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Occupational Medicine Advance Access originally published online on October 16, 2006
Occupational Medicine 2006 56(8):578-580; doi:10.1093/occmed/kql106
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© The Author 2006. Published by Oxford University Press on behalf of the Society of Occupational Medicine. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

Short Reports

Effects of overtime work on blood pressure and body mass index in Japanese male workers

Koji Wada1, Noritada Katoh2, Yutaka Aratake2, Yasushi Furukawa3, Toshio Hayashi4, Emi Satoh2, Katsutoshi Tanaka2, Toshihiko Satoh5 and Yoshiharu Aizawa5

1 Department of Occupational Health, Graduate School of Medical Sciences, Kitasato University, Sagamihara, Kanagawa, Japan
2 Department of Occupational Mental Health, Graduate School of Medical Sciences, Kitasato University, Sagamihara, Kanagawa, Japan
3 Ajinomoto Co., Inc., Kawasaki, Kanagawa, Japan
4 Toshiba Human Asset Service Co., Yokohama, Kanagawa, Japan
5 Department of Preventive Medicine and Public Health, Kitasato University School of Medicine, Sagamihara, Kanagawa, Japan

Correspondence to: Koji Wada, Department of Occupational Health, Graduate School of Medical Sciences, Kitasato University, 1-15-1 Kitasato, Sagamihara, Kanagawa 228-8555, Japan. Tel: +81 42 778 9311; fax: +81 42 778 9257; e-mail: kwada-sgy{at}umin.ac.jp


   Abstract
Top
 Abstract
Introduction
 Methods
 Results
 Discussion
 Conflicts of interest
 References
 
Background Effects of overtime work on health is a controversial issue.

Aims To determine the effects of overtime work on blood pressure and body mass index (BMI) for male workers in Japan.

Methods Participants were 323 male participants of three companies. Data were collected by occupational physicians at periodic physical examinations and additional examinations. The time courses for the development of definite hypertension and an increase in BMI were recorded. The Kaplan–Meier method was used to estimate the cumulative incidence rates of developing definite hypertension and increasing BMI, and the Cox proportional hazard model was used to determine the adjusted relative hazard of overtime work.

Results The age-adjusted incidence rates of developing definite hypertension and increasing BMI were significantly lower among the participants whose mean overtime was ≥50 h than among those whose mean overtime was <50 h/month (log-rank P < 0.05). The Cox proportional hazard model indicated that those who worked a mean overtime of ≥50 h/month had lower risks of developing definite hypertension (hazard ratio, 0.36; 95% CI, 0.15–0.88; P < 0.05) and increasing their BMI (hazard ratio, 0.44; 95% CI, 0.31–0.63; P < 0.01) after adjusting for age.

Conclusions The results of this study indicate that workers whose mean overtime was ≥50 h have lower risks of developing definite hypertension and increasing their BMI.

Keywords      Body mass index; hypertension; karoshi; overtime


   Introduction
Top
 Abstract
 Introduction
Methods
 Results
 Discussion
 Conflicts of interest
 References
 
Recently, ‘karoshi’, death from overtime work, has become a social concern in Japan [1]. Most cases of karoshi are the result of the sudden onset of a fatal cardiovascular or cerebrovascular event. In 2002, the Ministry of Health, Labor and Welfare in Japan announced a comprehensive program for the prevention of health problems due to overwork.

There are controversies over the effects of overtime work on health [2–5]. There are few reports on the direct influence of overtime work on the development of cardiovascular or cerebrovascular diseases. Kawakami et al. [6] reported that workers who did overtime work >50 h/month have higher risk of developing diabetes mellitus.

This epidemiological study was undertaken to determine whether overtime work is directly associated with the development of definite hypertension and increasing body mass index (BMI) for male workers in Japan.


   Methods
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 Abstract
 Introduction
 Methods
Results
 Discussion
 Conflicts of interest
 References
 
The eligible participants were all individuals who completed periodic physical examinations and/or ‘additional’ examinations at least twice between January 2002 and July 2004. Outcomes were compared between those who worked overtime of ≥50 h and those who worked <50 h. Normal working hours without overtime were defined as 160 h/month. Mean overtime work per month was calculated.

There are two sources of data for blood pressure and weight: periodic physical examinations and additional examinations. The periodic physical examinations were completed every 6 or 12 months depending on the company. Additional examinations were completed every month an employee exceeded 50 h of overtime work. Consequently, those who had more overtime work had more frequent additional examinations.

Hypertension was defined as a systolic blood pressure ≥140 mmHg or a diastolic blood pressure ≥90 mmHg on the basis of World Health Organization criteria [7].

Increasing BMI was defined as an increase in BMI between health examinations. The time courses for the development of definite hypertension and the increase in BMI were recorded.

Differences in baseline characteristics were compared using a t-test. Survival curves were constructed by the Kaplan–Meier method [8] with a difference in survival assessed by the log-rank test between the participants who worked a mean overtime of ≥50 h and those who worked a mean overtime of <50 h. The Cox proportional hazard model [9] was used to evaluate the associations of working hours with the development of definite hypertension and an increase in BMI. Data were adjusted for age. All analyses were performed using SPSS for Windows, version 10 [10].


   Results
Top
 Abstract
 Introduction
 Methods
 Results
Discussion
 Conflicts of interest
 References
 
The participants were 360 male workers of three companies (121 of a food company, 150 of an electronics manufacturing company and 89 of a steel engineering company) in the Kanto area, Japan. We excluded 37 participants who were already diagnosed as having hypertension at the baseline from the analyzed population. The overall mean age (SD) of the participants was 34.7 (8.6) years. The participants were followed up for a mean period (SD) of 23.0 (19.3) weeks. Two hundred and four participants (63%) worked a mean overtime of ≥50 h/month. There were no significant differences in baseline age, blood pressure or BMI between the participants who worked a mean overtime of <50 h and those who worked a mean overtime of ≥50 h (Table 1).


