A little exercise is good for you. Too much can be bad for you, and could even kill you.

Although it remains true that people who exercise are generally healthier than those who don't, a study published this month in Mayo Clinic Proceedings and a study presented last week to the American College of Sports Medicine find excessive training for ultramarathons, Olympic-length triathlons and other endurance events can cause long-term and potentially fatal damage to the heart and major arteries.

The Mayo report, a meta-study of previously published data, suggests chronic training for and participation in such events can cause abnormalities to the structure and function of the heart, including dilation of the chambers. Sustained endurance training releases the same biomarkers found during cardiac arrest.

These changes revert to normal within a week, but months or years of repetitive injury can cause patchy myocardial fibrosis, or scarring, and lead to an irregular heartbeat. One study reviewed by the authors found 12 percent of otherwise healthy marathoners showed evidence of scarring, and the rate of coronary disease among these runners during a two-year follow-up was "significantly" higher than in the control group.

"If you exercise for longer and excessive amounts of time, you're putting wear and tear on the body, including the heart, and you could be increasing your risk," said Dr. Chip Lavie of the University of Queensland School of Medicine and a co-author of both studies. "For a person who's exercising just for health, you get the benefits from a lot lower levels of activity."

Although elite endurance athletes often develop abnormal electrocardiograms and atrial or ventricular entropy, few have thought it would lead to serious arrhythmias or cardiac arrest. Evidence suggests it can. The report notes ultramarathoners and professional cyclists have been associated with as much as a five-fold increase in the prevalence of atrial fibrillation, the most common form of cardiac arrhythmia. Excessive endurance exercise also may be linked to coronary artery calcification, diastolic heart failure and stiffening of large-artery walls, but these findings remain largely hypothetical due to inconsistencies in the data.

Still, perspective is in order. Some two million people run a marathon in the United States each year, yet there is but one death for every 100,000 people who race. So it isn't as if marathoners and triathletes are dropping dead mid-stride. As in all things, moderation is key, researchers said, and 30 to 60 minutes is plenty. That jibes with Centers for Disease Control and Prevention recommendations that we get 2.5 hours of moderate exercise — brisk walking, riding a bike on level ground, etc. — each week. The feds say we ought to spend another 75 minutes per week engaging in vigorous activity like running.

A separate study, presented June 2 at the American College of Sports Medicine, found the health benefits of rigorous exercise plateau. The study followed 52,000 people, including 14,000 runners, for 15 years to examine the association between running and mortality. Lavie and his colleagues found the mortality rate was nearly 20 percent lower among runners, and particularly reduced among people who ran less than 20 miles per week, fewer than six times a week or at 6 to 7 mph.

"Not only did we find that more was not better, but the data suggests that more could be worse," Lavie said. "It's a U-shaped relationship. As you run more, mortality starts creeping back up. You'd expect it would plateau, but not only did this happen, it went the other way and they lost their survival advantage."

The finding is somewhat akin to studies examining the correlation between alcohol and heart disease. A glass or two of wine can reduce the risk, but a bottle or two will increase it.

"We're not trying to scare anyone," added Lavie. "Clearly the bigger problem for society is that not enough people exercise. We’re not trying to get people not to exercise. If they are exercising for their health, they are probably better off doing a heck of a lot less."