(CBS) - Heart attack survivors are encouraged to exercise regularly to improve their cardiac health, but new research suggests there's a point of diminishing returns.
"More isn't always better," said study researcher Paul Williams, staff scientist at the Lawrence Berkeley National Laboratory in Berkeley, Calif.
Williams tracked nearly 2,400 heart attack survivors from his long-term study of runners and walkers for about 10 years. In general, increased exercise reduced their risk of dying from a heart attack by up to 65 percent, he said.
But running more than 30 miles a week or walking beyond 46 miles weekly had the opposite effect, more than doubling heart attack risk, the study found.
Over the decade-long study, 526 people died, nearly three-quarters because of heart attacks and heart disease.
Because the study was limited to heart attack survivors, Williams can't say if the findings would apply to healthy adults who exercise intensively.
The heart attack survivors who exercised excessively were in the minority, with only 6 percent surpassing 30 miles of running or 46 miles walking a week, the study found.
For the majority of participants, increasing exercise but remaining within moderate limits significantly reduced the risk of heart-related deaths, Williams found.
The results aren't surprising, experts said.
The study shows "you don't have to do a lot of exercise to get a lot of benefit" from a health standpoint, said Dr. Carl Lavie, medical director of cardiac rehabilitation and preventive cardiology at the John Ochsner Heart and Vascular Institute, New Orleans.
According to the Physical Activity Guidelines for Americans, 150 minutes a week of moderate intensity exercise or 75 minutes of vigorous intensity exercise is advised.
"Exceeding the recommendation is better than meeting the recommendation" for reducing risk of heart disease death in heart attack survivors, Williams said, but only to the points he found.
He isn't sure why those who exercised at the highest levels had an increased risk of heart attack death. Nor can he say if the results would apply to activities other than walking or running.
The study did find that the heart benefits of walking compared to running were equivalent, as long as the energy output was the same -- walking will take about twice as long as running to burn the same number of calories.
The study is published online Aug. 12 in Mayo Clinic Proceedings.
In the same issue, Spanish researchers reviewed the findings of 10 published studies, looking at the effect of elite athletes' training on longevity.
That study included more than 42,000 athletes, mostly men, who had participated in football, baseball, track and field, and cycling. Elite athletes lived longer than the general population, the study found. This suggests that health effects of exercise, especially for reducing heart disease and cancer risk, are not always confined to moderate doses.
That finding isn't necessarily at odds with the other study, said Lavie, co-author of an editorial accompanying the studies.
"It's at least comforting to know that previously athletic, high-competition, high-level athletic activity does not seem to be associated with worsening of survival," but actually with benefits of survival, he said.
Also, the exercise done by the athletes was often not at the extreme level of marathoners, he said.
This is right in line with what heart doctors have recommended all along, said Dr. James O'Keefe, another editorial co-author and a cardiologist at St. Luke's Mid-America Heart Institute in Kansas City, Mo. "Exercise is the best thing you can do for your health," he said, if done in moderation.
"It's not good to be sedentary, but you can overdo it," he said.
Half of the U.S. population gets too little exercise, O'Keefe estimated, and "maybe one in 20 is getting too much."
According to O'Keefe, if you are exercising mostly for health (not fitness) benefits, 2.5 to 5 hours of vigorous exercise a week is plenty. He and his colleagues also noted that taking one or two days off from high-intensity exercise each week might pay off as well.
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