Kouichi Sakata

A Longitudinal Study on the Effect of Shift Work on Weight Gain in Male Japanese Workers

Objective: This study compared the effect of alternating shift work and day work on weight gain in Japanese male workers.

Long-term Longitudinal Study on the Relationship Between Alternating Shift Work and the Onset of Diabetes Mellitus in Male Japanese Workers

Objective: This study investigated the effect of alternating shift work (ASW) on the onset of diabetes mellitus in Japanese workers compared with onset in day-shift work (DSW). Methods: A longitudinal study was carried out on a DSW group (n = 3203) and ASW group (n = 2426) of a steel company who received their annual health checkups over a 10-year period between 1991 and 2001. The association between job schedule type and onset of diabetes mellitus (glycated hemoglobin A1c ≥6.0% or medication) was investigated by multivariate pooled logistic regression analyses. Results: The odds ratio (95% confidence interval) for the development of diabetes mellitus in the ASW group compared with the DSW group was 1.35 (1.05–1.75). Conclusions: Our study revealed that the ASW is an independent risk factor for the onset of diabetes mellitus.

Shift Work Is a Risk Factor for Increased Blood Pressure in Japanese Men. A 14-Year Historical Cohort Study

To clarify the effect of shift work on blood pressure in Japanese men, a 14-year historical cohort study was conducted in day workers (n=3963) and alternating shift workers (n=2748) who received annual health checkups between 1991 and 2005 in a Japanese steel company. The end points were a ≥10%, ≥15%, ≥20%, ≥25%, or ≥30% increase in systolic blood pressure or diastolic blood pressure from baseline during the period of observation. The association between shift work and an increase in blood pressure was investigated adjusting for age, body mass index, hemoglobin A1c, total serum cholesterol, creatinine, aspartate aminotransferase, &ggr;-glutamyl transpeptidase, uric acid, drinking habit, smoking habit, and habitual exercise by multivariate pooled logistic regression analyses. Shift work was significantly associated with the various end points. The odds ratios (and 95% CIs) were as follows: ≥10%, 1.15 (1.07 to 1.23); ≥15%, 1.21 (1.12 to 1.31); ≥20%, 1.15 (1.04 to 1.28); ≥25%, 1.20 (1.06 to 1.37); and ≥30%, 1.23 (1.03 to 1.47) for systolic blood pressure and ≥10%, 1.19 (1.11 to 1.28); ≥15%, 1.22 (1.13 to 1.33); ≥20%, 1.24 (1.13 to 1.37); and ≥25%, 1.16 (1.03 to 1.30) for diastolic blood pressure. Our study in male Japanese workers revealed that alternating shift work was a significant independent risk factor for an increase in blood pressure. Moreover, the effect of shift work on blood pressure was more pronounced than other well-established factors, such as age and body mass index.

The relationship between shift work and the onset of hypertension in male Japanese workers.

Learning ObjectivesRecall the prevalence of hypertension in this study of Japanese steel company employees during nearly 5 years of follow-up.Describe the observed differences between day workers and shift workers in blood pressure, body size, and life style.Specify the risk of hypertension in shift workers compared to day workers, and the life style and clinical factors that were significantly associated with its occurrence. We assessed the effect of shift work on the onset of hypertension. The design of this study was that of a cohort study from 1991–2001. In sum, 5338 workers comprised this cohort. The event was the onset of hypertension (systolic blood pressure ≥140 mm Hg and/or diastolic blood pressure ≥90 mm Hg and/or medication). Pooled logistic regression analysis was performed, including job schedule type, age, body mass index, lifestyle, and the results of blood chemistries as covariates. The odds ratio of the onset of hypertension in shift workers for daytime workers was 1.10 and significant. Our study revealed that shift work independently affected the onset of hypertension, and suggested that shift work is a risk factor for the onset of hypertension.

A longitudinal study on the relationship between shift work and the progression of hypertension in male Japanese workers.

