Women on Night Shift Have Increased Risk for Type 2 Diabetes

Elizabeth DeVita-Raeburn

December 07, 2011

December 7, 2011 — Women who worked irregularly scheduled night shifts had up to a 60% higher risk of developing type 2 diabetes than women who worked only day and evening hours, according to a study published online December 6 in PLOS Medicine.

The study, led by An Pan, PhD, a research fellow in the Department of Nutrition at the Harvard School of Public Health, Boston, Massachusetts, found that women who had worked rotating night shifts at least 3 times per month for 1 to 2 years had a 5% increased risk of developing type 2 diabetes. The risk rose the longer a woman had done rotating night-shift work, from 20% after 3 to 9 years to almost 60% for 20 or more years.

Dr. Pan and colleagues used prospective data collected over the course of 18 to 20 years on 177,184 women in the Nurses' Health Study. The women were aged from 25 to 67 years.

Using time-dependent Cox proportional models, adjusted for diabetes risk factors related to lifestyle, the pooled hazard ratios for women who reported no shift work compared with women who worked the night shift for 1 to 2, 3 to 9, 10 to 19, and 20 or more years were 1.05 (95% confidence interval [CI], 1.00 - 1.11), 1.20 (95% CI, 1.14 - .26), 1.40 (95% CI, 1.30 - 1.51), and 1.58 (95% CI, 1.43 - 1.74), respectively (P = .001 for the trend). Further adjustment for updated body mass index reduced the pooled hazard ratios to 1.03 (95% CI, 0.98 - 1.08), 1.06 (95% CI, 1.01- 1.11), 1.10 (95% CI, 1.02 - 1.18), and 1.24 (95% CI, 1.13 - 1.37), respectively (P = .001 for the trend).

In a secondary analysis of 107,663 of the study participants, night-shift work was also associated with weight gain, which might have contributed to the nurses' increased diabetes risk. However, that did not explain the results entirely, the researchers say.

Previous studies have associated night-shift work with obesity, metabolic syndrome, and glucose dysregulation.

Dr. Pan and colleagues theorized that the link between night shifts and diabetes could be the result of disrupted circadian clocks, which help regulate the gastrointestinal and endocrine systems. Sleep disruption is also known to contribute to insulin resistance, increases in appetite, and weight gain, and night-shift work has previously been linked to negative changes in health behaviors including smoking, irregular meals, and lack of exercise.

Limitations of the study include reliance on self-reports; a population that consisted of white women working as nurses; and the observational design, which precludes any inference about causality.

In a perspective also published online December 6 in PLOS Medicine, Mika Kivimåki, PhD, professor of social epidemiology in the Department of Epidemiology and Public Health at University College London, United Kingdom, and colleagues write that the current study represents "the most accurate estimate" to date of the association between shift work and type 2 diabetes. They say that the effect is comparable in size to that of work stress on coronary heart disease, and larger than the effect of work stress on diabetes.

The editorialists point out that "[w]e are increasingly residing in a '24/7' society; thus, the option to eradicate shift working is not realistic." If the observed association is studied further and randomized, controlled trials show that night-shift work causes type 2 diabetes, additional efforts should be made to promote healthy lifestyles, weight control, and early identification and treatment of prediabetes and diabetes in night-shift employees, Dr. Kivimåki and colleagues write.

The study was supported by the National Institutes of Health, and 2 authors received grants from the National Heart, Lung, and Blood Institute. The authors have disclosed no relevant financial relationships.

PLoS Med. Published online December 6, 2011. Article full text, Perspective full text

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