The first study, by researchers from the University of South Carolina and the Ochsner Health System, in New Orleans, was presented Saturday at the American College of Sports Medicine meeting, in San Francisco. It analyzed the link between running and cardiovascular-related deaths in nearly 53,000 adults. Research presented at medical meetings should be viewed as preliminary until published in a peer-reviewed medical journal.
The participants were between the ages of 20 and 100 and had undergone a medical exam between 1971 and 2003. All were free of heart disease, cancer and diabetes at the start of the study.
The researchers gleaned information from questionnaires in which the participants reported their leisure-time activities, including their running habits; about 27 percent said they ran.
The scientists used data from the National Death Index and found that the runners had about a 20 percent lower mortality rate than the non-runners, said lead researcher Dr. Chip Lavie, medical director of cardiac rehabilitation and prevention at John Ochsner Heart and Vascular Institute, in New Orleans.
Running lowered the risk for mortality when a person did not exceed more than 20 miles a week, log more than five to seven miles per hour, or run more than two to five times a week, the authors reported.
"Although higher doses [of running] are not associated with worse outcomes when compared with non-runners, those with higher doses of distance, frequency and speed seemed to lose the survival advantage gained at lower doses of running," Lavie noted.
Another study, published in the June issue of the Mayo Clinic Proceedings, reviewed the scientific literature on the effect of extreme endurance training — such as that performed by marathoners, triathletes, professional cyclists and ultra-marathon runners — and found it can lead to long-term heart damage.
"There's probably nothing better a person can do for themselves for their long-term health than daily exercise," said Dr. James O'Keefe, lead author of the second study and a professor of medicine at the University of Missouri and Saint Luke's Hospital in Kansas City, Mo. "But if you train more than the cardiovascular system is designed to handle, you can tax your heart and do damage."
He pointed out that the certain cardiovascular biomarkers become elevated during extreme training in some athletes.
"Even though they go back to normal within a week, over months and years, the elevations may lead to heart damage and increased susceptibility to certain types of arrhythmias," O'Keefe said.
He also noted that in one recent study, 12 percent of apparently healthy marathon runners showed "patchy myocardial scarring."
Ultra-marathon running and professional cycling have also been linked with as much as a fivefold hike in atrial fibrillation, an irregular heart rhythm, the researchers reported. There is also some suggestion that long-term excessive exercise may be linked with coronary artery calcification and large-artery wall stiffening, but more research is warranted, they added.
To help paint a clearer picture of how exercise affects the heart, O'Keefe explained, "You and I pump about five quarts of blood a minute around our bodies. If we exercise hard, it might go up to 25 quarts per minute. You're demanding a lot of your heart. While the heart is an extremely resilient organ and you can train it up and it can do amazing things, we believe that over the long haul, it's healthier for people to do more moderate exercise."
"Beyond 30 to 60 minutes a day, you reach a point of diminishing returns," he said.
If you crave high-intensity exercise, he said, try to stick to interval training — smaller bursts at a time, instead of prolonged intensity.
Marathon runner and cardiologist Dr. James Eichelberger, an associate professor of medicine in the cardiology division at the University of Rochester School of Medicine and Dentistry, in New York, said while it is true that athletes who train extremely can develop mild changes in heart chamber volumes and even small leakage of cardiac biomarkers into the bloodstream, "most of these changes normalize soon after cessation of extreme exercise. Long-lasting negative effects are not clear, and most likely do not negate the well-known long-term benefits."
His advice: "Be consistent and avoid extremes if you are exercising solely for health." In other words, "Don't be a weekend warrior."
When it comes to suggesting weekly mileage guidelines, Eichelberger hesitated. "I'm not sure there is really a correct distance that applies to everybody. The idea that running less than 20 miles a week is good, and running more than 20 is bad — I'd view that with a lot of skepticism. It probably truly varies depending on a lot of different factors — patient age, whether they're in shape or not and other comorbidities, among other factors," he said.
He also noted that the heart's not the only part of the body that's affected by extreme endurance training; it takes a toll on the musculoskeletal system as well.
Lavie, an author on the second study as well as the first, said the endurance athlete study shows that prolonged, intense exercise has its risks.
"We know people are going to want to run marathons and triathlons, and doing this occasionally is probably okay," Lavie said. "But from a health standpoint, it would be best not to become a very frequent long-endurance exerciser."