When the women enrolled in the trials, none had diabetes, cardiovascular disease or cancer.
During the 18- to 20-year study period, 6,165 women in the first group and almost 4,000 women from the second group developed type 2 diabetes.
When compared to women who hadn't done rotating shift work, women who did one to two years of shift work had a 5 percent increase in type 2 diabetes. Women who worked shifts for three to nine years had a 20 percent increased risk, while women who toiled 10 to 19 years on rotating shifts had a 40 percent greater risk of type 2 diabetes compared to women who didn't do shift work.
Women with more than 20 years on a rotating work schedule had the highest risk of all, with a 58 percent increase in the risk of type 2 diabetes, the study found.
When the researchers adjusted the data to account for body mass, the association between shift work and type 2 diabetes was reduced, but still present, they said.
Although the study wasn't designed to figure out why rotating shift work might increase the risk of type 2 diabetes, Hu said there are likely both biological and behavioral reasons. Rotating shift work disturbs the body's natural time clock (circadian rhythm), which, in turn, disrupts the body's ability to balance its need for energy. Hu said this can cause higher levels of glucose and insulin resistance, which are hallmarks of type 2 diabetes.
Working on rotating shifts also affects eating and sleeping behaviors, and women who worked rotating shifts also tended to smoke more.
"Shift work is an important risk factor for obesity and type 2 diabetes," Hu said. "This study increases the awareness of diabetes risk among people who work on a rotating shift, and the importance of diabetes screening, detection and prevention in this high risk group."
More research is needed to confirm the finding
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