Aug 16, 2014 04:36 AM EDT By Staff Reporter, UniversityHerald Reporter

American researchers have found evidence of an increase in cardiovascular deaths among heart attack survivors, who exercise too much.

This is a startling discovery as the importance of physical activity like brisk walking and jogging is always highlighted in the management and rehabilitation of cardiovascular disease. Exercise is also highly recommended to lower the risk of death from other diseases like hypertension, stroke, and type-2 diabetes.

For the study, the researchers examined the relationship between physical activity and deaths from cardiovascular diseases in about 2,400 physically active heart attack survivors.

They found that death rate from cardiovascular events was lowered by 65 percent among patients who were running less than 30 miles or walking less than 46 miles per week. However, the benefits of exercise were lost beyond this point, resembling a inverted J-curve pattern.

"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," Paul T. Williams of the Life Sciences Division at the Lawrence Berkeley National Laboratory and Paul D. Thompson from the Department of Cardiology at the Hartford Hospital, said in a press release.

"Results suggest that the benefits of running or walking do not accrue indefinitely and that above some level, perhaps 30 miles per week of running, there is a significant increase in risk. Competitive running events also appear to increase the risk of an acute event."

The researchers said that there is one drawback to the study. The participants comprised of only heart attack survivors and therefore cannot be generalized.

On the other hand, Spanish researchers conducted a meta-analysis of 10 cohort studies to determine mortality in elite athletes. The studies consisted of over 42,000 athletes (707 women) who had participated in a wide range of sports including football, baseball, track and field, cycling, Olympics and Tour de France.

"What we found on the evidence available was that elite athletes (mostly men) 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," said senior investigator Alejandro Lucia of the European University Madrid, Spain. "More research is needed however, using more homogeneous cohorts and a more proportional representation of both sexes."

The finding is published in Mayo Clinic Proceedings.

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