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It's no secret exercise comes with plenty of health benefits. But it turns out, there can be too much of a good thing.

Extreme exercise like marathons, triathlons and high-intensity interval training can raise the risk of cardiac arrest, abnormal heart rhythm and heart attack in people who are not active or accustomed, according to a new scientific statement from the 91ÊÓƵ.

"Exercise is medicine, and like medicine, it is possible to underdose and overdose on exercise – more is not always better and can lead to cardiac events, particularly when performed by inactive, unfit individuals with known or undiagnosed heart disease," Barry A. Franklin, chair of the statement's writing committee, said in a news release. Franklin is director of preventive cardiology and cardiac rehabilitation at Beaumont Health in Royal Oak, Michigan. "The purpose of this statement is to put the benefits and risks of vigorous exercise like marathons and triathlons in perspective."

This, of course, isn't to say that people should avoid exercise altogether. Authors of the statement – published Wednesday in the AHA's journal – encourage people to engage in aerobic exercises, which keep large muscles moving for sustained time. They can be done at low intensity or high intensity and include walking, brisk walking, running, bicycling and swimming.

After reviewing 300 studies, the authors concluded the benefits of exercise generally outweigh the risks. Physically active people, such as regular walkers, have up to a 50% lower risk of heart attack and sudden cardiac death. However, the committee also identified potential risks with intense exercise training.

The likelihood for sudden cardiac death or heart attack is low for people opting to take on high-intensity exercise goals, according to the authors' review of one small study. But over time, the risk of these events among men running marathons has increased, which suggests more high-risk participants who potentially have underlying cardiovascular issues are taking part. The risk for women, who made up just 15% of the study's population, was 3.5-fold less than in men.

First-time triathlon participants made up 40% of those who experienced cardiac events.

The authors also determined high altitudes lead to increased risk, and that half of cardiac events are suffered in the last mile of a marathon or half-marathon. Irregular heart rhythm, or heart arrhythmia, is highest among people on both sides of the fitness spectrum – those who are generally inactive and those who engage in high volumes of exercise training.

The new report suggests a slow and steady approach to training. Chest tightness and shortness of breath are signs to take it easy and, in extreme cases, seek medical help.

It also recommends warming up, cooling down, and increasing the amount of time spent on exercise from five to 10 minutes at the beginning, building up slowly to the desired time. Running and vigorous walking should be done on flat land, working up to hills over time if no concerning symptoms arise.

"It is important to start exercising," Franklin said. "But go slow, even if you were an athlete in high school."

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