Women who worked a rotating night shift had an increased risk of type 2 diabetes that was not completely explained by an increase in body mass index (BMI), according to results of a prospective study of women who were enrolled in the Nurses' Health Studies. Nurses who had 1 to 10 years of night shift work saw a 5% excess risk for type 2 diabetes compared to women who did minimal to no night shift work. That risk climbed to 40% after a decade of shift work, according to Frank Hu, MD, PhD, from Brigham and Women's Hospital/Harvard School of Medicine in Boston, and colleagues.
Excess risk rocketed to almost 60% for those who had put in 20 years or more, the group reported. Other studies have suggested that rotating night shift work is associated with an increased risk for obesity and metabolic syndrome, both of which are conditions related to type 2 diabetes, they wrote.
Hu's group examined the relationship between the duration of rotating night shift work and the risk of type 2 diabetes in U.S. women who participated in Nurses' Health Studies (NHS) I and II. They also looked at whether greater weight gain was linked to duration of shift work.
Collectively, NHS I and II enrolled nearly 240,000 women. For this study, the women who completed the NHS questionnaire in 1988 or 1989 served as the baseline for this particular study. Participants were excluded if they had diabetes, heart disease, stroke, or cancer at baseline. Follow-up took place at 18 to 20 years.
Rotating night shifts were defined as working at least three nights a month in addition to days and evenings in that same month. The control group consisted of women who did not report a history of rotating night shift work. In both cohorts, women who spent more years in night shift work were older, more likely to have a higher BMI, and be smokers.
In a secondary analysis, they found that night shift work was also associated with an elevated risk for obesity and excessive weight gain during the follow-up period. They suggested that, beyond BMI, a reason for the link between shift work and type 2 diabetes may be "chronic misalignment between the endogenous circadian timing system and the behavior cycles." This misalignment has been pegged as a reason for metabolic and cardiovascular disorders, including increases in glucose and insulin, they wrote.
In an accompanying commentary, Mika Kivimäki, PhD, from University College London, and colleagues said the study "probably represents the most accurate estimate of shift work-type 2 diabetes association available to date, suggesting this effect is comparable in size to that of work stress in coronary heart disease and larger than the effect of work stress on type 2 diabetes."
They suggested that in an increasingly "24/7" society, efforts need to be made to prevent type 2 diabetes among shift workers by promoting healthy lifestyle and weight control. Also, prediabetic and diabetic employees need to be identified early and treated accordingly.
Hu FB, et al PLoS Medicine 2011; 8(12).