High physical work demands are linked to an increased risk of death from ischemic heart disease—but only for men who aren't physically fit, reports a study in the November Journal of Occupational and Environmental Medicine, the official publication of the American College of Occupational and Environmental Medicine.

The increase in risk isn't explained by the higher rates of heavy work and health risk factors among men at lower socioeconomic levels, concludes the new research, led by Dr. Andreas Holtermann, of Bispebjerg University Hospital in Copenhagen.

A previous study of 5,250 Danish men found an increased risk of death from heart disease in men with high physical work demands and low physical fitness. However, social class was a potential confounding factor: Men at lower socioeconomic levels are more likely to have jobs involving heavy work. They also have higher rates of lifestyle risk factors, such as smoking and obesity.

To address this question, the researchers analyzed about 2,700 men in the lower socioeconomic levels. Thirty percent of men in this group had high physical work demands, compared with 3.5 percent for those at higher socioeconomic levels.

The long-term risk of death from heart disease was 14 percent for men in the lower social classes, compared with 9 percent in the higher social classes.

However, the main risk factor wasn't low socioeconomic status, but rather low physical fitness. Men with low fitness and high physical work demands were nearly three times more likely to die from heart disease, compared to those with low work demands.

Among men who did heavy work, risk was about 40 percent lower for those with high physical fitness.

Thus, socioeconomic factors don't seem to explain the link between heavy labor and heart disease risk.

"These observations indicate that physical fitness is a protector of or a risk modifier among men exposed to high physical loads on their cardiovascular system," Holterman and co-authors write.

The results suggest that by maintaining good physical fitness, men who engage in heavy labor can avoid increased risk, and possibly even lower their risk of death from heart disease.