Objective In this 1991–2001 cohort study of 6495 male workers in a Japanese steel company, we investigated whether shift work affects progression from mild hypertension to severe hypertension. Design A prospective cohort study. Participants Participants had mild hypertension at entry into the study with systolic blood pressure of 140–159 mmHg or diastolic blood pressure of 90–99 mmHg or both before treatment. In summary, 2911–2941 workers were included for each endpoint. Pooled logistic regression analysis was performed to adjust for the effect of contributing factors and annual variations. Main outcome Either severe hypertension (systolic blood pressure ≥ 160 mmHg and/or diastolic blood pressure ≥ 100 mmHg), severe systolic hypertension (systolic blood pressure ≥ 160 mmHg) or severe diastolic hypertension (diastolic blood pressure ≥ 100 mmHg) was defined as a different endpoint of observation. Results Job schedule was significantly associated with progression from mild hypertension to severe hypertension and severe diastolic hypertension. The odds ratios of shift workers compared with regular day workers were 1.23 (95% confidence interval: 1.05–1.44) and 1.28 (95% confidence interval: 1.07–1.52) for severe hypertension and severe diastolic hypertension, respectively. Age, body mass index, creatinine, γ-glutamyl transpeptidase, and glycosylated hemoglobin A1c were also significantly associated with severe hypertension, severe systolic hypertension and severe diastolic hypertension. Conclusion Our study revealed that shift work is a significant and independent risk factor for the progression of hypertension.

SHIFTWORK AND IMPAIRED GLUCOSE METABOLISM: A 14-YEAR COHORT STUDY ON 7104 MALE WORKERS

The aim of this study was to assess the effect of shiftwork on hemoglobin A1c (HbA1c) level, as an index of glucose metabolism. A 14 yr prospective cohort study was conducted on day (n = 4219) and alternating shiftworkers (n = 2885) who received annual health checkups between 1991 and 2005 at a Japanese steel company. The endpoints were either a 10%, 15%, 20%, 25%, or 30% increase in HbA1c during the period of observation, compared to HbA1c at entry to the study. The association between the type of job schedule and increase in HbA1c was investigated after adjusting for age, body mass index, mean arterial pressure, total serum cholesterol, creatinine, alanine aminotransferase, γ-glutamyl transpeptidase, uric acid, drinking habit, smoking habit, and habitual exercise using multivariate pooled logistic regression analyses. Shiftwork was significantly associated with the various HbA1c endpoints (≥10% HbA1c increase, odds ratio 1.35 [95% confidence interval 1.26–1.44]; ≥15% HbA1c increase, odds ratio 1.29 [95% confidence interval, 1.19–1.40]; ≥20% HbA1c increase, odds ratio 1.23 [95% confidence interval 1.11–1.37]; and ≥25% HbA1c increase, odds ratio 1.19 [95% confidence interval 1.03–1.36]). Age, body mass index, alanine aminotransferase, and γ-glutamyl transpeptidase were associated positively with all five HbA1c endpoints. Uric acid was associated negatively with all five HbA1c endpoints. Our study on male Japanese workers revealed alternating shiftwork (in addition to other established factors, such as age and body mass index) was a consistent risk factor for impaired glucose metabolism.

Shift work is a risk factor for increased total cholesterol level: a 14-year prospective cohort study in 6886 male workers

Objectives: The widespread adoption of 24 h continuous operations in a number of industries has resulted in an increase in shift work, which may influence lipid metabolism because of disturbed circadian rhythms, broken sleep and lifestyle problems. The objective of the present study was to assess the effect of shift work on serum total cholesterol as an index of lipid metabolism. Methods: A 14-year prospective cohort study was conducted in day workers (n = 4079) and alternating shift workers (n = 2807) who received annual health check-ups between 1991 and 2005 in a Japanese steel company. The end-points were either a 20%, 25%, 30%, 35%, 40% or 45% increase in serum total cholesterol during the period of observation, compared with serum total cholesterol at entry to the study. The association between the job schedule type and increase in serum total cholesterol was investigated using multivariate pooled logistic regression analyses. The odds ratios for the effect of shift work were obtained after adjustment for a number of potential confounders. Results: The significant odds ratios of alternating shift work (and 95% confidence intervals) were: ⩾20%, 1.16 (1.07 to 1.26); ⩾25%, 1.16 (1.05 to 1.28); ⩾35%, 1.23 (1.05 to 1.43); ⩾40%, 1.30 (1.07 to 1.58); and ⩾45%, 1.28 (1.01 to 1.63) for serum total cholesterol. Conclusion: Generally the odds ratios of alternating shift work tended to be higher for stricter cut-points of relative increase in serum total cholesterol level. Our study in male Japanese workers revealed that alternating shift work adversely affected lipid metabolism.

Three-shift system increases job-related stress in Japanese workers.

