Low-income workers were more likely to develop type-2 diabetes if they put in more than 55 hours per week than if they worked normal hours, researchers found. Work hours weren't tied to increased diabetes risks among wealthier people, however.
"Those who worked long hours in these jobs had 30 per cent increased risk of developing type 2 diabetes," says Mika Kivimäki, the study's lead author from the UK's University College London.
"One possible reason for this is that working long hours displaces health-restorative activities, in particular physical activity, sufficient sleep and healthy diet," says Kivimäki.
Type-2 diabetes, sometimes referred to as adult-onset diabetes, occurs when the body's cells are resistant to the hormone insulin, or the body doesn't make enough of it. Insulin gives blood sugar access to the body's cells to be used as fuel.
According to the latest National Health and Morbidity Survey 2011, Malaysia has 15.2 per cent or 2.6 million of adults 18 years and above have diabetes.
In 16 years, the National Center for Biotechnology Information (NCBI) in the US estimates that 7.7 per cent of the world population, or 439 million adults would live with diabetes. NCBI also predict that there will be a sharp rise of 69 per cent of adults in developing countries.
Previous studies suggested that working long hours is tied to an increased risk of developing diabetes, but more recent research had suggested the link is only true among the poorest workers, Kivimäki and his colleagues write in The Lancet Diabetes and Endocrinology.
For the analysis, they combined data from four previously published studies and 19 unpublished studies that looked at working long hours, which they defined as 55 hours or more per week, and the risk of developing diabetes.
The studies included more than 200,000 people, who were followed for an average of seven years, from the US, Japan, Australia and several European countries.
Out of every 10,000 study participants, about 29 developed diabetes each year during the study.
Overall, when the researchers compared people who worked long hours to people who worked a standard 35 to 40-hour work week, they found similar diabetes risks in both groups.
But when they focused on people who worked long hours, they saw a difference by wealth class.
Specifically, among every 10,000 of the lowest-paid workers, there were 13 extra cases of diabetes each year among those who worked longer hours, compared to those who worked normal hours.
There was no increased risk among the wealthiest people who worked long hours.
While the new study can't prove working long hours leads to diabetes among poor workers, Kivimäki says that it's good for health professionals to know of the link.
"Well targeted prevention and early diagnosis can reduce the number of diabetes cases and lower rates of developing complications," he says.
Orfeu Buxton, a researcher with The Pennsylvania State University in University Park, and Cassandra Okechukwu, a researcher from Harvard School of Public Health in Boston, suggest in a commentary accompanying the new analysis that the increased risk among the poorest group may stem from working longer shifts, late nights or split shifts that disrupt the body's so-called clocks - known as circadian rhythms.
"It's not the work hours themselves directly that are necessarily toxic - it's what they create or cause," says Buxton.
Circadian rhythms can slow down metabolism and cause the pancreas to secrete less insulin after meals, he said. This can lead to diabetes in some people.
"We don't think that everybody faces the same risks," Buxton says, adding that some people can handle long hours and shift work, but others may have problems within a few weeks.
Kivimäki says he hopes the study will prompt policy-makers and employers to think of ways the workplace can support healthy lifestyles. He also said there are a few ways individuals can lower their risks of developing diabetes.
"Diabetes prevention guidelines emphasise that 30 minutes of moderate-intensity physical activity on most days and a healthy diet can substantially reduce the risk of developing type-2 diabetes," he says.
Losing weight is also an excellent way of lowering diabetes risk, he says.