Men who alternate between daytime and nighttime shifts on their jobs have triple the normal rate of prostate cancer, according to a Japanese nationwide study.
A variable employment schedule, which can upset daily hormone-production cycles, had previously been linked to breast cancer and, in one study, to colorectal cancer in women. The new finding supports a longstanding expectation that disrupting 24-hour biological rhythms can cause tumors in men too.
The study is the first solid evidence tying shift work to prostate cancer, says neuroendocrinologist David Blask of the Bassett Research Institute in Cooperstown, N.Y.
To investigate the possibility that shift work might put men at risk, Japanese researchers led by urologist Tatsuhiko Kubo of the University of Occupational and Environmental Health in Kitakyushu gathered data in the late 1980s on some 14,000 cancerfree men who were between 40 and 65 years old. Eighty percent of them worked daytime schedules, 7 percent worked nights, and 13 percent rotated between day and night hours.
During the study, which tracked the men for nearly a decade, prostate cancer was diagnosed in 21 daytime workers, 3 nighttime workers, and 7 shift workers.
After taking into account the influence of body weight, smoking habits, alcohol consumption, and other variables, the researchers calculate that annual prostate-cancer incidence was three times as high among rotating-shift workers as among daytime workers. By contrast, nighttime-only and daytime-only workers didn’t have significantly different cancer rates, the researchers report in the Sept. 15 American Journal of Epidemiology.
“Those of us interested in circadian rhythms and cancer predicted this a long time ago,” says epidemiologist Richard Stevens of the University of Connecticut Health Center in Farmington.
One possible explanation for the shift-work effect is that the hormone melatonin might protect men against prostate cancer, says Stevens. People produce less of that hormone when they are exposed to light during their usual sleep periods.
Blask has already shown that melatonin slows the growth of human-breast tumors (SN: 1/7/06, p. 8: Bright Lights, Big Cancer), and he plans to test the hormone’s effects on prostate tumors.
Meanwhile, William Hrushesky of the Dorn Veterans Affairs Medical Center in Columbia, S.C., is examining whether melatonin supplements can fight prostate cancer.
Neither Blask nor Hrushesky is surprised that men in the Japanese study who consistently worked at night had normal cancer risk. “Their melatonin rhythm and circadian system have adapted to a reverse cycle,” Blask speculates.
Blask notes a caveat to the new analysis: Diet can influence prostate cancer, and the Japanese study didn’t determine whether the rotating-shift workers had less healthful diets than other men. Nevertheless, Blask says, “the shift-work issue isn’t just for women anymore.”