Shift workers - particularly male shift workers - may be at greater risk of type 2 diabetes, according to a new meta-analysis.

By the year 2025, the number of cases of type 2 diabetes is estimated to increase by 65%, with the condition affecting a projected total of 380 million individuals worldwide. Therefore, identifying modifiable risk factors is of significant public health importance.

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People who work rotating shifts have a 42% increased risk for type 2 diabetes, according to the meta-analysis.

Previously, studies have examined how shift work - irregular or unusual work schedules, such as working a combination of nights and days - might be associated with an increased risk for diabetes, but results have been inconsistent.

The new meta-analysis - conducted by researchers at Huazhong University of Science and Technology in China - compared the results of 12 international studies, involving more than 226,500 participants, 14,600 of whom had diabetes.

The researchers found that, compared with working normal office hours, any period of shift work is associated with a 9% increased risk of developing diabetes.

Looking more closely at the effects of gender, study design, study location, job, shift schedule, body mass index (BMI), family history of diabetes and physical activity levels, the researchers calculated the increased risk rose to 37% in men.

Although the authors are not certain about why the risk in men is so much higher than in women, they suggest that repeated disruption of the internal body clock may affect levels of the hormone testosterone in men. Previous research has shown that low levels of male hormones are associated with insulin resistance and diabetes.

People who work rotating shifts are 'most at risk'

The highest increased risk, though, was for people who work rotating shifts - regularly working different parts of the 24-hour cycle, rather than fixed hours. The authors calculated the increased risk for these workers to be 42%.

The nature of rotating shifts makes it hard for workers to maintain a regular sleep-wake cycle. This reduction in sleep or sleep quality may weaken insulin resistance, consider the authors.

The authors add that other research has shown that shift work is associated with weight gain and increased appetite, which are known risk factors for diabetes. Cholesterol levels and blood pressure may also be disturbed by working in shifts, which could contribute to increased diabetes risk.

However, not all variations of shift work were associated with an increased risk of diabetes.

"Due to limited number of relevant studies, we did not find a statistically significant association between mixed and evening shifts and diabetes," lead author Prof. Zuxun Lu told Medical News Today. "More original studies are needed to confirm the association between them."

The meta-analysis covered a large number of participants, but as it was observational, the researchers are unable to draw conclusions about cause and effect.

This week, the billionaire telecommunications entrepreneur Carlos Slim made global headlines when he suggested that workers' lives would be improved if employers adopted a 3-day week, with employees working 11 or 12 hours a day.

We asked Prof. Lu how, according to the results of the new meta-analysis, Slim's proposed overhaul of working hours may affect diabetes risk.

Prof. Lu replied:

"In our meta-analysis, we defined irregular or unusual hours of work, such as regular evening or night schedule, rotating shifts, irregular schedules, and so on, as shift work, no matter how many days the employers work for in each week.

Since it involves abnormal working hours, according to our definition, a 3-day working week with workers working 12 hours each day should be regarded as shift work. Therefore, a shift pattern such as this may be associated with an increased risk for diabetes type 2. However, because no available studies on this pattern of shift work were included in our review, whether it has a different effect on diabetes is uncertain."

Research in 2013 also linked shift work to increases in risk for reduced fertility, breast cancer and ovarian cancer.