adults to maintain theirence, the OR for new-onset depression in the high-health.

frequency group was 0.84 (95% CI, 0.64-1.10). Thus,

This study has somelimitations. The first is that thealthough frequent bathing in summer tended to besafety of bathing was not considered. Deaths duringassociated with the absence of new-onset depression,

bathing are frequently reported in Japan, but thethe difference was not significant. As for wintermechanism behind them remains largely unknown5),16).

bathing frequency, with the low frequency group as a

Safe bathing methods may be an important area ofreference, the OR for new-onset depression in the

study going forward. The second limitation is that few

high-frequency group was 0.76 (95% CI, 0.59-0.98), thus

other countries have a custom of daily tub bathing5),

Table 1
Participant background factors by summer and winter tub bathing frequency.

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Table 2 Association between tub bathing frequency and onset of depression.

which hinders comparisons with other countries.

administration, Funding acquisition.

Third, the influence of other unknown confounding
factors needs to be considered. Our JAGES groupreported that age, marital status and others are associ-Conflict of interests
The authors declare no conflict of interest.

ated with the onset of depression17). It is difficult toadjust for all unknown confounders. And in this study,

Acknowledgements

the participants were divided into two bathing fre-This study was supported by the following funding

quency groups in summer and winter respectively on

sources: JSPS KAKENHI (JP15H01972, JP18H03047,

the 2010 primary survey. There is a possibility that

JP20K10540), a Health and Labour Sciences Research

the participant’s bathing habits changed during the 6

Grant (H28-Choju-Ippan-002), the Japan Agency for

years. This is a limitation of cohort study design. De-

Medical Research and Development (AMED) (JP18

spite these limitations, the results of the present large-

dk0110027, JP18ls0110002, JP18le0110009, JP20dk

scale longitudinal study conceivably provide beneficial

0110034, JP21lk0310073, JP21dk0110037, JP22lk0310087),

information for preventing the onset of depression in

Research Funding for Longevity Sciences from National

older adults.

Center for Geriatrics and Gerontology (29-42), and theV

Conclusions

Japan Health and Research Institute (2018-4). We
would like to take this opportunity to express our

New-onset depression was shown to be infrequent ingratitude. The sponsors had no role in study design, inolder adults who bathe in a tub frequently. Tub bath-the collection, analysis, or interpretation of data, in theing was suggested to potentially contribute to the pre-writing of the report, or in the decision to submit thevention of depression in the elderly.

article for publication.

References

Author contributions
SH: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Writing-Original Draft,
Funding acquisition. TO: Methodology, Software, Validation, Investigation, Writing-Review & Editing, Supervision, Funding acquisition. AY: Methodology, Software,
Validation, Investigation, Writing-Review & Editing,Funding acquisition. KK: Conceptualization, Investigation, Writing-Review & Editing, Supervision, Project

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高齢者の浴槽入浴頻度と抑うつ発症との関連：
JAGES プロジェクトによる 6 年間のコホート研究
早坂抄

信哉 1）＊，尾島俊之 2），八木明男 3），近藤克則 3），4）録【背景・目的】高齢者において抑うつの発症は様々な疾患のリスクとなり，要介護状態に陥るきっかけとなる．一方，日本においては浴槽の湯につかる特有の入浴法が多くの国
民の生活習慣となっているが，この生活習慣としての浴槽入浴と長期的な抑うつ発症との関連は明らかではなかった．本研究は，大規模な 6 年間にわたる縦断研究によって生活習慣としての浴槽入浴が長期的な抑うつ発症との関連を明らかにすることを目的とした．
【方法】Japan Gerontological Evaluation Study（以下，JAGES）の一環として 2010 年，
2016 年に調査対象となった 11,882 人のうち，自立しておりかつ Geriatric Depression
Scale（以下，GDS）4 点以下で抑うつがなく，夏の入浴頻度の情報のある 6,452 人，および冬の入浴頻度の情報がある 6,465 人をそれぞれ解析した．コホート研究として週 0 ～ 6
回の浴槽入浴と週 7 回以上の浴槽入浴の各群の 6 年後の GDS による抑うつ発症割合を求め，浴槽入浴との関連をロジスティック回帰分析によって年齢，性別，治療中の病気の有
無，飲酒の有無，喫煙の有無，婚姻状況，教育年数，等価所得を調整して多変量解析を行いオッズ比を求めた．
【結果】週 0 ～ 6 回の浴槽入浴に対する週 7 回以上の浴槽入浴の抑うつ発症の，調整後の多変量解析によるオッズ比は夏の入浴頻度 0.84（95％信頼区間：0.64 ～ 1.10），冬の入浴頻度 0.76（95％信頼区間：0.59 ～ 0.98）であり，冬に週 7 回以上浴槽入浴することは抑うつを発症するリスクが有意に低かった．
【結論】習慣的な浴槽入浴の温熱作用を介した自律神経のバランス調整などによる抑うつ予防作用による結果の可能性があり，健康維持のため高齢者へ浴槽入浴が勧められることが示唆された．
キーワード：浴槽入浴，抑うつ，高齢者，コホート研究，JAGES

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