Abstract
Background
While bathing styles vary among countries, most Japanese people prefer tub bathing to showers and saunas. However, few studies have examined the relationship between tub bathing and health outcomes. Accordingly, in this prospective cohort study, we investigated the association between tub bathing frequency and the onset of functional disability among older people in Japan.
Methods
We used data from the Japan Gerontological Evaluation Study (JAGES). The baseline survey was conducted from August 2010 through January 2012 and enrolled 13,786 community-dwelling older people (6,482 men and 7,304 women) independent in activities of daily living. During a 3-year observation period, the onset of functional disability, identified by new certification for need of Long-Term Care Insurance, was recorded. Tub bathing frequencies in summer and winter at baseline were divided into three groups: low frequency (0–2 times/week), moderate frequency (3–6 times/week), and high frequency (≥7 times/week). We estimated the risks of functional disability in each group using a multivariate Cox proportional hazards model.
Results
Functional disability was observed in a total of 1,203 cases (8.7%). Compared with the low-frequency group and after adjustment for 14 potential confounders, the hazard ratios of the moderate- and high-frequency groups were 0.91 (95% confidence interval [CI], 0.75–1.10) and 0.72 (95% CI, 0.60–0.85) for summer and 0.90 (95% CI, 0.76–1.07) and 0.71 (95% CI, 0.60–0.84) for winter.
Conclusion
High tub bathing frequency is associated with lower onset of functional disability. Therefore, tub bathing might be beneficial for older people’s health.
Key words: tub bathing, functional decline, care, older people, prevention
INTRODUCTION
Population aging is a critical issue in most developed and developing countries.1,2 With a proportion of individuals 65 years or older of 27.8% (in August 2017), Japan has the most aged population in the world.3 This proportion is expected to increase rapidly in the near future.4 A growing number of older people have functional disability requiring care in daily life, and there are increasingly fewer young people to support them. Therefore, prevention of functional disability is an important issue for public health in Japan and other counties.
Although the Japanese have one of the longest life expectancies in the world,4 the reasons for their longevity are not well understood. The Japanese diet5 and social cohesion6,7 appear to play some part. Some other aspects of the Japanese lifestyle might also have a protective effect on health.
The Japanese prefer to take baths, especially in a bathtub, rather than shower or sauna bath, not only for cleanliness, but also to feel warm and refreshed and to aid sleep.8,9 The relationship between bathing and health outcomes has been reported in several studies. In cross-sectional studies, tub bathing frequency was reported to be associated with good sleep quality, low perceived stress, and good self-rated health.10–12 Only one longitudinal study in Japan, with a 5-year observational period, has examined this issue,13 finding that tub bathing frequency was inversely associated with the onset of functional disability. However, that study was limited in generalizability because it specifically enrolled outpatients and had a relatively small sample size (n = 610). In a large cohort study in Finland, Laukkanen et al14 identified a strong negative association between sauna bathing frequency and cardiovascular and all-cause mortality, although sauna bathing is clearly distinct from tub bathing.
Here, we conducted a large prospective cohort study targeting the general population to evaluate the association between tub bathing frequency and the onset of functional disability among Japanese older adults.
METHODS
Study population and setting
We used data from the Japan Gerontological Evaluation Study (JAGES).15 The baseline survey was performed from August 2010 through January 2012. The target population was community-dwelling older adults, aged 65 or older, not certified to need care under Long-Term Care Insurance (LTCI), and living in 18 municipalities in eight prefectures in Japan. A total of 110,447 randomly selected people were mailed self-report questionnaires asking about their health status, habits, and lifestyle; 72,760 completed questionnaires were returned (response rate, 65.9%). For 69,408 of them (95.4%), we were able to refer to the LTCI database, which contains the information necessary for follow-up. In a randomly selected one-fourth or one-fifth of the questionnaires (the rate varied depending on municipality), participants were asked about bathing habits (n = 16,416). We excluded individuals who were not independent in activities of daily living (ADL) (n = 896) or missing information on bathing frequency in summer or winter (n = 1,734). This left 13,786 individuals (6,482 men and 7,304 women) for analysis; their mean age was 73.4 (standard deviation, 6.0) years.
We observed the participants for 3 years from the baseline survey and recorded the onset of functional disability (defined in the next section), movement out of the municipality, and death. This information was obtained from the LTCI database after the end of the observational period; this database is maintained by the local authority of each municipality.
This study was approved by the human research ethics committee of Nihon Fukushi University (No. 10-05). All individuals enrolled in the baseline survey were informed that their participation in the study was voluntary and that completion and return of the questionnaire indicated their consent to participate.
