Their age-adjusted risk is three times higher than that of day workers.
Rotating-shift work is associated with an increased risk of prostate cancer, according to a Japanese study.
Shift workers are known to be at increased risks for sleep and GI disturbances, obesity, hypertension, and breast and other cancers, but a link with prostate cancer has not been previously described. One explanation for the link could be that circulating levels of melatonin, a hormone closely associated with circadian rhythms, are decreased in some patients with breast or prostate cancer, and melatonin has been shown to inhibit the proliferation of breast and prostate cancer cell lines.
The new finding comes from study of 14,052 working men enrolled throughout Japan between 1988 and 1990. At baseline, participants completed a self-administered questionnaire that provided information on type of work schedule, job type, activities at work, and other variables.
Of the 14,052 men, 11,269 (80.2%) reported day work, 982 (7.0%) reported fixed-night work, and 1,801 (12.8%) reported rotating-shift work. Compared with men who engaged in day work, those who performed rotating-shift work had a higher prevalence of high BMI (34.4% vs. 30.8%), sitting work (48.4% vs. 28.1%), and indoor work (44.5% vs. 38.6%), and a lower prevalence of office work (14.3% vs. 22.7%).
All three groups were similar in terms of the prevalence of family history of prostate cancer, current smoking, current alcohol drinking, frequent stress, marriage, and higher educational level, according to Tatsuhiko Kubo, MD, of the University of Occupational and Environmental Health in Kitakyushu, and his colleagues. There were no significant differences between day workers and rotating-shift workers in mean age at baseline (58.5 vs. 59.3 years), mean number of years of follow-up (6.7 vs. 6.3 years), mean age at study end point (65.2 vs. 65.6 years), family history of prostate cancer, and mean BMI at baseline.
Compared with day workers, rotating-shift workers had a threefold higher age-adjusted risk for prostate cancer, Dr. Kubo’s group reported in the American Journal of Epidemiology (2006;164:549-555). The association was not influenced by geographic area, family history of prostate cancer, BMI, smoking, alcohol drinking, job type, physical activity at work, workplace, perceived stress, educational level, or marriage status.
The researchers noted that melatonin suppression is believed to increase the level of sex hormones, and that the melatonin pathway may be a relevant mechanism in the growth and differentiation of prostate cancer. They concluded that disruption of the circadian rhythm associated with rotating-shift work may have greater effects on melatonin production and prostate tumorigenesis than shortening exposure to daylight associated with fixed-night work.