In an article in the July issue of the American Journal of Medicine, the research team discussed the latest findings from the Los Angeles Atherosclerosis Study, in which investigators followed the physical activity of 500 middle-aged employees while monitoring progression of their atherosclerosis.
Atherosclerosis, the gradual build-up of fatty material along the inner lining of artery walls, can eventually result in life-threatening heart attacks and stroke. The disease causes one of every two deaths in the United States.
They found that while arteries thickened more slowly in those who exercised more during their leisure time, arteries actually thickened faster among those who were very physically active during the workday.
But when the researchers measured psychological stress related to work (asking how much sleep participants lost over worries about work, for example) they found an interesting connection.
"We found that atherosclerosis progressed significantly faster in people with greater stress, and people who were under more stress also were the ones who exercised more in their jobs," says James Dwyer, Ph.D., professor of preventive medicine at the Keck School and the study's principal investigator.
When investigators grouped participants according to the amount of stress at their workplace, they no longer saw an adverse relationship between physical activity in the workplace and atherosclerosis progression.
"This suggests that the apparent harmful effect of physical activity at work on atherosclerosis-and heart disease risk-may be due to the tendency of high-activity jobs to be more stressful in modern workplaces," Dwyer says.
"It appears from our findings that the psychological stresses associated with physically active jobs overcomes any biological benefit of the activity itself. It's also possible that physical activity in the context of a body activated by psychological stress does not have the same benefits as exercise conducted in the reduced stress usually associated with leisure time."
The study found that leisure-time exercise benefits were considerable.
Researchers asked study participants how often they exercised vigorously (jogging, biking or performing other aerobic activities long enough to speed up heart beat and work up a sweat).
They found that atherosclerosis progression slowed dramatically as the level of physical activity during leisure increased. This effect on atherosclerosis was stronger than that for most other risk factors, such as smoking and LDL cholesterol levels.
The team also found that moderate exercisers, such as regular walkers, had slower atherosclerosis progression than those who were couch potatoes. And those who exercised vigorously three or more times a week benefited the most. Relative to vigorous exercises, arteries thickened at twice the rate in moderate exercisers and at three times greater rate in the couch potatoes.
"These results are important because they demonstrate the very substantial and almost immediate-within one or two years-cardiovascular benefit of greater physical activity," Dwyer says. "These benefits are seen in moderately active persons relative to sedentary persons, and they are seen for regular aerobic activity relative to moderate activity."
Researchers measured atherosclerosis progression by using ultrasound to measure the thickness of the main arteries in the neck, called carotids. They took measurements at 18 months and three years after the study began.
Study subjects were selected from healthy employees of a utility company ages 40 to 60 years during 1995 to 1996.
The study, which was supported by the National Institutes of Health, differs from other recent studies on workplace and leisure-time exercise because it measured progression of atherosclerosis during its development and monitored exercise at the same time; other recent studies relied on memories of activity in the past, after heart attacks had occurred. This study also looked at stress as a possible link between workplace exercise and atherosclerosis.
Notes Dwyer: "The results of our epidemiologic study show that randomized trials of the effects of physical activity on the development of cardiovascular disease are now feasible. We now have the technology to unambiguously answer many of the difficult questions about the form of physical activity that will maximize cardiovascular health in studies of one or two years duration."
Cheryl K. Nordstrom, Kathleen M. Dwyer, C. Noel Bairey Merz, Anne Shircore and James H. Dwyer, "Leisure Time Physical Activity and Early Atherosclerosis: The Los Angeles Atherosclerosis Study," American Journal of Medicine. Vol. 115, No. 1, pp. 19-25.