Since time immemorial, running for exercise has been associated with a healthy lifestyle that includes control over weight, reduction of stress and better management of blood pressure and cholesterol.

But, can one ever end up doing too much running? Well, yes, says new research conducted by the Cardiovascular Research Institute at the Lehigh Valley Health Network in Allentown, Pa., and presented this last weekend at the American College of Cardiology's annual meeting in Washington, D.C.

Martin Matsumura, co-director of the institute, explained in a report by HealthDay that several prior studies have suggested "moderate" running -- defined by one expert as a total of two to three hours per week -- appears ideal for longevity.

The newest study asserts high-mileage runners, like those who get no exercise at all, tend to have shorter lifespans than do moderate runners, although the reasons for that are not yet known.

"Our study didn't find any differences that could explain these longevity differences," said Matsumura in the HealthDay report, although the data appeared to negate cardiac risk or the use of certain medicines as factors.

Matsumura said he and his colleagues collected data from more than 3,800 men and women runners, with an average age of 46. The participants were recruited online via the Masters Running Study, a web-based study of training and health information on runners aged 35 and above.

Nearly 70 percent of the study subjects said thay typically ran more than 20 miles a week.

The runners explained their use of the common painkillers called NSAIDs -- nonsteroidal anti-inflammatory medications such as ibuprofen and naproxen/Aleve) which have been linked to heart problems -- as well as aspirin, considered to be heart-protective.

The runners were also asked about known heart risk factors such as high blood pressure, high cholesterol, diabetes, family history of heart disease and smoking.

None of these factors, Matsumura noted, explained the shorter lives of high-mileage runners; as it turned out, the use of NSAIDs was more common in runners who ran less than 20 miles weekly.

All that considered, "What we still don't understand is defining the optimal dose of running for health and longevity," said Matsumura. "I certainly don't tell patients 'Don't run,' " But, he added, runners should be advised to stay informed about new research, especially as more becomes understood about the relationship between running mileage and one's lifespan.

James O'Keefe, director of preventive cardiology at the Mid-American Heart Institute in Kansas City, told HeathDay the study may have simply underscored the fact there's just "too much wear and tear" on the bodies of high-mileage runners.

He said his own research has led him to speculate that extreme exercise may cause a "remodeling" of the heart that could take away some of the benefits of more moderate activity.

O'Keefe's says to reap the health rewards of jogging, the "sweet spot" runners should aim for is a slow to moderate pace, only two or three times a week, for a total of one to 2.5 hours.

"If you want to run a marathon," O'Keefe said in the HealthDay report, "run one and cross it off your bucket list."