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 (HbA1c), 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 HbA1c levels during the observation period, compared to HbA1c at entry to the study. The associations between years of shift work and increases in HbA1c 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.
- Shift work
- glucose metabolism
- glycated haemoglobin A1c
- cohort studies
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