MONDAY, March 2, 2020 (HealthDay News) -- For most people, aerobic exercise is great. However, high-intensity exercise like running in marathons and triathlons can pose heart risks for those who have inadequate training.

Sudden cardiac arrest, atrial fibrillation and heart attacks are among these risks, according to a new scientific statement from the American Heart Association (AHA).

"Exercise is medicine, and there is no question that moderate to vigorous physical activity is beneficial to overall cardiovascular health," said Barry Franklin, chair of the writing committee for the new scientific statement.

"However, 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," said Franklin, director of preventive cardiology and cardiac rehabilitation at Beaumont Health in Royal Oak, Mich.

"More people are running marathons, participating in triathlons and doing high-intensity interval training. The purpose of this statement is to put the benefits and risks of these vigorous exercise programs in perspective," he said in an AHA news release.

The committee reviewed more than 300 studies and concluded that improving physical fitness is beneficial for most. People who exercise have up to a 50% lower risk of heart attack and cardiac death.

However, while the risk of sudden cardiac death or heart attack is low among people engaging in high-intensity exercise, it is still possible. Nearly 40% of cardiac events in triathlons occur in first-time participants, suggesting that poor training or underlying conditions may be the culprit.

Before embarking on an intense training program, the AHA encourages people to start a light exercise program and slowly build up. But if you have symptoms such as chest pain, chest pressure or severe shortness of breath while exercising, check with a doctor before starting any program.

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

The scientific statement was published Feb. 26 in the AHA journal Circulation.

More information

The Mayo Clinic has more on benefits of exercise.