This finding offers new evidence in support of the “physical activity paradox” — the observation from some earlier studies that the health effects of occupational physical activity are different from those of leisure-time physical activity.
Regular physical activity is, of course, widely acknowledged as one of cornerstones of a healthy lifestyle. Not getting enough exercise has been linked to dying prematurely from a host of medical problems, including heart disease, type 2 diabetes and stroke.
U.S. health guidelines recommend that people engage in at least 150 minutes of moderate-intensity physical activity each week (a goal most American adults fail to meet). Those guidelines do not distinguish, however, between occupational and leisure-time activity.
The new study suggests that such a distinction may need to be made. “Meeting current general [physical activity] guidelines through occupational [physical activity] instead of leisure time [physical activity] may not provide the intended health benefits or may even confer health risks,” the study’s authors warn.
For the study, an international team of researchers conducted a meta-analysis of the best evidence to date on the association between on-the-job physical activity and early death from all causes. They identified 17 studies, which involved almost 194,000 people, and then pooled the results for their analysis. The studies followed their participants for an average of 20 years.
Those pooled results revealed that men whose work involved a high level of physical activity were 18 percent more likely to die during the studies than men whose work was sedentary. The men most at risk were blue-collar workers, or those involved in manual labor.
The finding held even after adjusting for the men’s leisure-time physical activity and other factors that can affect longevity, such as age, smoking, alcohol use, body mass index (BMI) and socioeconomic status.
The finding did not apply to women, however. In fact, women in physically demanding jobs were slightly less likely to die during the studies than their counterparts whose work involved a low level of physical activity. That may be because the difference between work-related high and low levels of activity was not very wide among women.
“This evidence indicates that physical activity guidelines should differentiate between occupational and leisure time physical activity,” the study’s authors conclude.
The researchers did not investigate why strenuous on-the-job physical activity might be detrimental to workers’ health. One possible explanation is that manual laborers have, overall, less healthy lifestyles than sedentary workers.
But Pieter Coenen, the study’s lead author and a public health researcher at VU University Medical Center in Amsterdam, believes other factors are involved.
“If you go out for a run for half an hour in your leisure time, that increases your heart rate and you feel well afterwards,” he told Ian Sample, a science editor at the Guardian. “But when you are physically active at work, it’s a very different type of activity. You are working for eight hours a day and have limited rest periods. You are lifting, doing repetitive movements, and manual handling.
“Our hypothesis,” he added, “is that these kinds of activities actually strain your cardiovascular system rather than help you to improve the fitness of your cardiovascular system.”
Plenty of caveats
The meta-analysis comes with several limitations. Most notably, the studies on which the analysis is based were observational. They, therefore, cannot prove a direct causal relationship between physically demanding jobs and early death. Other factors, not addressed in the studies (or, at least, not in all of them), might explain the correlation — a greater exposure to dangerous work conditions, for example.
Also, the people in the studies self-described the physical demands of their jobs. Such self-reports can be unreliable.
Still, the findings are provocative. They suggest that physically demanding jobs may need to be restructured to allow for the body to recover. They also suggest that people with such jobs may need to make sure they remain physically active during their non-working hours.
As Coenen told CBS News: “This may help these workers to balance the negative health effects of occupational physical activity with the positive effects of leisure-time physical activity.”
FMI:You can read the study in full on the British Journal of Sports Medicine’s website.