Shift work and the risk of diabetes mellitus among Japanese male factory workers

TLDR

The study examined whether shift work increases the risk of developing diabetes mellitus. A prospective cohort of 2,860 Japanese male factory workers was followed for 8 years, with diabetes defined by HbA1c ≥ 6.1 % or physician diagnosis, and relative risks were estimated using Cox proportional hazards models. During the 8‑year follow‑up, 87 new diabetes cases occurred (incidence 4.41 per 1,000 person‑years); two‑shift workers had a significantly higher risk (RR ≈ 2.0) compared with white‑collar workers, while three‑shift and fixed‑day workers did not show significant increases, indicating that shift work—especially two‑shift schedules—raises diabetes risk.

Abstract

Objectives This study investigated whether shift work is a risk factor for the development of diabetes mellitus. Methods The workers, 2860 men in a sash and zipper factory in the Toyama prefecture of Japan, were followed for 8 years, and the incidence rate of diabetes mellitus was determined. The cohort contained fixed daytime blue-collar workers, shift blue-collar workers, and white-collar workers. The workers were considered to have diabetes mellitus if, in their annual health examination, they had a glycated hemoglobin level of ≥6.1 or if the diagnosis had been made by a hospital physician. The relative risks were estimated by Cox's proportional hazards regression model. Results Among the 2860 workers, there were 87 cases of new-onset diabetes mellitus, resulting in an incidence rate of 4.41 per 1000 person-years. The age-adjusted incidence was highest for the two-shift workers and lowest for the white-collar workers. The relative risk of diabetes mellitus for the two-shift workers and the three-shift workers compared with the fixed daytime workers was 1.73 and 1.33, respectively, after adjustment for all the confounding factors, but these values were not statistically significant. When the white-collar workers were used as a reference group, a significantly increased risk of diabetes mellitus was found for the two-shift workers (relative risk was 2.01 after adjustment for all confounding factors), but not for the three-shift workers or the fixed daytime blue-collar workers. Conclusions The study suggests that shift work is a risk factor for the onset of diabetes mellitus and that there is a different risk associated with different shift schedules.