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, gamma-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 (> or =10% HbA1c increase, odds ratio 1.35 [95% confidence interval 1.26-1.44]; > or =15% HbA1c increase, odds ratio 1.29 [95% confidence interval, 1.19-1.40]; > or =20% HbA1c increase, odds ratio 1.23 [95% confidence interval 1.11-1.37]; and > or =25% HbA1c increase, odds ratio 1.19 [95% confidence interval 1.03-1.36]). Age, body mass index, alanine aminotransferase, and gamma-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.