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  		Table 1 Baseline characteristics of participants who did overtime for ≥50 h and for <50 h/montha

 
Over the studied time courses, 23 participants developed definite hypertension and 135 showed an increase in BMI. The median time for the development of definite hypertension was 26 weeks and that for the increase in BMI was 26 weeks for the participants who worked a mean overtime of ≥50 h/month. The median time for the development of definite hypertension was 11 weeks and that for an increase in BMI was 10.5 weeks for the participants who worked a mean overtime of <50 h/month.

The stratified Kaplan–Meier survival estimates are shown in Figure 1. Survival rate differed between the participants whose mean overtime was ≥50 h and those whose mean overtime was <50 h/month in terms of the development of definite hypertension and an increase of their BMI (log-rank test P < 0.05).


Figure 1
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  		Figure 1 Kaplan–Meier survival curve. Left: definite hypertension, right: an increase in BMI. Overtime <50 h, overtime ≥50 h. The pluses on the plots are censored observations.

 
The evaluation using the Cox proportional hazard model indicated that the participants whose mean overtime work was ≥50 h/month had lower risks of developing definite hypertension (hazard ratio, 0.36; 95% CI, 0.15–0.88; P < 0.05) and increasing their BMI (hazard ratio, 0.44; 95% CI, 0.31–0.63; P < 0.01) after adjusting for age (Table 2).


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  		Table 2 Cox regression model for definite hypertension and an increase in BMI with age and overtime work

 

   Discussion
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
Conflicts of interest
 References
 
In this study, we found that mean monthly overtime work of ≥50 h was negatively associated with the development of definite hypertension and an increase in BMI. Workers who worked overtime of ≥50 h had more frequent examinations and these examinations were staggered, hence the greater variability in the timing of the observations. In comparison, the workers who worked <50 h of overtime underwent a periodic health examination, thereby resulting in more clustered observations.

Findings on the effect of overtime work on hypertension are inconsistent across other studies [2–5]. Hayashi et al. [2] showed an increase in blood pressure among workers whose mean overtime was 84–96 h as compared with those working 25–43 mean hours of overtime per month. Our results are well in line with those of Nakanishi et al. [3], which indicated that overtime work is negatively associated with the development of hypertension during a 5-year follow-up.

Nakamura et al. [4] showed that working overtime is associated with increasing BMI as determined by correlation analysis. To the best of our knowledge, our study is the first to assess the effect of overtime work on BMI by survival analysis.

This study had a few limitations. First, we only assessed mean overtime hours per month and age. Differences in job-related strain, occupations and lifestyle among workers should be studied in future studies. Second, we did not consider working hours before the start of this study. Some participants might have become used to working overtime. Third, we were able to assess only men, even though these are not specific problems to men. Fourth, the follow-up time was not sufficiently long to accurately assess the development of hypertension even though we focused on the trend of blood pressure. Further study should be carried out with more variables and for longer periods to assess the effects of overtime work on hypertension.


   Conflicts of interest
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conflicts of interest
References
 
None declared.


   Acknowledgements
 
This study was partly funded by a grant from the Ministry of Health, Labour and Welfare, Japan.


   References
Top
 Abstract
 Introduction
 Methods
 Results
 Discussion
 Conflicts of interest
 References
 

  1. Uehata T. (1991) Karoshi due to occupational stress-related cardiovascular injuries among middle-aged workers in Japan. J Sci Labour 67:20–28.

  2. Hayashi T, Kobayashi Y, Yamaoka K, Yano E. (1996) Effect of overtime work on 24-hour ambulatory blood pressure. J Occup Environ Med 38:1007–1111.[CrossRef][Web of Science][Medline]

  3. Nakanishi N, Yoshida H, Nagano K, Kawashimo H, Nakamura K, Tatara K. (2001) Long working hours and risk for hypertension in Japanese male white collar workers. J Epidemiol Community Health 55:316–322.[Abstract/Free Full Text]

  4. Nakamura K, Shimai S, Kikuchi S, et al. (1998) Increases in body mass index and waist circumference as outcomes of working overtime. Occup Med (Lond) 48:169–173.

  5. Park J, Kim Y, Cho Y, et al. (2001) Regular overtime and cardiovascular functions. Ind Health 39:244–249.[Web of Science][Medline]

  6. Kawakami N, Araki S, Takatsuka N, Shimizu H, Ishibashi H. (1999) Overtime, psychosocial working conditions, and occurrence of non-insulin dependent diabetes mellitus in Japanese men. J Epidemiol Community Health 53:359–363.[Abstract]

  7. Whitworth JA. (2003) World Health Organization, International Society of Hypertension Writing Group. 2003 World Health Organization (WHO)/International Society of Hypertension (ISH) statement on management of hypertension. J Hypertens 21:1983–1992.[CrossRef][Web of Science][Medline]

  8. Kaplan E and Meier P. (1958) Nonparametric estimation from incomplete observations. J Am Stat Assoc 53:457–481.[CrossRef][Web of Science]

  9. Cox D. (1972) Regression models and life tables (with discussion). J R Stat Soc Ser B 34:187–220.

  10. SPSS for Windows, Version 10.0J. (1999) (SPSS Inc, Chicago, IL).


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