While aerobic exercise generally benefits health, extreme endurance exercise – like marathon and triathlon – can raise a person’s risk for sudden cardiac arrest and atrial fibrillation, according to a new study.

After reviewing more than 300 scientific studies, the research team found that physically active people, such as regular walkers, have up to a 50 percent lower risk of heart attack and sudden cardiac death.

However, the team also identified potential risks with intense exercise training, according to the study published in the journal circulation.

“Exercise is medicine, and there is no question that moderate to vigorous physical activity is beneficial to overall cardiovascular health. 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 Barry A Franklin, Professor at Oakland University in the US.

“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 programmes in perspective,” Franklin added.

The research group also reviewed a small study that concluded that the risk of sudden cardiac death or heart attack is low among people participating in high-intensity exercise such as marathons and triathlons.

However, over time, the risk of heart attack or sudden cardiac death among male marathon participants has risen, suggesting that these events are attracting higher risk participants (those who may have an underlying or undiagnosed cardiovascular condition such as heart rhythm abnormalities or a prior heart attack).

For women, who comprised only 15 percent of the study’s population, the occurrence of sudden cardiac death was 3.5 fold less than in men.

Among participants in triathlons, almost 40 percent of cardiac events occurred in first-time participants, indicating that inadequate training or underlying heart problems may be involved.

The research team also found that half of cardiac events occur in the last mile of a marathon or half-marathon, so maintaining a steady pace rather than sprinting is advised.

The risk of cardiac events is greater at high altitudes, but can be decreased by spending at least one day acclimating to the elevation prior to strenuous activity, the study said.

For people who want to become more active, the researchers suggested that most people can start a light programme of exercise and build up slowly to a moderate to vigorous exercise regimen.