SAN ANTONIO, March 2, 2001 -- A new study of utility workers finds that work stress can nullify the cardiovascular benefits of a physically demanding job, according to researchers reporting today at the American Heart Association's 41st Annual Conference on Cardiovascular Disease Epidemiology and Prevention.

"If you think being physically active at work is helping your heart, think again if you also have workplace stress," says James H. Dwyer, Ph.D., a professor at the Keck School of Medicine at the University of Southern California, Los Angeles.

Researchers followed 447 utility company workers for three years in the late 1990s. This was immediately after deregulation of the industry resulted in increased competition among utility companies. The workers were between ages 40 and 60 and held jobs such as managers, meter readers or administrative assistants.

Researchers gathered information on the amount of physical activity employees got at work as well as how frequently they worked up a sweat in their free time. They used non-invasive ultrasound tests to measure the thickness of the workers' carotid arteries at the start of the study and again three years later. The thickening of the walls of the carotid arteries - which are located in the neck and carry blood to the brain - is thought to strongly correspond to the build up of plaque in the larger arteries, including those in the heart.

Carotid arteries of workers in the top 20 percent for on-the-job activity increased in thickness by about 13 micrometers per year. Those who reported the least exercise on the job showed carotid thickening of about 7 micrometers per year. Those with the most physical activity at work also reported the most job stress.

The situation was the opposite for leisure time exercise. Subjects who exercised at least four times a week during their time off showed carotid artery thickening of 6.5 micrometers each year, while individuals with the least active leisure time had increases in carotid thickness of about 11 micrometers per year. Dwyer says the leisure time results indicate a protective effect from exercise.

"We were surprised to find that the physical activity level during work was related to carotid thickening rather than being neutral or protective," Dwyer says. "The independent effects of physical activity and psychological stress at work are difficult to separate. However, this study suggests that the protective effects of physical activity may be blocked or counteracted when activity is performed in a psychologically stressful context."

Dwyer says he does not consider the workers' stress unique.

"It was not so much the danger of the job as it was the demands of the job, the uncertainties, the difficulties working with other people. These are the kinds of stresses that develop in any workplace. They are not particular to utility workers," he says.

"It's going to be difficult to sort out the effects of stress and physical activity and how they interact."

Stress is difficult to measure because it is dependent on individual perception. Some workers liked the increased competition that deregulation caused while others felt threatened, he says. So rather than focus on specific stresses, the researchers asked subjects whether their job stress affected other aspects of their life, such as disrupting sleep.

The leisure time exercise results seem more clear-cut, Dwyer adds. Based on these findings, it appears that if the goal is prevention of cardiovascular disease, then people in their 40s and 50s should exercise four or more times per week with enough intensity to produce heavy breathing and sweating.

Co-authors include lead author Cheryl K. Nordstrom, M.P.H.; Kathleen M. Dwyer, Ph.D.; C. N. Bairey Merz, M.D.; and Anne M. Shircore, M.S.

---American Heart Association

Back to The Science of Mental Health