Q: I have been jogging regularly for several years; but a friend my age just had a heart attack while running, and I now wonder whether it is safe for me to continue my jogging routine.
A: Two reports in the December issue of the New England Journal of Medicine, one from this country and one from Germany, shed considerable light on your question.
The authors found some bad news: The risk of a heart attack during or in the hour after strenuous exertion, such as jogging or shoveling snow, was two times greater in the German study and six times greater in the American study than during periods of rest or less strenuous activity.
The good news for joggers is that the heart attacks occurring during exercise were almost entirely limited to those individuals who did not exercise regularly.
They also found that most heart attacks took place when people were sedentary or only moderately active -- about 1 in 20 appeared to be triggered by exertion.
The increases in heart rate and blood pressure that accompany exercise may provoke a heart attack by disrupting an atherosclerotic plaque and thus promote a blood clot that blocks a coronary artery.
Individuals whose regular exercise keeps them physically fit may be relatively protected from exercise-induced heart attacks because they have smaller rises in their heart rate and blood pressure during exercise.
These studies provide further evidence for the benefits of physical fitness but also reinforce the warning that sedentary people must be careful about undertaking activities that are unusually strenuous. Other studies have shown that regular exercise can slow or prevent the development and progression of coronary artery disease.
Dr. Margolis is professor of medicine and biological chemistry at the Johns Hopkins School of Medicine.