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Researchers find evidence of association between rotating night shift work, increased risk of diabetes in women

Friday, December 30 2011 | Comments
Evidence Grade 0 What's This?
An analysis of data from two large prospective cohort studies shows that an extended period of rotating night shift work is associated with a modestly increased risk of diabetes in women, which appears to be partly mediated through body weight.

Using data from the Nurses' Health Study (NHS) I and NHS II, researchers followed 69,269 women aged 42 to 67 years in NHS I and 107,915 women aged 25 to 42 years in NHS II who had no history of diabetes, cardiovascular disease, stroke or cancer at baseline. The women provided information about whether they had worked rotating night shifts (defined as a minimum of three nights per month in addition to 19 days and evenings during that month) and if so, for how long; this information was updated every two to four years in NHS II.

The two studies had 18 to 20 years of follow-up data available for analysis. Participants who reported a diagnosis of diabetes during follow-up were mailed a supplementary questionnaire to confirm their condition.

Of the NHS I cohort, 59 percent reported having engaged in one or more years of rotating night shift work at some point, and 11.3 percent reported having this type of work schedule for 10 years or longer. In NHS II, at baseline, 61.9 percent of the women reported having engaged in one or more years of rotating night shift work and 4.4 percent reported having done so for 10 years or longer. At 12 years postbaseline, the corresponding percentages of NHS II participants reporting one or more years and 10 or more years increased to 69 percent and 7.9 percent, respectively.

In all, 6,165 cases of diabetes were documented during 1.3 million person-years of follow-up in NHS I, and 3,961 cases of diabetes were documented during 1.9 million person-years in NHS II.

In models that adjusted for age and survey questionnaire cycle, the duration of rotating night shift work was monotonically associated with an increased risk of diabetes in both study cohorts. As compared with the women who had never worked rotating night shifts, those in NHS I who had done so for at least three years and those in NHS II who had done so for at least one year were more likely to report a diagnosis of diabetes, and the likelihood increased as the duration of rotating night shift work increased. In the NHS II cohort, those who had such work schedules for 20 years or longer were two and a half times more likely to report a diagnosis of diabetes relative to the women who had not worked rotating night shifts.

The association between rotating night shift work and diabetes risk was not as strong in a model that further adjusted for other covariates, including ethnicity, family history of diabetes, smoking status, alcohol intake, physical activity, menopausal status, use of aspirin, hormone replacement therapy or oral contraceptives and diet. In a model that adjusted for all of these factors and body mass index (BMI), the association was further attenuated, although it was still monotonic and statistically significant.

In the model that adjusted for all factors except BMI, each five-year increase of rotating night shift work was associated with an 11 percent increased risk of diabetes in the NHS I cohort and an 18 percent increased risk of diabetes in the NHS II cohort. In the model that also adjusted for BMI, the estimated risks were reduced to 5 percent in both cohorts.

In a secondary analysis, the researchers found that rotating night shift work was associated with a significantly higher risk of obesity and excessive weight gain during follow-up. In the fully adjusted model, each five-year increase in shift work duration was associated with an increase of 0.17 unit in BMI and an increase of 0.45 kg of weight gain.

The authors of the study pointed out that the NHS I and NHS II cohorts had fairly consistent outcomes in this analysis, despite the difference in the age groups, so the increased risk of diabetes did not appear to be limited to a particular age group. They also noted that both of these cohorts consisted predominantly of white females and recommended that additional studies be conducted to confirm their findings in different populations and to further explore "the underlying mechanisms" for the observed association.

In an accompanying commentary, a separate group of researchers remarked that this analysis probably represents the most accurate estimate thus far of the association between shift work and type 2 diabetes and the results suggest that this effect "is comparable in size to that of work stress on coronary heart disease and [is] larger than the effect of work stress on type 2 diabetes."

"If the observed association between rotating shift work and type 2 diabetes is causal, as it may be, additional efforts to prevent type 2 diabetes among shift workers through promotion of healthy lifestyles, weight control and early identification and treatment of prediabetic and diabetic employees are needed," they advised.

The results of the analysis and the commentary were published in the December issue of the journal PLoS Medicine.

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