Physical aerobic strain on the job is bad for your heart. New findings published by Dr. Niklas Krause of the University of California, San Francisco, and a group of international collaborators indicate the processes that lead to atherosclerosis—a major underlying cause of heart disease and death—are linked to physical activity at work.
The study involved 612 Finnish men aged 42 to 60 years at baseline. The participants were followed for 11 years with repeated measurements of energy expenditure at work and ultrasound measurements of the carotid arteries’ intima media thickness (IMT), as a marker of atherosclerotic progression. Carotid IMT is a measure of the thickening, and therefore narrowing, of the main arteries that bring blood to the brain. Relative aerobic strain was measured as energy expenditure at work relative to the aerobic capacity of each participant.
Atherosclerosis progressed much faster in all men with high energy expenditure at work; furthermore, men with pre-existing cardiovascular disease were especially vulnerable. Krause and colleagues reported that for men who worked at maximum levels of relative aerobic strain, IMT increased by 90% in subjects who had ischemic heart disease (IHD) compared to a 46% increase in those without IHD. The increase was only 29% among healthy men working at minimum levels of aerobic strain.
Thirty percent of the men studied were exposed to excessive relative aerobic strain based on current recommendations of work physiologists, but in certain sectors the percentage was even higher. Krause said he was “surprised to see that for such a large proportion of the aging work force, the job demands don’t really change, although their cardio-respiratory fitness goes down each year by 1% or 2%. They should actually have less demanding work when they get older, but that doesn’t seem to be the case. Energy expenditure of those who are still working was on average only 130 kilocalories less than when they were 11 years younger.”
The study participants exercised on average 120 hours per year, or 20 minutes of leisure time physical activity a day. Krause notes that, “although this population clearly has the benefits of relatively high levels of leisure time physical activity, this was not enough to overcome the detrimental effects of aerobic strain at work.”
Krause emphasized the biologic plausibility of these findings. “Whenever your heart rate increases, the flow characteristics in the arteries change, and they change in a way that is not beneficial for the arteries. The arterial system is very well suited for an occasional increase in the heart rate for a high bout of work, but if the heart rate is on average higher than resting heart rate, then the arteries are exposed to less optimal blood flow for longer periods of time than if you have a lower heart rate. In addition, physically demanding work may increase blood pressure, another known risk factor for arteriosclerosis.”
The strength of this study is its longitudinal design with precise markers of preclinical atherosclerosis, and the ability to control for “virtually all known cardiovascular risk factors,” according to Krause. The evidence on the progression of this underlying cause of heart disease could have important ramifications for worker safety and population health.
In their paper Krause and his colleagues recommend that, “Regulatory statutes dealing with work time and rest schedules need to assure that workers are protected from excessive aerobic strain even if individual monitoring is not available. Such prevention measures are especially needed for older workers with age- or disease-related reduced cardiorespiratory fitness, existing IHD, or known atherosclerosis.”
The paper titled, “Occupational Physical Activity, Energy Expenditure, and 11-year Progression of Carotid Atherosclerosis” will appear in the upcoming issue of the respected Scandinavian Journal of Work Environment and Health. The online-first version can be downloaded directly from the journal’s website together with an editorial by Dr. Kukkonen-Harjula entitled: “Physical activity and cardiovascular health – work and leisure differ” and a companion article by Dr. Virkkunen et al. showing that high physical workload leads to elevated blood pressure.