Though built to run, the human body may reach a maximum health benefit at 30 miles per week of such endurance exercise, after which the costs weigh heavily.

An increasing amount of physiology research supports the possibility of "exercise overdose," in which high doses of exercise have fewer health benefits than moderate bouts of running, researchers say. Recent research shows that the extra six years of longevity gained by running regularly may disappear beyond the 30-mile per week mark. Other improvements to blood pressure, cholesterol levels, and cardiac health are offset at least partially by susceptibility to atrial fibrillation and plaque formation in the coronary artery, studies show.

"The lesson I've learned from 40 years of cardiology is that when there's this much smoke, there's often some fire," said Dr. Paul Thompson, a sports medicine specialist and chief of cardiology at Hartford Hospital in Connecticut.

Like Thompson, some in sports medicine are alarmed by the growing body of anecdotal evidence in recent years. One high-profile example is Normann Stadler, two-time winner of the Ironman Triathalon World Championship, who needed emergency surgery for an aortic aneurysm, a condition that may have been aggravated by his endurance athleticism. New research shows a connection between endurance athletics and enlarged aortic roots, which in Stadler's case was described as "enormous."

Other studies show elevated levels of coronary plaque in veteran marathoners, which could theoretically be caused by intensive exercise, researchers say. Dr. John Mandrola, a cardiac electrophysiologist at Baptist Hospital East in Louisville, Ky., believes the link may be important. "Heart disease comes from inflammation and if you're constantly, chronically inflaming yourself, never letting your body heal, why wouldn't there be a relationship between over-exercise and heart disease?"

However, other health studies contradict the idea that endurance exercise causes health hazards, suggesting the benefits themselves are unlimited. The only limitation is the number of miles one man or woman is capable of running in a week.

Benjamin Levine, director of the Institute for Exercise and Environmental Medicine in Dallas, falls into this camp of theorists. "It's true that the majority of cardiovascular protection comes from exercise at more moderate levels, but there is compelling evidence that there's no upper limit."

Mandrola, who is a marathoner and avid cyclist, qualifies his opinions on the link between endurance exercise and health problems by recommending — as with so many other human endeavors — moderation. "I don't want anyone to read that exercise can be bad for you," he said. "Some folks do tons of exercise and are protected. Some folks probably have some individual susceptibility to it. I'm a big believer in short intervals of high intensity."

And it may be that combination of bouts of high-intensity endurance exercise with periods of rest that is what makes a human animal. Paleoanthropologists during the past decade have argued that humans evolved as long-distance runners, lacking the sprinting speed of their predators, but capable of persisting beyond any other swift animal on the planet, including dogs, wolves, hyenas, and antelope. With abundant sweat glands to cool the body and an architecture optimized with the Achilles tendon, big knee joints, and a muscular glutei maximi, the human is endowed with capability to move beyond mere walking or short bursts of speed.

Human anatomy, paleoanthropologists say, suggests that long-distance running once ensured the survival of the human species — and, more broadly, hominids millions of years ago — on the African savanna.