High risk of type 2 diabetes for nurses working intermittent night shifts, leading unhealthy lifestyles

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Nurses who work intermittent night shifts and lead an unhealthy lifestyle face an especially high risk of type 2 diabetes, according to a new study. The findings from the study are based on two long-term health studies on female nurses - Nurses’ Health Study (1988-2012) and Nurses’ Health Study II (1991-2013).

Previous research has shown that unhealthy habits such as smoking, being overweight and not exercising increase the risk of type 2 diabetes. Meanwhile, shift work, especially doing night shifts, has also been linked to a higher risk of developing the condition. This study, Rotating night shift work and adherence to unhealthy lifestyle in predicting risk of type 2 diabetes: results from two large US cohorts of female nurses published in the BMJ, looks at the impact of an unhealthy lifestyle and irregular night shifts combined.

A research team led by the Harvard School of Public Health looked at data from two major nursing health studies, which enrolled nurses in the late 1980s and early 1990s. The team focused on more than 143,000 female nurses who did not have type 2 diabetes, cardiovascular disease or cancer and had completed regular medical, food and lifestyle questionnaires.

For the purposes of study, rotating night shift work was defined as working at least three night shifts per month in addition to day and evening shifts that month.

Meanwhile, an unhealthy lifestyle was characterised by four factors – being overweight or obese, ever having smoked, doing less than half an hour of moderate to vigorous exercise per day, and a poor diet high in fat, sugar and salt.

Over the 22 to 24 years nurses were followed up, 10,915 out of the 143,410 included in the study reported being diagnosed with type 2 diabetes. The researchers found those who worked intermittent night shifts were more likely to have type 2 diabetes.

For every five years working rotating night shifts the nurses were almost a third (31%) more likely to develop the condition. Meanwhile, each unhealthy lifestyle factor more than doubled their risk of being diagnosed, making it 2.3 times more likely.

However, the findings show that when working intermittent night shifts and an unhealthy lifestyle were combined the risk of type 2 diabetes shot up even further. Nurses who worked rotating night shifts and reported any one of the four unhealthy lifestyle factors had a 2.83 times higher risk of type two diabetes for each of those factors.

The researchers found the risk of type 2 diabetes was higher than simply adding the risks of an unhealthy lifestyle and irregular night shifts together. They calculated that rotating night shift work account for about 17% of the combined higher risk of type 2 diabetes while an unhealthy lifestyle accounted for about 71%. They suggested the remaining 11% of extra risk was down to some kind of interaction between the two.

The findings indicate efforts to promote healthy lifestyles could pay dividends among nurses, and others, when it comes to preventing type 2 diabetes and there may be particular benefits for shift workers.

However, the researchers stress the fact all the nurses in the study were female and mostly white may mean the findings do not apply to men or other racial or ethnic groups. “In two large US cohorts of female nurses, both rotating night shift work and unhealthy lifestyle were associated with a higher risk of type 2 diabetes, and the joint effect was higher than the addition of the risks associated with each individual factor,” concluded the authors. “Our 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.”

The findings could not prove a cause-and-effect relationship between rotating shift work and type 2 diabetes. But, other recent studies have also found associations between these work schedules and diabetes, along with heart disease and cancer.

Harvard’s findings are supported by a previous study, Night Shift Work, Genetic Risk, and Type 2 Diabetes in the UK Biobank, published in April 2018 in the Diabetes Care journal.

For the study, researchers looked at data on more than 272 217 participants in the UK Biobank database, 77 000 of whom provided a lifetime history of employment. and found that those who worked irregular or rotating shifts that included night shifts were 44% more likely to have type 2 diabetes than those who worked only days.

"Shift work, particularly night shifts, disrupts social and biological rhythms, as well as sleep, and has been suggested to increase the risk of metabolic disorders, including type 2 diabetes," said study co-first author Celine Vetter. She directs the University of Colorado's Circadian and Sleep Epidemiology Laboratory.

The more often a person worked an irregular night shift, the greater their risk for type 2 diabetes, the findings showed. For example, working nights less than three times a month increased the risk by 24%, but working nights more than eight times a month increased the risk by 36%.

"Our study is one of the first to show a dose-response relationship, where the more often people work nights, the greater their likelihood of having the disease," Vetter said.

However, working a permanent night shift was not linked to an increased risk of diabetes. The study authors suggested that these people might adapt to a consistent night-shift schedule, or perhaps they were "night owls" who had a natural tendency to be awake at night.

Similarly the authors concluded that if a person can't avoid working nights, they may be able to reduce their health risks by eating a healthy diet, watching their weight, and getting enough exercise and sleep.