Outcome
We defined functional disability as being dependent in ADL by physical or cognitive difficulty, which was identified by certification of need for LTCI (including the “need support” level).16–18 The Japanese LTCI was established to improve older people’s welfare by promoting public care. LTCI is a compulsory coverage insurance, and its benefits are obtained by older people when they apply and are certified to need care. The certification is standardized and based on information on the applicant’s ADL gathered by a qualified investigator and on comments from the family physician.18
Exposure
In the questionnaire, participants were asked, “How many times a week do you take a bath in a bathtub in summer and in winter?” The original question is shown in eFigure 1. We divided tub bathing frequencies into three groups: low frequency (0–2 times/week), moderate frequency (3–6 times/week), and high frequency (≥7 times/week) for both summer and winter.11,12
In the Japanese style of bathing, the water temperature is usually 39–42°C. Bathers tend to spend 5–15 minutes in the bathtub, soaking deeply up to the shoulder level.19 Bathing is usually performed in the late evening.9 Because changes in the seasons are clear in Japan (hot and wet in summer and cold and dry in winter),20 the associations between bathing and health outcomes were expected to differ among seasons. Therefore, we measured tub bathing frequency separately in summer and winter.
Covariates
In the baseline survey, we recorded participants’ demographic factors (age, sex, and marital status) and asked about socioeconomic status (employment, equivalized income, and years of education), heath-related behaviors (smoking status, alcohol consumption, and body mass index), and self-reported health status (treatment for any disease, physical strength, cognitive function, depression, and instrumental ADL).
These variables were divided into the following categories: age (65–69, 70–74, 75–79, 80–84, or ≥85 years old), sex (male or female), marital status (married or single), employment status (not employed or employed), equivalized income (≤1.99, 2.00–3.99, or ≥4.00 million yen/year), years of education (0–9, 10–12, or ≥13 years), smoking status (never smoker, former smoker, or current smoker), alcohol consumption (nondrinker or drinker), body mass index (≤18.4, 18.5–24.9, or ≥25.0 kg/m2), treatment for any disease (without any disease or with any disease), physical strength (normal or low), cognitive function (normal or a decline), depression (not depressed or depressed), and instrumental ADL (independent or dependent). Physical strength and cognitive function were assessed based on the Kihon Checklist,21,22 which was developed to identify older people who are at risk of functional disability. Depression was assessed by the shorter version of the Geriatric Depression Scale,23 which includes 15 questions, with a cutoff point of ≥5 indicating “depressed”. Instrumental ADL was evaluated using the Tokyo Metropolitan Institute of Gerontology Index of Competence,24 which consists of five questions; participants who missed ≥1 point were regarded as being “dependent”.
Statistical analysis
After describing the baseline characteristics of the study participants, we used a multivariate Cox proportional hazards model to estimate the risks of functional disability according to tub bathing frequency. In the multivariate-adjusted model, we adjusted for 14 covariates: age, sex, marital status, employment, equivalized income, years of education, smoking status, alcohol consumption, body mass index, treatment for any disease, physical strength, cognitive function, depression, and instrumental ADL. All of these covariates were assumed to be potential confounders. If the information on a covariate was missing, we classed the participant into a “missing” category when performing the analysis. Participants who moved out or died without functional disability were censored.
Next, we performed sensitivity analysis in the multivariate-adjusted model, excluding participants whose follow-up period was less than 1 year in order to take account of non-observable risk factors. Subgroup analysis was then performed with stratification by sex, age (65–74 or ≥75 years old), and baseline health status, such as treatment for any disease, physical strength, cognitive function, depression, and instrumental ADL. When performing this analysis, we used the variables in the multivariate-adjusted model except for the variable used for the stratification.
The proportional hazards assumption was graphically verified by plotting the log [−log] transformation of the cumulative survival curve of each exposure group.25
We used SPSS Statistics version 24.0 (IBM Inc., Armonk, NY, USA) for all analyses. A P-value less than 0.05 was considered significant.
RESULTS
Baseline characteristics of the study participants are described in Table 1. The numbers of individuals in each bathing frequency group (low, moderate, and high) were 1,448, 2,777, and 9,561 for summer and 1,347, 4,021, and 8,418 for winter, respectively. Compared with the other groups, the people classed in the high-frequency group were younger and more likely to be married, not depressed, and independent in instrumental ADL and have a moderate-to-high equivalized income, normal physical strength, and normal cognitive function. This tendency was clearer in winter than in summer.
Table 1. Baseline characteristics of study participants according to the frequency of tub bathing in summer and in winter.
