Those who stay abreast of the latest health news are well accustomed to the ever-expanding list of health risks—high-fructose corn syrup, carbs, sitting, etc. But when it comes to exercise, most of the public-health focus is dedicated to identifying the risk factors that are associated with too little exercise, not too much. That’s simply because most Americans don’t meet the recommended weekly dose of 150 minutes of cardiovascular exercise. So the idea that you can exercise too much, while not completely novel, has traditionally received far less attention in the popular media and scientific literature.

Regular physical exercise has been associated with decreased risk of chronic disease, improved cardiovascular health, and longer life. But while science has a fairly good handle on the minimum level of exercise needed to promote good health, far less is known about a potential upper limit on the value of physical activity, past which endurance exercise can become harmful.

Just as hitting the gym for bicep curls and bench presses strengthens and enlarges the muscles of the arms and chest, regular endurance exercise can cause the muscles of the heart to grow. The development of these changes, clinically referred to as “athlete’s heart,” requires years of intensive exercise and is characterized by thickening of the heart’s muscular chambers responsible for pumping blood out to working muscles. Normally benign, these changes are not associated with increased risk of cardiac events or death.

“In recent years, some cardiologists and researchers have tried to make the case of comparing exercise to a drug, saying that a moderate amount of exercise is very good, but too little or too much is ‘toxic,’” says Dr. Michel Accad, a cardiologist and director of Athletic Heart, San Francisco’s first clinic dedicated to the athlete’s heart. “While that isn’t supported by the research, there is certainly evidence that some cardiovascular complications occur more commonly in relation to habitual endurance effort in a subset of people.”

The controversy over heart safety and prolonged or intensive exercise is often re-ignited when tragedy befalls a sporting event, like marathoner Ryan Shay’s death during the U.S. Olympic Marathon Trials. In fact, Accad says, “Exercise may not be the cause of the underlying heart problem, but it may be the trigger.”

Endurance exercise may set off an existing cardiac condition, but can it cause one? The results of a 2011 study in the Journal of Applied Physiology suggested that years of intense endurance training was associated with fibrosis, or scarring, of the heart muscle. The male subjects of the study, former members of the British national and Olympic teams in distance running and rowing, had all exercised vigorously throughout their adult lives.

The prevalence of heart scarring in veteran athletes was significantly associated with the number of years spent training, number of competitive marathons and ultra-endurance events completed. “This data suggests a link between lifelong endurance exercise and myocardial fibrosis that requires further investigation,” the authors of the study state. While they could not definitively establish cause and effect between long hours of endurance exercise and heart damage, the study offered sobering evidence that long-term endurance exercise could potentially have adverse effects.

This exercise-mediated heart stress may start a chain of events going from scarring to arrhythmias—asynchronous heartbeats. One study documents a five-fold increase in one kind of arrhythmia, atrial fibrillation, among middle-aged male endurance athletes, and the repetitive micro-trauma of high-intensity training has been speculated as a potential cause. While evidence of this process was found during the autopsy of Ryan Shay, it’s unclear if the scarring found on his heart was the actual cause of the arrhythmia.

But despite these possible concerns, many cardiologists involved in this area of research aren’t ready to recommend that distance runners cut back on exercise. “The epidemiological data does not show that those who exercise more have increased mortality,” Accad says. “On the contrary, most studies show that athletes tend to have excellent longevity, and do not support an increase in mortality with higher exercise intensity.”

But researchers are concerned about more than just a handful of elite athletes—541,000 runners finished marathons in the United States last year. These runners are frequently carrying out high levels of training and competition once reserved for more serious athletes.

Studies have also found indications of cardiac stress after marathons or long distance triathlons. The altered heart function seems to be transient—lasting 24 to 48 hours in most cases—so researchers question whether participating in such events could have any long-term effects. After this kind of intense exercise, researchers have noted that a heart protein called troponin is released into the blood stream. This is also seen after heart attacks. However, the troponin rise following endurance exercise quickly normalizes, unlike the troponin rise seen after more serious heart problems. Researchers speculate that these changes are probably better described as a sign of the heart’s adaptation to exercise.

A 2010 review study in the journal Echocardiography describes this short-term exercise-induced reduction in heart function as “cardiac fatigue.” Dr. Chad Asplund, author of a just published review article on endurance exercise and the heart, emphasizes, “The data presented to date is interesting, but as of yet there is no causal link to prove for certain that more then X amount of exercise will lead to changes to the heart.”

As further evidence of this, Asplund highlights studies of former Tour de France riders and Olympians, both groups of extreme exercisers, who on average live 10 to 12 years longer than non-exercisers.

“Statistically, the number of cardiac complications associated with long distance events is very low (approximately 1 in 200,000) and in that that sense one should consider participation in long distance events to be quite safe,” Accad states. “But the heart does need time to repair itself after competition.”

There may well be an upper limit of exercise for a subset of exercise junkies. The problem is, research hasn’t identified that threshold, likely because it varies for each individual. Accad states, “I think that it may be better to monitor veteran endurance athletes for early signs of problems rather than declare a specific threshold unsafe.”

“I think there is a ‘sweet spot’ of exercise, but that amount per week is going to be different for each person,” Asplund says. “My advice for people is to be balanced in their training, and if training takes over their life, then it can become an addiction like anything else, and not a good thing.”