Objectives The aim of this study was to calculate the benchmark doses (BMD) and their 95% lower confidence boundary (BMDL) for the threshold number of years of shift work associated with a relative increase in haemoglobin A1c (HbA_1c), an index of glucose metabolism.

Methods A 14-year prospective cohort study was conducted in male workers at a Japanese steel company (n=7104) who had received annual health check-ups between 1991 and 2005. The endpoints were either a 10%, 15%, 20%, 25% or 30% increase in HbA_1c levels during the observation period, compared to HbA_1c at entry to the study. The associations between years of shift work and increases in HbA_1c were investigated using pooled logistic regression, adjusted for age, body mass index, mean arterial pressure, total serum cholesterol, creatinine, alanine aminotransferase, γ-glutamyl transpeptidase, uric acid, drinking habits, smoking habits and habitual exercise.

Results The BMDL/BMD for years of shift work were calculated using benchmark responses (BMRs) of 5% or 10% and parameters for duration of shift work and other covariates. Assuming a mean age of 53 years in workers aged 50 years or older, the BMDL/BMD for years of shift work with a BMR of 5% were 17.8/23.9 (≥15%), 15.7/18.7 (≥20%), 18.9/22.7 (≥25%) and 25.2/31.7 (≥30%). With a BMR of 10%, the respective values were 29.5/39.7 (≥15%), 24.3/28.9 (≥20%), 27.3/32.7 (≥25%) and 34.1/42.9 (≥30%).

Conclusion These results suggest that special attention should be paid to middle-aged workers whose years of shift work exceeds these threshold times.