Rotating night-shift work together with an unhealthy lifestyle significantly increases the risk of type 2 diabetes, researchers say.

“Most cases of type 2 diabetes could be prevented by adherence to a healthy lifestyle, and the benefits could be larger in rotating night-shift workers,” said study authors led by Zhilei Shan. He is a nutrition researcher at Harvard School of Public Health in Boston.

The new study was published in the BMJ.

Combined effects

The researchers noted that previous studies have shown smoking, a poor diet, inactivity, and being overweight/obese increase the risk of type 2 diabetes. Shift work has also been linked with an increased risk, the investigators added in a journal news release.

But the authors said they believe this is the first study to examine the combined effect of an unhealthy lifestyle and shift work on type 2 diabetes risk. The results show it’s especially important for shift workers to follow a healthy lifestyle.

The researchers analysed data from more than 143 000 US women who did not have type 2 diabetes, heart disease or cancer when they enrolled in the Nurses’ Health Study (NHS) and NHS II in 1976 and 1989.

Over 22 to 24 years of follow-up, nearly 11 000 of the women were diagnosed with type 2 diabetes. For every five years of working rotating night shifts, there was a 31% higher risk of a type 2 diabetes diagnosis.

Each unhealthy lifestyle factor – ever smoking, being overweight or obese, having a poor diet, being inactive – was linked with a 2.3 times higher risk of being diagnosed with type 2 diabetes. The study authors explained that a poor diet was low in fruits, vegetables, nuts and whole grains, and high in processed meat, trans fats, sugar and salt.

Rotating night-shift work

Women who had any of the four unhealthy lifestyle factors and also worked rotating night shifts had the highest risk of developing type 2 diabetes, the findings showed. Among this group, each individual unhealthy lifestyle factor was associated with a 2.8 times increased risk.

This suggests that some kind of interaction between rotating night shifts and an unhealthy lifestyle factor adds further risk, according to the authors. In this study, rotating night shifts were defined as working at least three night shifts a month, in addition to day and evening shifts that month.

The researchers concluded that rotating night-shift work accounted for about 17% of the combined higher risk of type 2 diabetes, unhealthy lifestyle accounted for about 71%, and the remaining 11% was additional risk associated with the interaction of the two.

According to the report, the increased risk of type 2 diabetes among rotating night-shift workers with an unhealthy lifestyle may be due to disrupted sleep and body-clock rhythms affecting hormones, other metabolic pathways or the balance of bacteria in the gut.

Because this was an observational study, no firm conclusions can be drawn about cause and effect. Also, since all of the nurses were female and mostly white, the findings may not apply to men and other racial or ethnic groups, the researchers noted.

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