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

Methods and Procedures: A longitudinal cohort study was conducted in day workers (n = 4,328) and alternating shift workers (n = 2,926) of a steel company who received annual health checkups over a 14-year period between 1991 and 2005. The association between the type of job schedule and weight gain was investigated using multivariate pooled logistic regression analyses. The endpoints in the study were either a 5, 7.5, or 10% increase in BMI during the period of observation, compared to the BMI at entry.

Results: The type of job schedule was significantly associated with all three BMI endpoints (5% increase in BMI; odds ratio (OR) for comparison between alternating shift workers and regular day workers, 1.14; 95% confidence interval (CI), 1.06–1.23): (7.5% increase in BMI; OR, 1.13; 95%CI, 1.03–1.24: 10% increase in BMI; OR, 1.13; 95%CI, 1.00–1.28). BMI at study entry was also positively associated with the 5, 7.5, and 10% increases in BMI during the study. On the other hand, age and drinking habits were negatively associated with 5, 7.5, and 10% increases in BMI.

Discussion: Our study revealed that alternating shift work was an independent risk factor for weight gain in male Japanese workers. Efficient health screening and regular checkups, combined with support to control unhealthy lifestyle factors, would be of considerable benefit for maintaining the health of Japanese shift workers.