| Variables | Summer | Winter | ||||||||||
| Frequency of tub bathing, times/week | Frequency of tub bathing, times/week | |||||||||||
| 0–2 | 3–6 | ≥7 | 0–2 | 3–6 | ≥7 | |||||||
| n | (%) | n | (%) | N | (%) | n | (%) | n | (%) | N | (%) | |
| Participants | 1,448 | 2,777 | 9,561 | 1,347 | 4,021 | 8,418 | ||||||
| Age, years | ||||||||||||
| 65–69 | 469 | (32.4) | 785 | (28.3) | 3,209 | (33.6) | 285 | (21.2) | 1,172 | (29.1) | 3,006 | (35.7) |
| 70–74 | 394 | (27.2) | 820 | (29.5) | 2,766 | (28.9) | 345 | (25.6) | 1,197 | (29.8) | 2,438 | (29.0) |
| 75–79 | 305 | (21.1) | 616 | (22.2) | 2,038 | (21.3) | 331 | (24.6) | 887 | (22.1) | 1,741 | (20.7) |
| 80–84 | 194 | (13.4) | 373 | (13.4) | 1,083 | (11.3) | 256 | (19.0) | 539 | (13.4) | 855 | (10.2) |
| ≥85 | 86 | (5.9) | 183 | (6.6) | 465 | (4.9) | 130 | (9.7) | 226 | (5.6) | 378 | (4.5) |
| Sex | ||||||||||||
| Male | 732 | (50.6) | 1,395 | (50.2) | 4,355 | (45.5) | 699 | (51.9) | 1,806 | (44.9) | 3,977 | (47.2) |
| Female | 716 | (49.4) | 1,382 | (49.8) | 5,206 | (54.5) | 648 | (48.1) | 2,215 | (55.1) | 4,441 | (52.8) |
| Marital status | ||||||||||||
| Married | 911 | (62.9) | 1,798 | (64.7) | 6,880 | (72.0) | 812 | (60.3) | 2,518 | (62.6) | 6,259 | (74.4) |
| Single | 452 | (31.2) | 850 | (30.6) | 2,341 | (24.5) | 452 | (33.6) | 1,319 | (32.8) | 1,872 | (22.2) |
| Missing | 85 | (5.9) | 129 | (4.6) | 340 | (3.6) | 83 | (6.2) | 184 | (4.6) | 287 | (3.4) |
| Employment | ||||||||||||
| Not employed | 969 | (66.9) | 1,895 | (68.2) | 6,368 | (66.6) | 921 | (68.4) | 2,734 | (68.0) | 5,577 | (66.3) |
| Employed | 297 | (20.5) | 511 | (18.4) | 2,069 | (21.6) | 212 | (15.7) | 764 | (19.0) | 1,901 | (22.6) |
| Missing | 182 | (12.6) | 371 | (13.4) | 1,124 | (11.8) | 214 | (15.9) | 523 | (13.0) | 940 | (11.2) |
| Equivalized income, million yen/year | ||||||||||||
| Low (≤1.99) | 598 | (41.3) | 1,299 | (46.8) | 3,546 | (37.1) | 619 | (46.0) | 1,820 | (45.3) | 3,004 | (35.7) |
| Middle (2.00–3.99) | 428 | (29.6) | 788 | (28.4) | 3,244 | (33.9) | 336 | (24.9) | 1,170 | (29.1) | 2,954 | (35.1) |
| High (≥4.00) | 112 | (7.7) | 148 | (5.3) | 1,079 | (11.3) | 70 | (5.2) | 247 | (6.1) | 1,022 | (12.1) |
| Missing | 310 | (21.4) | 542 | (19.5) | 1,692 | (17.7) | 322 | (23.9) | 784 | (19.5) | 1,438 | (17.1) |
| Years of education | ||||||||||||
| 0–9 | 675 | (46.6) | 1,381 | (49.7) | 4,455 | (46.6) | 720 | (53.5) | 1,941 | (48.3) | 3,850 | (45.7) |
| 10–12 | 453 | (31.3) | 827 | (29.8) | 3,282 | (34.3) | 330 | (24.5) | 1,276 | (31.7) | 2,956 | (35.1) |
| ≥13 | 235 | (16.2) | 438 | (15.8) | 1,499 | (15.7) | 209 | (15.5) | 624 | (15.5) | 1,339 | (15.9) |
| Missing | 85 | (5.9) | 131 | (4.7) | 325 | (3.4) | 88 | (6.5) | 180 | (4.5) | 273 | (3.2) |
| Smoking status | ||||||||||||
| Never smoker | 742 | (51.2) | 1,385 | (49.9) | 5,334 | (55.8) | 669 | (49.7) | 2,154 | (53.6) | 4,638 | (55.1) |
| Former smoker | 402 | (27.8) | 768 | (27.7) | 2,457 | (25.7) | 346 | (25.7) | 1,040 | (25.9) | 2,241 | (26.6) |
| Current smoker | 167 | (11.5) | 333 | (12.0) | 888 | (9.3) | 167 | (12.4) | 433 | (10.8) | 788 | (9.4) |
| Missing | 137 | (9.5) | 291 | (10.5) | 882 | (9.2) | 165 | (12.2) | 394 | (9.8) | 751 | (8.9) |
| Alcohol consumption | ||||||||||||
| Non-drinker | 823 | (56.8) | 1,666 | (60.0) | 6,001 | (62.8) | 851 | (63.2) | 2,475 | (61.6) | 5,164 | (61.3) |
| Drinker | 547 | (37.8) | 939 | (33.8) | 3,039 | (31.8) | 418 | (31.0) | 1,325 | (33.0) | 2,782 | (33.0) |
| Missing | 78 | (5.4) | 172 | (6.2) | 521 | (5.4) | 78 | (5.8) | 221 | (5.5) | 472 | (5.6) |
| Body mass index, kg/m2 | ||||||||||||