Three‐Shift System Increases Job‐Related Stress in Japanese Workers: Hideto Harada, et al. Department of Occupational and Environmental Medicine, Graduate School of Medicine, Chiba University—We assessed the relationship between shift work and job stress. The target subjects were 4,962 male workers (3,078 day workers and 1,884 shift workers) aged 18 to 60 yr who work in a Japanese steel company. We used the “Brief Job Stress Questionnaire”, which was developed by a research group organized by the Japanese Ministry of Labour. We evaluated the effect of shift work on job stress using logistic regression analysis including age, lifestyle factors, work conditions, marital status, and living arrangements in the model. Job schedule type was significantly associated with job control, with an odds ratio of 2.22 for shift workers compared to day workers. The logistic regression analysis revealed that the odd ratios for having one or more stressor items in an unfavorable condition were significantly higher for shift workers compared to day workers. Increase in the amount of overtime and decrease in the number of holidays led to a significant deterioration in job stress. Our study reveals that the 3‐shift system of employment increases work‐related stress, and that job control is low among shift workers. To reduce job stress in this occupational population, a reduction in the amount of overtime and an increase in the number of holidays seem to be useful interventions.

ESTIMATION OF THE BENCHMARK DURATION OF SHIFTWORK ASSOCIATED WITH WEIGHT GAIN IN MALE JAPANESE WORKERS

The authors estimated the benchmark durations (BMDs) and their 95% lower confidence limit (BMDL) for the reference duration of shiftwork for weight gain. A 14-yr prospective cohort study was conducted in male workers at a Japanese steel company (n = 7254) who had received annual health check-ups between 1991 and 2005. The endpoints in the study were either a 5%, 7.5%, or 10% increase in body mass index (BMI) during the period of observation, compared to the BMI at entry. The association between the duration of shiftwork and weight gain was investigated using multivariate pooled logistic regression analyses with stepwise selection of covariates, including age, BMI measured during the study, drinking and smoking habits, and habitual exercise. The BMDL/BMD for shiftwork in subjects aged in their 40s or ≥50 yrs was estimated using benchmark responses (BMRs) of 5% or 10% and parameters for the duration of shiftwork and other covariates. For workers aged in their 40s, the BMDL/BMD for shiftwork with a BMR of 5% was 18.6/23.0 yrs (≥7.5%) and 16.9/19.4 yrs (≥10%). For workers aged ≥50 yrs, the BMDL/BMD with a BMR of 5% was 22.9/28.2 yrs (≥7.5%) and 20.6/23.6 yrs (≥10%). The reference duration of shiftwork that associated with weight gain was shown to be at least 17 yrs in middle-aged workers. Special attention should be paid to prevent weight gain at an earlier stage and not when this increase in weight has become apparent. (Author correspondence: suwa@faculty.chiba-u.jp)

Benchmark duration of work hours for development of fatigue symptoms in Japanese workers with adjustment for job-related stress.

The objective of this study was to calculate benchmark durations and lower 95% confidence limits for benchmark durations of working hours associated with subjective fatigue symptoms by applying the benchmark dose approach while adjusting for job-related stress using multiple logistic regression analyses. A self-administered questionnaire was completed by 3,069 male and 412 female daytime workers (age 18-67 years) in a Japanese steel company. The eight dependent variables in the Cumulative Fatigue Symptoms Index were decreased vitality, general fatigue, physical disorders, irritability, decreased willingness to work, anxiety, depressive feelings, and chronic tiredness. Independent variables were daily working hours, four subscales (job demand, job control, interpersonal relationship, and job suitability) of the Brief Job Stress Questionnaire, and other potential covariates. Using significant parameters for working hours and those for other covariates, the benchmark durations of working hours were calculated for the corresponding Index property. Benchmark response was set at 5% or 10%. Assuming a condition of worst job stress, the benchmark duration/lower 95% confidence limit for benchmark duration of working hours per day with a benchmark response of 5% or 10% were 10.0/9.4 or 11.7/10.7 (irritability) and 9.2/8.9 or 10.4/9.8 (chronic tiredness) in men and 8.9/8.4 or 9.8/8.9 (chronic tiredness) in women. The threshold amounts of working hours for fatigue symptoms under the worst job-related stress were very close to the standard daily working hours in Japan. The results strongly suggest that special attention should be paid to employees whose working hours exceed threshold amounts based on individual levels of job-related stress.