A new study from Denmark shows that while leisure-time physical activity is associated with reduced heart attack risk, occupational physical activity is associated with increased risk (European Heart Journal, April 14, 2021;42(15):1512–1515). The Copenhagen General Population Study followed 104,046 participants for 10 years and found that compared to low levels of leisure-time physical activity, moderate, high and very high volumes of leisure-time physical activity reduced risk for heart attacks and death significantly, but the opposite was true for physical activity on the job. Participants with leisure-time physical activity had a 15 percent reduced risk for heart attacks, while those with heavy physical activity at work had a 35 percent increased risk.
Differences Between Leisure-Time and Work-Time Physical Activity
People who have physically demanding jobs often believe that they do not need to exercise because their work keeps them moving much of the time. However, there are differences between work and leisure activity:
• Recreational exercise is usually more intense and sustained for aerobic endurance, whereas work activity is usually repetitive motions of short duration at low intensity, such as standing for hours, walking up and down steps, moving arms to load boxes and so forth.
• Leisure time exercise improves cardiorespiratory and metabolic fitness and health, while physical activity on the job is associated with fatigue, insufficient recovery, elevated blood pressure and elevated heart rates.
• Heavy manual job workers are more likely to be stressed by noise, air pollution and other poor working conditions, and they may be too tired from their jobs to exercise after work.
Stable Plaques: A Possible Explanation
The authors of the Copenhagen study did not pursue reasons why there would be higher rates of heart attacks among people with high physical activity at work, but here is my opinion. Heart attacks are not caused by plaque buildup on the inner lining of arteries; they are caused by a sudden complete obstruction of the blood flowing to part of the heart muscle. The sudden complete obstruction of blood flow to a part of the heart muscle is usually caused by a plaque breaking off, followed by bleeding where the plaque broke off. Then clots form at the bleeding site, and the clot extends to block completely the flow of blood to the part of the heart muscle supplied by that artery. The part of the heart muscle deprived completely of blood flow then dies, to cause a heart attack.
Older endurance athletes may have more plaques in their arteries than non-exercisers, but they often have the type of plaques that are far less likely to break off to cause heart attacks (Circulation, April 27, 2017;136:138-148; May 2, 2017;136:126-137). They have:
• low 10-year-history risk scores for suffering heart attacks (Framingham study data),
• greater plaque thickness,
• more calcium in their plaques,
• wider and more dilated arteries that are far less likely to be blocked, and
• more stable plaques that are far less likely to break off to cause heart attacks.
A study of 21,758 men, average age 51.7 years, followed for an average 10.4 years, showed that men who exercised the most can have more plaques in their arteries, but did not suffer more heart attacks or deaths than those with less heart artery calcification (JAMA Cardiol, Jan 30, 2019). The least active men with excessive arterial plaques were twice as likely to die of heart disease. Compared to low-level exercisers, the heavy exercisers without increased plaques had half the risk of dying during the study period. See Stable Plaques: Why Exercisers Have Fewer Heart Attacks
How to Measure Plaque Stability
Your doctor can help to predict your risk for a heart attack with a test called a Calcium Score, which is a special CT X ray or sound wave test that measures the thickness of plaques on the inner lining of arteries. A CT scan can also show how stable a plaque is. It can show the difference between stable plaques that are usually safe and those that are unstable and more likely to break off to cause heart attacks (American Journal of Roentgenology, March, 2015;204(3):W249-W260). Signs of plaque stability include extensive calcification, less lipid-rich areas, increased fibrous areas and structural changes.
We do not know why leisure-time exercise appears to give you stronger protection against suffering a heart attack than having a job that requires you to be physically active. It is my opinion that the recreational activity is usually more intense and sustained than repetitive-motion movements at work and therefore may be more likely to stabilize plaques so they do not break off to cause a heart attack. The Copenhagen study suggests that even if you have a job that involves a lot of physical effort, you should still have a program of recreational exercise that gives you the benefits of aerobic activity.