Runners beware, if you're speeding past the 20 mile-a-week mark, you may be doing your body more harm than good.

These findings were released in a new study, but some runners say the benefits outweigh the risks.

The Cardiovascular Research Institute study looked at 4,800 people who ran more than 20 miles a week, or faster than eight minute miles.

A doctor for Lehigh Valley Health Network was among those looking at the numbers.

"We were interested to see if that particular group of runners we're doing something different in their daily life," said co-director, Dr. Martin Matsumura.

The study found that people getting no exercise and people running over 20 miles a week tend to have shorter lifespans than moderate runners, but why isn't clear.

"In general found absolutely no reason in terms of backgrounds, medicines, compliance of medicines, or risk factors that would explain why," added Matsumura.

Runner's World's Chief Running Officer, Bart Yasso, has developed a marathon training regime and he's run thousands of miles training.

"I would have to see more numbers and a study done on a longer period of time than just a few marathoners," added Yasso.

Some runners don't believe high mileage on the trails will lead to health problems.

They say they run because it's a way of life, as long as they are doing it in moderation.

"I don't know about over 20 miles a week," said Yasso. "I believe in moderation for everything. So I believe in what I would call crazy mileage, people running insane distances all year long and for many years. I could see some harm there."

Some new to running say the study won't sway them either.

"Running is a very competitive sport," said Tera Bouchard, who runs around 20 miles a week. "First of all, you're competing with yourself and it's natural to want to go faster and go farther."

"We have a lot of other analysis we're doing of this four thousand runner database," said Matsumura. "We're going to do some follow-up studies."

Further testing may reveal the answer and possibly give people a reason to slow down.