| ≤18.4 | 105 | (7.3) | 217 | (7.8) | 616 | (6.4) | 123 | (9.1) | 299 | (7.4) | 516 | (6.1) |
| 18.5–24.9 | 953 | (65.8) | 1,766 | (63.6) | 6,460 | (67.6) | 833 | (61.8) | 2,595 | (64.5) | 5,751 | (68.3) |
| ≥25.0 | 296 | (20.4) | 631 | (22.7) | 2,049 | (21.4) | 294 | (21.8) | 881 | (21.9) | 1,801 | (21.4) |
| Missing | 94 | (6.5) | 163 | (5.9) | 436 | (4.6) | 97 | (7.2) | 246 | (6.1) | 350 | (4.2) |
| Treatment for any disease | ||||||||||||
| Without any disease | 365 | (25.2) | 612 | (22.0) | 2,242 | (23.4) | 295 | (21.9) | 882 | (21.9) | 2,042 | (24.3) |
| With any disease | 957 | (66.1) | 1962 | (70.7) | 6,614 | (69.2) | 922 | (68.4) | 2,840 | (70.6) | 5,771 | (68.6) |
| Missing | 126 | (8.7) | 203 | (7.3) | 705 | (7.4) | 130 | (9.7) | 299 | (7.4) | 605 | (7.2) |
| Physical strengtha | ||||||||||||
| Normal | 972 | (67.1) | 1,801 | (64.9) | 6,760 | (70.7) | 808 | (60.0) | 2,652 | (66.0) | 6,073 | (72.1) |
| Low | 301 | (20.8) | 606 | (21.8) | 1,685 | (17.6) | 344 | (25.5) | 862 | (21.4) | 1,386 | (16.5) |
| Missing | 175 | (12.1) | 370 | (13.3) | 1,116 | (11.7) | 195 | (14.5) | 507 | (12.6) | 959 | (11.4) |
| Cognitive functionb | ||||||||||||
| Normal | 836 | (57.7) | 1,605 | (57.8) | 5,964 | (62.4) | 727 | (54.0) | 2,390 | (59.4) | 5,288 | (62.8) |
| Decline | 519 | (35.8) | 992 | (35.7) | 2,991 | (31.3) | 525 | (39.0) | 1,376 | (34.2) | 2,601 | (30.9) |
| Missing | 93 | (6.4) | 180 | (6.5) | 606 | (6.3) | 95 | (7.1) | 255 | (6.3) | 529 | (6.3) |
| Geriatric depression scale | ||||||||||||
| 0–4 (not depressed) | 798 | (55.1) | 1,559 | (56.1) | 6,034 | (63.1) | 686 | (50.9) | 2,245 | (55.8) | 5,460 | (64.9) |
| 5–15 (depressed) | 392 | (27.1) | 765 | (27.5) | 1,962 | (20.5) | 420 | (31.2) | 1,075 | (26.7) | 1,624 | (19.3) |
| Missing | 258 | (17.8) | 453 | (16.3) | 1,565 | (16.4) | 241 | (17.9) | 701 | (17.4) | 1,334 | (15.8) |
| Instrumental ADLc | ||||||||||||
| Independent | 1,051 | (72.6) | 2,015 | (72.6) | 7,201 | (75.3) | 893 | (66.3) | 3,026 | (75.3) | 6,348 | (75.4) |
| Dependent | 294 | (20.3) | 576 | (20.7) | 1,774 | (18.6) | 350 | (26.0) | 744 | (18.5) | 1,550 | (18.4) |
| Missing | 103 | (7.1) | 186 | (6.7) | 586 | (6.1) | 104 | (7.7) | 251 | (6.2) | 520 | (6.2) |
ADL, activities of daily living.
aConsists of 5 questions on participants’ self-reported physical strength. Participants who missed ≥3 points were regarded as being “low”.
bConsists of 3 questions on participants’ self-reported cognitive function. Participants who missed ≥1 point were regarded as showing a “decline”.
cConsists of 5 questions on participants’ self–reported instrumental ADL, such as use of public transport and management of money. Participants who missed ≥1 point were regarded as being “dependent”.
The total observation time was 36,619 person-years (average, 2.7 years/participant). Of the 13,786 participants, 1,203 cases (8.7%) of functional disability, 90 cases (0.7%) of movement out of the municipality, and 335 cases (2.4%) of death were recorded. The main results of our research are shown in Table 2, with a description of the hazard ratios of each bathing frequency group, analyzed by both crude and multivariate-adjusted models. In the multivariate-adjusted model, compared with the low-frequency group, the hazard ratios of the moderate- and high-frequency groups were 0.91 (95% confidence interval [CI], 0.75–1.10) and 0.72 (95% CI, 0.60–0.85) in summer and 0.90 (95% CI, 0.76–1.07) and 0.71 (95% CI, 0.60–0.84) in winter. Significant risk reduction was seen in the high-frequency group in both summer and winter. The hazard ratios were almost the same in summer and winter.
