March 15, 2013

expert reaction to nightshift work and risk of ovarian cancer

Research published in Occupational and Environmental Medicine showed a potential link between working night shifts and risk of developing ovarian cancer, with the risk lower for  night types (“owls”) than for morning types (“larks”).

Prof Patrick Wolfe, Royal Society Research Fellow and Professor of Statistics at University College London, said:

“It is important to remember that neither this study by Bhatti et al nor the commentary by Erren is claiming that working nights over the long term directly causes cancer–rather that there may be an association or hidden linkage between the two (such as a continued disruption in circadian rhythms affecting the body’s production of melatonin).  Both articles recommend that follow-up work should try to refine this understanding by assessing in detail whether participants are ‘lark’-like or ‘owl’-like (referred to as ‘chronotype’)–the idea being that this may provide greater insight into the degree of disruption of circadian rhythms experienced by these two types of nightshift work.” 

“There are huge challenges in progressing from laboratory experiments with rats to large-scale epidemiological studies, and as the authors note, their work should be seen as a first step in teasing out the possibility of an association.  They are correctly cautious in noting 3 factors in particular: first, the only previous similar study did not find evidence of an association, which might be because it did not collect data on long-term night shift working; second, they saw evidence of an effect only in the set of subject who were 50 years of age or older; and third, they did not see an effect that increased with increasing duration of long-term nightshift work.”

Dr Paul Pharoah, Professor of Cancer Epidemiology at the University of Cambridge, said:

“This paper reports that night shift work might be associated with a small increase in risk of ovarian cancer (from a lifetime risk of 18 per 1000 to lifetime risk of 24 in 1000).  However, the evidence is weak.  

“In this study it would not have been possible to control for all possible factors that might be associated with ovarian cancer risk.  The fact that there was no evidence that increasing amounts of night shift work were associated with increasing risk brings into question the causal nature of the association.  

“Further work is required, but night shift work and the associated changes in sleep patterns and hormones is incredibly complex and studies that will generate robust results are exceptionally hard to do.  I’d be very surprised if this reported association were confirmed in the next ten years.”

Prof Valerie Beral, Professor of Epidemiology at the University Of Oxford, said:

“The authors themselves state that this evidence is weak and should not be used to conclude that shift work causes cancer.  I agree.   There could be a number of factors at play.  One important one is that women who do shift work have fewer children; the pattern is very marked, and it matters to ovarian cancer even more than breast cancer. 

“We already know that having children reduces risk of ovarian cancer by an additional 15-20% for every birth.  If shift workers are having fewer children it could partly account for their higher incidence of ovarian cancer.  In addition, in a study like this it is quite possible that those women with cancer are more likely to recall shift work patterns, especially if they are looking for causes of their cancer.  So we must be ultra-cautious about interpreting these results.

“But shift work does merit further investigation, and in our own study, funded by the HSE, we will look specifically at women doing shift work and track their health over time.  This should give a more reliable indication.”

‘Nightshift work and risk of ovarian cancer’ by Parveen Bhatti et al. published in Occupational and Environmental Medicine  on Thursday 14th March.