Authors' response to Letter to the Editor

Abstract

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 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.