Table 2. Hazard ratios of functional disability onset according to the frequency of tub bathing in summer and in winter.
| Frequency of tub bathing, times/week | Crude model | Multivariate-adjusted modela | |||||
| HR | 95% CIs | P-value | HR | 95% CIs | P-value | ||
| Summer | 0–2 | reference | reference | ||||
| 3–6 | 1.00 | (0.83–1.21) | 0.995 | 0.91 | (0.75–1.10) | 0.323 | |
| ≥7 | 0.64 | (0.54–0.75) | <0.001 | 0.72 | (0.60–0.85) | <0.001 | |
| Winter | 0–2 | reference | reference | ||||
| 3–6 | 0.66 | (0.56–0.78) | <0.001 | 0.90 | (0.76–1.07) | 0.246 | |
| ≥7 | 0.41 | (0.35–0.48) | <0.001 | 0.71 | (0.60–0.84) | <0.001 | |
ADL, activities of daily living; HR, hazard ratio; CIs, confidence intervals.
aAdjusted for age, sex, marital status, employment, equivalized income, years of education, smoking status, alcohol consumption, body mass index, treatment for any disease, physical strength, cognitive function, depression, and instrumental ADL.
The results from the sensitivity analysis are shown in Table 3. After the exclusion of individuals whose follow-up period was less than 1 year, the inverse association between bathing frequency and functional disability onset remained consistent. Subgroup analysis results are presented in eTable 1, eTable 2, eTable 3, eTable 4, eTable 5, eTable 6, and eTable 7. The results were mostly consistent among subgroups.
Table 3. Sensitivity analysis: hazard ratios of functional disability onset, after the exclusion of participants whose follow-up period was <1 year.
| Frequency of tub bathing, times/week | Multivariate-adjusted modela | ||||||
| HR | 95% CIs | P-value | |||||
| Summer | 0–2 | reference | |||||
| 3–6 | 0.93 | (0.73–1.17) | 0.517 | ||||
| ≥7 | 0.75 | (0.61–0.93) | 0.007 | ||||
| Winter | 0–2 | reference | |||||
| 3–6 | 0.94 | (0.76–1.17) | 0.592 | ||||
| ≥7 | 0.72 | (0.59–0.89) | 0.002 | ||||
ADL, activities of daily living; HR, hazard ratio; CIs, confidence intervals.
aAdjusted for age, sex, marital status, employment, equivalized income, years of education, smoking status, alcohol consumption, body mass index, treatment for any disease, physical strength, cognitive function, depression, and instrumental ADL.
DISCUSSION
Our main finding is that individuals who frequently took baths in a bathtub were less likely to be functionally disabled after adjustment for potential confounders. A seasonal difference in the association was not seen in the multivariate-adjusted model. The results from the sensitivity analysis weaken the possibility of a reverse-causation bias because potentially vulnerable individuals might be functionally disabled earlier. In addition, the results from the subgroup analysis imply that the association between exposure and outcome is consistent regardless of sex, age, or baseline health status.
Two pathways may explain the association between bathing and older people’s health status. First, tub bathing promotes good sleep26,27 and decreases sympathetic nerve activity.28 These changes may be beneficial for the mental status of older people and may prevent depression or cognitive function decline. Second, tub bathing raises the body temperature,29 which leads to increased expression of heat shock proteins (HSPs).30 HSPs have cytoprotective, anti-apoptotic, and anti-inflammatory effects.31 Additionally, HSPs are believed to play therapeutic roles in type 2 diabetes mellitus32 and Alzheimer’s disease.33 Thus, HSPs may improve or maintain the health conditions of older people. In Japan, the direct causes of older people’s functional disability are mainly dementia, cerebrovascular disease, arthrosis, fracture, and malignancy.34 The relationship between tub bathing and these diseases should be researched in a future study.
Our study results are consistent with those obtained in the aforementioned Finnish study,14 which indicated a robust negative association between sauna bathing frequency and cardiovascular and all-cause mortality, as well as several Japanese cross-sectional studies10–12 reporting a positive association between tub bathing frequency and self-rated health, which is a good predictor of functional disability in older people.35,36 Our results are also consistent with the only Japanese longitudinal study to examine this issue,13 which found a negative association between tub bathing and functional disability onset, although that study had a smaller sample size than ours and specifically targeted outpatients.
