The study found “clear evidence” of an increase in deaths among heart attack survivors who exercise to excess.
Researchers studied the relationship between exercise and cardiovascular disease-related deaths in about 2,400 physically active heart attack survivors.
Their study confirmed previous reports indicating that the cardiovascular benefits for walking and running were equivalent, as long as the energy expenditures were the same.
Reductions in deaths from cardiovascular “events” of up to 65 per cent were seen among patients who were running fewer than 30 miles or walking fewer than 46 miles per week.
However, beyond that point much of the benefit of exercise was lost.
Study co-author Dr Paul Williams, of the Lawrence Berkeley national laboratory in the, said: “These analyses provide what is to our knowledge the first data in humans demonstrating a statistically significant increase in cardiovascular risk with the highest levels of exercise.
“Results suggest that the benefits of running or walking do not accrue indefinitely and that above some level there is a significant increase in risk.
“Competitive running events also appear to increase the risk of an acute event.”
However, he added: “Our study population consisted of heart attack survivors and so the findings cannot be readily generalised to the entire population of heavy exercisers.”
Commenting on the findings, Dr James O’Keefe, of the Mid America Heart Institute, said: “Extrapolation of the data would suggest that approximately one out of 20 people is overdoing exercise.”
However, he said around half of people are not getting the minimum recommended amount of physical activity.
Analysts said that a weekly cumulative dose of vigorous exercise of not more than about five hours has been identified in several studies to be the safe upper range for long-term cardiovascular health and life expectancy.
They propose that people from either end of the exercise spectrum – sedentary people and over-exercisers – would probably reap long-term health benefits from changing their physical activity levels to be in the moderate range.
Meanwhile, in the same issue of the journal Mayo Clinic Proceedings, investigators in Spain reported on a major analysis of ten studies aimed at providing an accurate overview of mortality in elite athletes.
The studies included more than 42,000 top athletes.
Senior investigator Dr Alejandro Lucia, of the European University Madrid, said: “We found that elite athletes live longer than the general population, which suggests that the beneficial health effects of exercise, particularly in decreasing cardiovascular disease and cancer risk, are not necessarily confined to moderate doses.”