Objective Research indicates that shiftwork may be associated with increased risks of adverse health outcomes, including some cancers. However, the evidence of an association between shiftwork and colorectal cancer risk is limited and inconclusive. Further, while several possible pathways through which shiftwork might result in cancer have been proposed, few studies have taken these factors into account. We investigated the association between two types of shiftwork (graveyard shiftwork and early-morning shiftwork) and six mechanistic shiftwork variables (including light at night and phase shift) and the risk of colorectal cancer among females in an Australian population-based case–control study. Graveyard shiftwork was the primary exposure of interest.
Methods Participants (350 cases and 410 controls) completed a lifetime occupational history, and exposure to each of the eight shiftwork variables was assigned to participants through a job exposure matrix. We used logistic regression to calculate odds ratios (OR) and corresponding 95% confidence intervals (CI) for the association between different shiftwork variables and the risk of colorectal cancer, adjusting for potential demographic, lifestyle and medical confounders.
Results Working in an occupation involving long-term exposure (>7.5 years) to graveyard shiftwork was not associated with colorectal cancer risk (adjusted OR 0.95, 95% CI 0.57 to 1.58). Similarly, no increased risks of colorectal cancer were seen for any of the other seven shiftwork variables examined.
Conclusions No evidence of an increased risk of colorectal cancer among females who had worked in occupations involving shiftwork was observed in this study.
- public health
- shift work
- gi tract
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Contributors WMW and TB drafted the manuscript, prepared the data and conducted all statistical analyses and data interpretation. RNC, SS, LF, JSH and RCF contributed to data preparation, data analysis and/or interpretation of data, and provided feedback on the draft of the manuscript. LF and JSH were also involved in the conception and design of the case–control study. All authors have read and approved of the submitted manuscript.
Funding The WABOHS was supported by Australian National Health and Medical Research Council (NHMRC; grant #353568). RF is supported by an Australian Postgraduate Award from the University of Adelaide. LF is supported by the NHMRC (fellowship #37614900) and Cancer Council Western Australia (Fellowship). TB is supported by the NHMRC (early career fellowship #1072266) and the Canadian Institutes of Health Research (fellowship #300068).
Competing interests None declared.
Ethics approval The WABOHS was approved by the Human Research Ethics Committee of The University of Western Australia and the Confidentiality of Health Information Committee within the Western Australian Department of Health, and the current analysis was approved by the Curtin University Human Research Ethics Committee. All procedures performed in this study involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki Declaration and its later amendments or comparable ethical standards. The study reported in this manuscript complies with the current laws of Australia.
Provenance and peer review Not commissioned; externally peer reviewed.