Strengths of our work include a large sample size, the enrollment of a general population in different regions in Japan, and the use of appropriate statistical methodology to adequately control for confounders. There are also several limitations. First, the certification of need for care did not always reflect functional disability. However, because this misclassification was expected to occur equally in each exposure group, we might actually have underestimated the association between exposure and outcome.37 Second, because we surveyed only tub bathing frequency, other types of bathing (ie, sauna or shower) were not taken into account, which limits the generalizability of our study when our findings are applied to people living in cultures without tub bathing. These issues should be investigated in a future study. Third, we could not examine the safety or risks of tub bathing in this study design. Sudden death or accidents during bathing are often reported in Japanese society.38,39 Therefore, safety issues related to tub bathing are important and should be studied appropriately. The relationship between bathing habit and mortality should be researched in a subsequent study, to take account of the risk of bathing-related death. Additionally, if our results are applied to clinical practice or recommended as an intervention, careful attention should be paid to safety. Considerations include advice for people with fever or excessive hypertension to avoid bathing39 and a reminder that the bathroom and dressing room should be warm.40 Lastly, we could not completely eliminate the reverse-causation bias, even after controlling for confounders and performing sensitivity analysis, because healthier people might bathe more frequently. However, this limitation is inevitable in an observational study.
In conclusion, our study indicates that a high frequency of tub bathing is associated with lower onset of functional disability among Japanese older adults. Further studies investigating the mechanisms linking tub bathing and older people’s health are warranted.
ACKNOWLEDGEMENTS
Funding: This study used data from JAGES, which was supported by MEXT (Ministry of Education, Culture, Sports, Science and Technology-Japan) – Supported Program for the Strategic Research Foundation at Private Universities (2009–2013); JSPS (Japan Society for the Promotion of Science) KAKENHI Grant Numbers (JP18390200, JP22330172, JP22390400, JP23243070, JP23590786, JP23790710, JP24390469, JP24530698, JP24683018, JP25253052, JP25870573, JP25870881, JP26285138, JP26882010, JP15H01972); Health Labour Sciences Research Grants (H22-Choju-Shitei-008, H24-Junkanki [Seishu]-Ippan-007, H24-Chikyukibo-Ippan-009, H24-Choju-Wakate-009, H25-Kenki-Wakate-015, H25-Choju-Ippan-003, H26-Irryo-Shitei-003 [Fukkou], H26-Choju-Ippan-006, H27-Ninchisyou-Ippan-001, H28-choju-Ippan-002); AMED (Japan Agency for Medical Research and Development), the Research Funding for Longevity Sciences from National Center for Geriatrics and Gerontology (24-17, 24-23, 29-42); and World Health Organization Centre for Health Development (WHO Kobe Centre) (WHO APW 2017/713981). The views and opinions expressed in this article are those of the authors and do not necessarily reflect the official policy or position of the respective funding organizations.
Conflicts of interest: None declared.
Author contributions: AY analyzed and interpreted the data and drafted the article. SH, TO, and KK designed the study, interpreted the data, and revised the article. YS, TT, YM, YN, and TN interpreted the data and revised the article.
APPENDIX A. SUPPLEMENTARY DATA
The following is the supplementary data related to this article:
eFigure 1. Original question on tub bathing frequency (in Japanese)
eTable 1. Subgroup analysis: hazard ratios of functional disability onset stratified by sex
eTable 2. Subgroup analysis: hazard ratios of functional disability onset stratified by age
eTable 3. Subgroup analysis: hazard ratios of functional disability onset stratified by treatment for any disease
eTable 4. Subgroup analysis: hazard ratios of functional disability onset stratified by physical strength
eTable 5. Subgroup analysis: hazard ratios of functional disability onset stratified by cognitive function
eTable 6. Subgroup analysis: hazard ratios of functional disability onset stratified by depression
eTable 7. Subgroup analysis: hazard ratios of functional disability onset stratified by instrumental ADL
REFERENCES
- 1.Christensen K, Doblhammer G, Rau R, Vaupel JW. Ageing populations: the challenges ahead. Lancet. 2009;374:1196–1208. 10.1016/S0140-6736(09)61460-4 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Shetty P. Grey matter: ageing in developing countries. Lancet. 2012;379:1285–1287. 10.1016/S0140-6736(12)60541-8 [DOI] [PubMed] [Google Scholar]
- 3.Statistics Bureau, Ministry of Internal Affairs and Communications, Japan. Result of the Population Estimates, Monthly Report. http://www.stat.go.jp/english/data/jinsui/tsuki/index.html; 2018 Accessed 18.05.08.
- 4.UN Population Division. World Population Prospects: the 2017 revision population database. https://esa.un.org/unpd/wpp/; 2018 Accessed 18.05.08.
