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.

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.

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 combined data on more than 140,000 nurses who took part in two long-term studies.

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.

However, shift work, particularly at night, has been linked to chronic diseases such as type 2 diabetes, cardiovascular disease, and several forms of cancer.