The overall association for long-term shift work showed no interaction by tumor hormone receptor status, while shorter durations working nights didn't significantly correlate with breast cancer risk.
Source/Credit: MedPage Today |
By Crystal Phend | July 3, 2013
Working the night shift long term raises breast cancer risk, likely across job types, researchers affirmed.
Breast cancer risk was 2.21-fold higher with 30-plus years of night-shift work versus none in a general population sample (95% CI 1.14-4.31), Kristan Aronson, PhD, of Queen's University Cancer Research Institute in Kingston, Ontario, and colleagues found.
The odds ratios for breast cancer were particularly strong with 30-plus years of night work in the health occupations, at 3.11 versus no night work, the group reported online in Occupational and Environmental Medicine.
The odds also reached 2.25-fold with 30 or more years working nights in nonhealth jobs but without statistical significance (95% CI 0.92 to 5.52).
"However, it is important to note that relationships between night-shift work duration and breast cancer risk were similar to those of the main analysis for those employed in both health and nonhealth occupations," Aronson's group wrote.
"Long-term night-shift work in a diverse mix of occupations is associated with increased breast cancer risk and not limited to nurses, as in most previous studies."
The overall association for long-term shift work showed no interaction by tumor hormone receptor status, while shorter durations working nights didn't significantly correlate with breast cancer risk.
"As shift work is necessary for many occupations, understanding of which specific shift patterns increase breast cancer risk, and how night-shift work influences the pathway to breast cancer is needed for the development of healthy workplace policy," the researchers wrote.
The biological mechanism has yet to be discovered but is suspected to involve melatonin, which regulates circadian rhythm based on light exposure but may also have cancer-protective protective properties, Aronson's group explained.
"While increased light exposure during night shifts is thought to decrease production of melatonin, thereby increasing cancer risk, other mechanisms are also possible," they wrote.
The study included 1,134 breast cancer cases from the British Columbia Cancer Registry and 1,179 age-matched controls from the screening mammography program in that province or Kingston, Ontario, who had no history of cancer other than nonmelanoma skin cancer.
About one-third of women in both groups had worked a job where at least 50% of their time was reported to have been spent on evening or night shifts or both, which would include both rotating and permanent night-shift schedules.
Proportions were similar in the 3 months to 14 years and 15 to 29 years categories of night-shift work, but cancer cases accounted for more of the small proportion who had worked nights for 30 years or more (2.5% versus 1.1%).
Limitations were lack of data on number of consecutive night shifts in each job and the possibility of recall and selection bias, though the researchers called the likelihood low.
Also, "although there were differences in the characteristics of cases and controls, for instance in ethnicity, education and so on, none of these factors were identified as confounders of the night-shift work/breast cancer association," they noted.
Other studies have shown a breast cancer impact of working after dark among women in the military, which was greater with higher exposure; though a larger general population cohort in Sweden didn't show an association over nearly 20 years of follow-up.
The study was funded by a grant from the Canadian Institutes of Health Research.
The researchers reported no conflicts of interest.
Reviewed by Robert Jasmer, MD; Associate Clinical Professor of Medicine, University of California, San Francisco and Dorothy Caputo, MA, BSN, RN, Nurse Planner
Primary source: Occupational and Environmental Medicine
Source reference: Grundy A, et al "Increased risk of breast cancer associated with long-term shift work in Canada" Occup Environ Med 2013; DOI: 10.1136/oemed-2013-101482.
Read original post here: Night Work Again Tied to Breast Cancer Risk
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