- 5.Shimazu T, Kuriyama S, Hozawa A, et al. Dietary patterns and cardiovascular disease mortality in Japan: a prospective cohort study. Int J Epidemiol. 2007;36:600–609. 10.1093/ije/dym005 [DOI] [PubMed] [Google Scholar]
- 6.Marmot MG, Syme SL. Acculturation and coronary heart disease in Japanese-Americans. Am J Epidemiol. 1976;104:225–247. 10.1093/oxfordjournals.aje.a112296 [DOI] [PubMed] [Google Scholar]
- 7.Saito M, Kondo N, Kondo K, Ojima T, Hirai H. Gender differences on the impacts of social exclusion on mortality among older Japanese: AGES cohort study. Soc Sci Med. 2012;75:940–945. 10.1016/j.socscimed.2012.04.006 [DOI] [PubMed] [Google Scholar]
- 8.Takehara H, Yanase T, Nishikawa K, Murakami K. The Survey on Bathroom Enviroment and Residential Consciousness (Part 2): On the Residential Consciousness and Interest in Bathing. J Home Economics Jpn. 2001;52:1005–1013 (in Japanese). [Google Scholar]
- 9.Yano T, Hiro M, Imanishi J, et al. Comparison of the habit of bathing between Japanese living in Kyoto and Japanese Americans living in Los Angeles. J Balneol Climatol Phys Med. 2017;80:80–92 (in Japanese) 10.11390/onki.2305 [DOI] [Google Scholar]
- 10.Hayasaka S, Shibata Y, Goto Y, Noda T, Ojima T. Bathing in a bathtub and health status: a cross-sectional study. Complement Ther Clin Pract. 2010;16:219–221. 10.1016/j.ctcp.2010.05.002 [DOI] [PubMed] [Google Scholar]
- 11.Goto Y, Hayasaka S, Nakamura Y. Health effects of seasonal bathing in hot water, seasonal utilization of hot spring facilities, and high green tea consumption. J Balneol Climatol Phys Med. 2014;77:171–182. 10.11390/onki.77.171 [DOI] [Google Scholar]
- 12.Goto Y, Hayasaka S, Nakamura Y. Bathing in hot water, bathing in japanese style hot spring and drinking green tea may contribute to the good health status of Japanese. J Balneol Climatol Phys Med. 2012;75:256–267. 10.11390/onki.75.256 [DOI] [Google Scholar]
- 13.Organization of Balneotherapist in Japanese Association of Balneology Climatology and Physical Medicine Prospective 5yrs cohort study on the correlation between bathing habit and need for long term care insurance of aged subjects. J Balneol Climatol Phys Med. 2011;74:200–206 (in Japanese) 10.11390/onki.74.200 [DOI] [Google Scholar]
- 14.Laukkanen T, Khan H, Zaccardi F, Laukkanen JA. Association between sauna bathing and fatal cardiovascular and all-cause mortality events. JAMA Intern Med. 2015;175:542–548. 10.1001/jamainternmed.2014.8187 [DOI] [PubMed] [Google Scholar]
- 15.Kondo K. Progress in aging epidemiology in Japan: The JAGES Project. J Epidemiol. 2016;26:331–336. 10.2188/jea.JE20160093 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 16.Aida J, Kondo K, Kawachi I, et al. Does social capital affect the incidence of functional disability in older Japanese? A prospective population-based cohort study. J Epidemiol Community Health. 2013;67:42–47. 10.1136/jech-2011-200307 [DOI] [PubMed] [Google Scholar]
- 17.Saito E, Ueki S, Yasuda N, Yamazaki S, Yasumura S. Risk factors of functional disability among community-dwelling elderly people by household in Japan: a prospective cohort study. BMC Geriatr. 2014;14:93. 10.1186/1471-2318-14-93 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 18.Tsutsui T, Muramatsu N. Care-needs certification in the long-term care insurance system of Japan. J Am Geriatr Soc. 2005;53:522–527. 10.1111/j.1532-5415.2005.53175.x [DOI] [PubMed] [Google Scholar]
- 19.Ishizawa T, Watanabe S, Yano S, et al. Relationship between bathing habits and physical and psychological state. J Balneol Climatol Phys Med. 2012;75:227–237. 10.11390/onki.75.227 [DOI] [Google Scholar]
- 20.Ogura Y. Weather in Japan: Its Diversity and Mechanism. Tokyo: University of Tokyo Press; 2015:37–54 (in Japanese). [Google Scholar]
- 21.Shinkai S, Watanabe N, Yoshida H, et al. Research on screening for frailty: development of “the Kaigo-Yobo Checklist”. Nihon Koshu Eisei Zasshi. 2010;57:345–354 (in Japanese). 10.11236/jph.57.5_345 [DOI] [PubMed] [Google Scholar]
- 22.Fukutomi E, Okumiya K, Wada T, et al. Relationships between each category of 25-item frailty risk assessment (Kihon Checklist) and newly certified older adults under Long-Term Care Insurance: a 24-month follow-up study in a rural community in Japan. Geriatr Gerontol Int. 2015;15:864–871. 10.1111/ggi.12360 [DOI] [PubMed] [Google Scholar]
