November 23, 2018
Working night shifts and having an unhealthy lifestyle appear to have an additive effect on the likelihood of developing type 2 diabetes, and women with both have a greater risk than simply adding the impact of either factor alone, suggests a pooled analysis of two major studies.
Zhilei Shan, PhD, Tongji Medical College, Huazhong University of Science and Technology, Wuhan, China, and Harvard T.H. Chan School of Public Health, Boston, Massachusetts, and colleagues combined data on more than 140,000 nurses who took part in two long-term prospective health studies.
The research, which was published online on November 21 in BMJ, indicates that every 5 years of rotating night shift work increased the risk of type 2 diabetes by around 30%.
And having a combination of several unhealthy lifestyle factors, such as being a smoker or having a high body mass index (BMI) or poor diet, more than doubled the risk of developing the disease.
However, women with both an unhealthy lifestyle and more than 5 years of rotating night shift work were 2.83 times more likely than women without these factors to develop type 2 diabetes, with the two factors together accounting for 11% of additional risk.
The researchers say their findings "suggest that most cases of type 2 diabetes could be prevented by adherence to a healthy lifestyle, and the benefits would be larger in rotating night shift workers."
They believe that there are "several possible mechanisms" to explain their results. "Sleep loss and circadian misalignment could disrupt the intestinal microbiota," and lifestyle behaviors such as diet and physical activity "could affect gut microbial diversity and metabolites," they hypothesize.
Shift Work Already Linked to Increased Risk of Chronic Diseases
Labor force surveys indicate that approximately one in five US employees works nonstandard working hours or shifts. Health workers account for one third of shift workers, with nurses making up the largest part, in efforts to ensure round-the-clock services.
Shift workers also have higher rates of excess adiposity and smoking compared with other workers, both of which are linked to type 2 diabetes risk.
To examine the joint associations of night shift work and unhealthy lifestyle factors on type 2 diabetes risk, the team examined data from the Nurses' Health Study (NHS), which started in 1976, and NHS II, initiated in 1989.
In both studies, which included a total of 238,278 female registered US nurses, questionnaires to update medical and lifestyle information were sent every 2 years following a baseline survey. In addition, the participants completed a food frequency questionnaire, which was updated every 4 years. Follow-up rates were over 90%.
Excluding nurses with a self-report of diabetes, coronary heart disease, stroke, or cancer at baseline and all those with missing shift work and key clinical and demographic data, 55,324 nurses were selected from NHS and 88,086 from NHS II.
In both studies, women who spent more years in rotating shift work, defined as three or more night shifts per month, in addition to day and evening shifts in the same month, were more likely than other women to be current smokers and have a higher BMI.
Women who spent more time in rotating night shift work were, in the NHS, more likely to be older and, in NHS II, more likely to be unmarried or living alone.
In the NHS, there were 5474 incident cases of type 2 diabetes, which were identified through self-report and validated by a supplementary questionnaire during 1,139,597 person-years of follow-up. In NHS II, there were 5441 incident cases during 1,778,721 person-years of follow-up.
Significant Additive Effect of Shift Work and Unhealthy Lifestyles
Pooled multivariate regression analysis, adjusted for BMI, indicates that women with rotating night shift work had an increased risk of developing type 2 diabetes versus women not performing rotating night shifts, at a hazard ratio of 1.04 for 1 to 5 years of night shifts, 1.09 for 5 to 9 years, and 1.16 for 10 years or longer (P for trend < .001).
The team also looked at unhealthy lifestyle factors, which included current smoking, activity levels less than 30 minutes/day at moderate to vigorous intensity, diet in the bottom three fifths of the Alternate Healthy Eating Index score, and a BMI of 25 kg/m2 or higher.
They found that, compared to women with one or no unhealthy lifestyle factors, those with three or more unhealthy factors had an increased risk of type 2 diabetes, at a hazard ratio of 5.39.
Moreover, women with at least 10 years of rotating night shift work and three or more unhealthy lifestyle factors had, compared with other women, an adjusted hazard ratio for type 2 diabetes of 7.04.
Researchers say there was a significant additive interaction between the duration of rotating night shift work and number of unhealthy factors (P < .001).
This was calculated at an adjusted hazard ratio of 1.31 for each 5-year increment of rotating night shift duration, 2.30 per unhealthy lifestyle factor, and 2.83 for their joint effect, at a relative excess risk due to interaction of 0.20.
It was therefore estimated that the attributable portions of the joint risk were 17.1% for rotating night shift work alone, 71.2% for unhealthy lifestyle alone, and 11.3% for their interaction.
The relationships remained significant after adjusting for hypertension, hyperlipidemia, and hypertension medication, and were similar when restricting cases of type 2 diabetes to those with symptoms.
The researchers note that there are several limitations to their study, including the fact that a statistical additive interaction does not necessarily imply causality and that having an all-female and largely white study population limits generalizability of the results.
The studies also relied on self-report for both rotating night shift work and lifestyle factors, and there were no data on actual night working time and stress.
But the analysis has several strengths, the team says, including the large sample size and repeated measures. "To our knowledge, this is the first study to investigate how rotating night shift work and modifiable lifestyle factors are jointly related to risk of type 2 diabetes," they note.
They add, "Further studies are needed to explore the pathways underlying the interaction between rotating night shift work and lifestyle factors on risk of type 2 diabetes."
The study was supported by research grants from the National Institutes of Health (NIH). Shan was supported by the Young Scientists Fund of the National Natural Science Foundation of China and China Postdoctoral Science Foundation. The authors have reported no relevant financial relationships.