IJC International Journal of Cancer

Authors’ response to Letter to the Editor Manolis Kogevinas1,2,3,4 and Kyriaki Papantoniou1,2,3,5 1

Centre for Research in Environmental Epidemiology (CREAL), Barcelona, Spain IMIM (Hospital Del Mar Medical Research Institute), Barcelona, Spain 3 blica (CIBERESP), Madrid, Spain CIBER Epidemiologıa Y Salud Pu 4 National School of Public Health, Athens, Greece 5 Universitat Pompeu Fabra (UPF), Barcelona, Spain

Letter to the Editor

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We thank W.B. Grant for his comments on our paper on night shift work and prostate cancer risk and his suggestion that lack of vitamin D may be one of the causes for higher cancer risk in night workers.1 The association between night shift work and cancer including prostate cancer is complex. Several pathways are involved, including circadian disruption, melatonin suppression, sleep deprivation, circadian gene deregulation and lifestyles changes.2 It has been suggested that inadequate vitamin D levels related to reduced time spent outdoors among night workers may be one of the multiple underlying mechanisms and this is certainly a potential pathway linking night work and cancer that should be further examined. Our study is the first study to consider sun exposure as a possible confounding factor of the night work–prostate cancer association.3 We assessed present and past sun exposure summer habits through a questionnaire, as proxies for vitamin D levels at least for the brightest months of the year, although we agree that it would have been preferable if more details (e.g., unbathing, protection cream) were available. All models were adjusted for past sun exposure that was not a major confounder of the night shift work–prostate cancer association. Studies on breast cancer that have accounted for sun exposure in their analyses reported only marginal changes in their night work risk estimates.4,5 The role of sunlight exposure and vitamin D has been long debated and the evidence from observational studies, on levels of serum 25-hydroxyvitamin D and incidence of prostate cancer is inconclusive.6,7 Furthermore, although counterintuitive, it is not clear that night shift or rotating night shift workers spend less time or receive less sunlight exposure compared to day workers. In the present study, we did not find differences in sunlight exposure between day and night workers.3 Similarly in a recent nested case–control study, night shift workers reported more frequent sun exposure

C 2015 UICC Int. J. Cancer: 137, 1000 (2015) V

than those with day work, that often spend most of the daytime hours working indoors.5 More data are needed on sun exposure and vitamin D levels among current and past night workers, in order to confirm if this biologically plausible hypothesis can explain part of the increased risk for prostate cancer observed among night shift workers.

Author Contributions MK and KP wrote the letter to the editor, read and approved the final manuscript.

References 1. 2. 3.

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Grant WB. Low ultraviolet-B exposure may explain some of the link between night shift work and increased risk of prostate cancer. Int J Cancer, in press. Fritschi L, Glass DC, Heyworth JS, et al. Hypotheses for mechanisms linking shiftwork and cancer. Med Hypotheses 2011;77:430–6. Papantoniou K, Casta~ no-Vinyals G, Espinosa A, et al. Night shift work, chronotype and prostate cancer risk in the MCC-Spain case–control study. Int J Cancer, in press. Fritschi L, Erren TC, Glass DC, et al. The association between different night shiftwork factors and breast cancer: a case-control study. Br J Cancer 2013;109: 2472–80. Hansen J, Lassen CF. Nested case-control study of night shift work and breast cancer risk among women in the Danish military. Occup Environ Med 2012; 69:551–6. World Health Organization International Agency for Research on Cancer. IARC working group reports, vol. 5: Vitamin D and cancer. Lyon, France: World Health Organization International Agency for Research on Cancer, 2008. Key TJ. Nutrition, hormones and prostate cancer risk: results from the european prospective investigation into cancer and nutrition. Recent Results Cancer Res 2014;202:39–46.

DOI: 10.1002/ijc.29460 History: Received 12 Jan 2015; Accepted 14 Jan 2015; Online 28 Jan 2015 Correspondence to: Manolis Kogevinas, Centre for Research in Environmental Epidemiology (CREAL), Barcelona, Doctor Aiguader, 88 08003 Barcelona, Spain. E-mail: [email protected]

Authors' response to Letter to the Editor.

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