The study, led by Andreas Holtermann of the National Research Centre for the Working Environment in Copenhagen, found that physically fit men were 45% less likely to die of heart disease and 38% less likely to die of other causes than their unfit colleagues, according to a Bloomberg report.  The most active men either did heavy manual labor such as digging and shoveling, or were active athletes who ran, played tennis or badminton for at least three hours a week, the study said.

Men who weren’t exactly tri-athletes but were still fit cut their death risk compared to unfit male workaholics. However, males who were both unfit and worked more than 45 hours a week fared the worst: they were more than twice as likely to die of heart disease as men working under 40 hours.

Meanwhile, unfit men who worked 41 to 45 hours a week were 59% more likely to die of heart disease than men working less, the study found.

The research is the first to show that fitness can offer protection from some detrimental effects of working long hours.

“Long hours are not a problem if you are physically fit,” Holtermann told Bloomberg. “The bad news is that if you aren’t fit, you much more likely to die if you work longer than 45 hours a week.”

According to Bloomberg, the researchers studied about 5,000 healthy men enrolled in a long-term epidemiological project called the Copenhagen Male Study. The scientists used bicycle ergometers to test participants’ fitness levels and questionnaires to determine their work hours and activity levels.

The researchers admitted that the study was limited by the fact that the men reported both their activity levels and work hours, making the data susceptible to errors, and by the fact that it only includes Caucasian men, Bloomberg reported.