Researchers say people who work long hours doing jobs of low socioeconomic status - such as manual work - are much more likely to develop type 2 diabetes that those who work fewer hours.
Long working hours have previously been associated with poor health outcomes. A study recently reported by Medical News Today, for example, found a higher risk of coronary heart disease among people who work more than 60 hours a week.
The researchers of this latest study, led by Mika Kivimäki, professor of epidemiology at University College London in the UK, notes that past studies have also linked long working hours to stress, unhealthy lifestyles, depressive symptoms and disturbed sleep, which they say are factors that can contribute to development of diabetes.
"However, the direct association between long working hours and incident type 2 diabetes has been assessed in only a few studies," the researchers add.
With this in mind, the team conducted the largest study so far to investigate how long working hours influence the risk of type 2 diabetes.
Long hours in low socioeconomic status jobs 'increases type 2 diabetes risk by 30%'
To reach their findings, the team analyzed data from a selection of published and unpublished studies looking at the effect of long working hours on type 2 diabetes. The data involved 222,120 men and women from the US, Europe, Japan and Australia, and participants were followed up for an average of 7.6 years.
When the researchers compared people who worked 55 hours or more each week with those who worked the standard 35-40 hours a week, they found no significant difference in the risk of developing type 2 diabetes.
However, when they compared results by socioeconomic status, the team found that participants with jobs of low socioeconomic status - such as jobs involving manual labor - who worked 55 hours or more each week were 30% more likely to develop type 2 diabetes, compared with those who worked 35-40 hours a week.
"The strong socioeconomic patterning in the results was surprising," Prof. Kivimäki told MNT. "The higher the person's position in the socioeconomic hierarchy was, the less working long hours was linked to diabetes risk. So, high socioeconomic position appears to protect against the 'diabetogenic' effects of long working hours."
The researchers note that this finding remained even after accounting for age, sex, obesity status, smoking and physical activity. It also remained when the researchers excluded participants who do shift work, which has previously been linked to increased risk of obesity and type 2 diabetes.
'Working long hours in such groups could be marker of other risk factors'
Although the team did not investigate the reasons for this association, they hypothesize that it could be down to a number of factors. Those who work long hours in low socioeconomic status jobs may be more likely to have less sleep and less time to engage in physical exercise, for example, which may increase the risk of type 2 diabetes.
"It is also possible that working long hours in low socioeconomic groups is a marker of other risk factors, such as low pay and financial constraints," said Prof. Kivimäki. "Those with a high socioeconomic status are less likely to have such hardships."
Commenting on their overall findings, Prof. Kivimäki says:
"The pooling of all available studies on this topic allowed us to investigate the association between working hours and diabetes risk with greater precision than has been previously possible.
Although working long hours is unlikely to increase diabetes risk in everyone, health professionals should be aware that it is associated with a significantly increased risk in people doing low socioeconomic status jobs."
When MNT asked Prof. Kivimäki if people in low socioeconomic status jobs should reduce their working hours to reduce diabetes risk, he replied that no studies have suggested that lowering working hours would have such an effect.
"For this reason, my recommendation for people who wish to decrease their risk of type 2 diabetes is to eat and drink healthfully, be physically active, avoid overweight, do not smoke, and keep blood glucose and lipids levels within the normal range. This applies both to individuals who work long hours and those who work standard hours."
He added that in future research, the team plans to investigate the link between long working hours and other health outcomes, such as coronary heart disease, stroke and excessive alcohol consumption.
In an editorial linked to the study, Dr. Orfeu Buxton, of Pennsylvania State University, and Dr. Cassandra Okechukwu, of Harvard School of Public Health in Boston, MA, say that this study offers a "solid foundation" for further research into the risks and interventions for diabetes.
"The results remained robust even after controlling for obesity and physical activity," they add, "which are often the focus of diabetes risk prevention, suggesting that work factors affecting health behaviors and stress may need to be addressed as part of diabetes prevention."
MNT recently reported on a study from the Centers for Disease Control and Prevention (CDC), revealing that rates of diabetes in the US leveled off between 2008 and 2012.