Is running marathons bad for your health?

Last Updated Mar 13, 2015 5:30 PM EDT

With long, lean bodies and serious cardiovascular endurance, marathon runners are seen as the pinnacle of health. But is running marathons really good for you?

Health benefits of moderate running include weight control, stress reduction, better blood pressure and lower cholesterol. A study published in the British medical journal Heart, however, showed that people who work out too hard for too long may actually erase some of the healthy benefits of moderate exercise.

Their research showed that high-intensity exercise sessions lasting longer than one to two hours can overload the heart. After years of excessive exercise, thickening of the heart tissue may develop. In some cases, this scar tissue can create the possibility of dangerous irregular heartbeat and even sudden death.

Not only can too much long distance running damage the heart, but marathon runners also have to endure challenging weather conditions that may contribute to heat stroke, dehydration and exhaustion. At this weekend's L.A. Marathon, runners may encounter record-breaking heat -- with temperatures nearing 90, about 20 degrees above normal.

All that said, however, the risk of dropping dead during a marathon is minimal -- about 0.5 to 1 in 100,000, writes the author of the Heart study, James H. O'Keefe M.D. And, if you have just decided to run your first marathon, the benefits of training for one may outweigh the risks.

A study published in the Journal of the American College of Cardiology followed 55,000 people, aged 18 to 100, over 15 years, and found that runners lived an average of three years longer than people who didn't run at all.

With the current American lifestyle centering around commuting to work in a car and sitting in an office most of the day, as many as 50 million Americans are failing to get any daily physical activity. Sedentary lifestyles are linked to increased risk of health problems and even early death, according to the American Psychological Association.

Running or even just walking a modest amount a few times each week was still associated with solid health benefits.

There does seem to be a point of diminishing returns with exercise. The health benefits seem to drop among people who run more than 20 miles a week, more than six days a week, or faster than eight miles an hour, according to an article published in the journal Medicine & Science in Sports & Exercise.

Many marathoners, however, are not running 26.2 miles simply to meet physical fitness guidelines.

Cancer survivor Ethan Zohn has competed in four marathons and one ultra-marathon. He says he runs to be a part of something bigger than himself.

"Running long distances is a little bit like my form of meditation. It was a way to get myself back to health after cancer. I started by walking half a mile, then moved on to jog half a mile, then run a few miles. Meeting daily goals helped me build my strength back."

Zohn also runs on behalf of GrassRootsSoccer, a charity he co-founded.

"Running for something bigger than myself, for a charity, gives me a sense of purpose."

Studies have shown that people who volunteer to help others are happier, have greater self-esteem, less depression and lower stress levels than those who don't.

Marathons are massive events that attract millions of people throughout the world. Running aside, these participants want to be part of a community of others who are challenging themselves, overcoming limitations, and supporting charitable causes.

In 2011, the New York City Marathon raised $34 million for 190 non-profits organizations, and as of Friday, the L.A. Marathon has raised over $2 million in charity donations.

Bottom line: If you've got the time and the drive, one marathon probably won't kill you. But you may want to stop at just one.

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    Parvati Shallow covers health and wellness for CBSNews.com