- 23.Yesavage JA, Sheikh JI. Geriatric Depression Scale (GDS): recent evidence and development of a shorter version. Clin Gerontol. 1986;5:165–173. 10.1300/J018v05n01_09 [DOI] [Google Scholar]
- 24.Koyano W, Shibata H, Haga H, Suyama Y, Nakazato K. Measurement of competence in the elderly living at home: development of an index of competence. Nihon Koshu Eisei Zasshi. 1987;34:109–114 (in Japanese). [Google Scholar]
- 25.Hess KR. Graphical methods for assessing violations of the proportional hazards assumption in Cox regression. Stat Med. 1995;14:1707–1723. 10.1002/sim.4780141510 [DOI] [PubMed] [Google Scholar]
- 26.Liao WC. Effects of passive body heating on body temperature and sleep regulation in the elderly: a systematic review. Int J Nurs Stud. 2002;39:803–810. 10.1016/S0020-7489(02)00023-8 [DOI] [PubMed] [Google Scholar]
- 27.Yasuda T, Kubo T, Masamitsu Y, et al. Behavior change from the shower bathing to bath bathing affects sleep and working efficiency. J Balneol Climatol Phys Med. 2015;78:341–352 (in Japanese) 10.11390/onki.78.341 [DOI] [Google Scholar]
- 28.Yamamoto K, Nagata S. Physiological and psychological evaluation of the wrapped warm footbath as a complementary nursing therapy to induce relaxation in hospitalized patients with incurable cancer: a pilot study. Cancer Nurs. 2011;34:185–192. 10.1097/NCC.0b013e3181fe4d2d [DOI] [PubMed] [Google Scholar]
- 29.Dorsey CM, Teicher MH, Cohen-Zion M, et al. Core body temperature and sleep of older female insomniacs before and after passive body heating. Sleep. 1999;22:891–898. 10.1093/sleep/22.7.891 [DOI] [PubMed] [Google Scholar]
- 30.Minowada G, Welch WJ. Clinical implications of the stress response. J Clin Invest. 1995;95:3–12. 10.1172/JCI117655 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 31.Kalmar B, Greensmith L. Induction of heat shock proteins for protection against oxidative stress. Adv Drug Deliv Rev. 2009;61:310–318. 10.1016/j.addr.2009.02.003 [DOI] [PubMed] [Google Scholar]
- 32.Krause M, Ludwig MS, Heck TG, Takahashi HK. Heat shock proteins and heat therapy for type 2 diabetes: pros and cons. Curr Opin Clin Nutr Metab Care. 2015;18:374–380. 10.1097/MCO.0000000000000183 [DOI] [PubMed] [Google Scholar]
- 33.Hoshino T, Suzuki K, Matsushima T, Yamakawa N, Suzuki T, Mizushima T. Suppression of Alzheimer’s disease-related phenotypes by geranylgeranylacetone in mice. PLoS One. 2013;8:e76306. 10.1371/journal.pone.0076306 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 34.Ministry of Health, Labour and Welfare, Japan. Comprehensive Survey of Living Condition, 2016. https://www.mhlw.go.jp/toukei/saikin/hw/k-tyosa/k-tyosa16/index.html; 2017. Accessed 18.08.26 (in Japanese).
- 35.Stuck AE, Walthert JM, Nikolaus T, Büla CJ, Hohmann C, Beck JC. Risk factors for functional status decline in community-living elderly people: a systematic literature review. Soc Sci Med. 1999;48:445–469. 10.1016/S0277-9536(98)00370-0 [DOI] [PubMed] [Google Scholar]
- 36.Hirai H, Kondo K, Ojima T, Murata C. Examination of risk factors for onset of certification of long-term care insurance in community-dwelling older people: AGES project 3-year follow-up study. Nihon Koshu Eisei Zasshi. 2009;56:501–512 (in Japanese). 10.11236/jph.56.8_501 [DOI] [PubMed] [Google Scholar]
- 37.Wacholder S, Hartge P, Lubin JH, Dosemeci M. Non-differential misclassification and bias towards the null: a clarification. Occup Environ Med. 1995;52:557–558. 10.1136/oem.52.8.557 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 38.Suzuki M, Shimbo T, Ikaga T, Hori S. Sudden death phenomenon while bathing in Japan—mortality data. Circ J. 2017;81:1144–1149. 10.1253/circj.CJ-16-1066 [DOI] [PubMed] [Google Scholar]
- 39.Hayasaka S, Haraoka T, Ojima T. Relationship of bathing care-related illness or incident with blood pressure and body temperature: a case-control study. J Balneol Climatol Phys Med. 2016;79:112–118 (in Japanese) 10.11390/onki.2294 [DOI] [Google Scholar]
- 40.Takasaki Y, Nagai Y, Inoue K, Maki M, Ohnaka T, Tochihara Y. Bating habits of the elderly in winter and factors affecting regional differences in bathing death rates. J Human Living Environment. 2011;18:99–106 (in Japanese). [Google Scholar]
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