Can too much high-intensity exercise pose heart risks?

Along with following a nutritious diet and avoiding risky behavior like smoking, part of a healthy lifestyle includes exercising regularly. But two new studies published in the journal Heart suggest doing too much high-intensity exercise may increase risks of dying from a heart attack or developing an irregular heart rhythm later in life.

In a linked editorial to the studies, two writers say the results reveal a "J-shaped curve" for health benefits of exercise, whereby more does not always mean better. They also say the research raises questions about intensity and duration of physical activity at different points in life.

The first study was conducted by researchers in Germany, who, for 10 years, assessed the frequency and intensity of physical activity in more than 1,000 individuals with stable coronary artery heart disease for 10 years.

Participants were mostly in their 60s and had participated in a cardiac rehabilitation program to help them exercise regularly. The researchers tracked survival of all participants as part of the study.

Currently, heart disease patients are advised to participate in up to an hour of moderate intensity aerobic activity at least five times a week. The breakdown of study participant activity was as follows:

• 40% were physically active two to four times per week
• 30% were physically active more than four times per week
• 30% were physically active less than two times per week
• 10% rarely or never did any exercise.

Though the most physically inactive were twice as likely to have a heart attack or stroke as those who were physically active and were four times as likely to die of all causes, results showed that those who did the most strenuous daily exercise were more than twice as likely to die of a heart attack or stroke.

The researchers note that they took into account other influential factors that could have affected the results.

Regular high-intensity exercise linked with atrial fibrillation

In the second study, conducted by researchers in Sweden, more than 44,000 men between the ages of 45 and 79 were asked about their physical activity tendencies during the ages of 15, 30, 50 and during the past year of their life.

Men who exercised intensely for over 5 hours a week were more likely to develop an irregular heartbeat by 60 years of age, one study found.

For an average of 12 years, the researchers tracked the participants' heart health to determine how many developed an irregular heart rhythm, also known as atrial fibrillation, which is a known stroke risk factor.

Results showed that men who exercised intensely for more than 5 hours a week were 19% more likely to develop an irregular heartbeat by 60 years of age, compared with those who exercised less than 1 hour a week.

However, those who did slightly less-intense exercises - such as cycling or briskly walking - for an hour a day or more at the age of 60 were 13% less likely to develop an irregular heartbeat than those who did not exercise at all.

In the linked editorial, authors Eduard Guasch and Lluís Mont say these results may have to do with exercise intensity likely being greater at age 30 than at age 60:

"In general," they write, "the intensity of exercise performed by 30-year-olds is higher than that performed by 60-year-olds, meaning that the same degree of exercise might be reported as moderate at 30 years of age and intense at 60."

Intensity, duration of exercise are 'critical factors'

The researchers note that though common knowledge holds that physical activity has positive benefits for heart health, other studies have shown that endurance training, especially, is linked with a pro-inflammatory state.

They say both studies suggest that intensity and duration of exercise are "critical factors" in the links that were found, adding that "maximum cardiovascular benefits are obtained if performed at moderate doses, while these benefits are lost with (very high) intensity and prolonged efforts."

Still, they point to a limitation in both studies: the self-administered questionnaires. They note that taking data that was previously collected on exercise performance and correlating them with clinically relevant outcomes "is the main drawback of both studies."

However, they add that such questionnaires are commonly used in large registries, and they can "provide fairly good estimates of total exercise."

Despite the findings of the studies, the researchers say:


"The benefits of exercise are definitely not to be questioned; on the contrary, they should be reinforced. The studies reviewed here, and future studies, will serve to maximise benefits obtained by regular exercise while preventing undesirable effects - just like all other drugs and therapies."

Clinical experts recently reported on a study that suggested starting exercise at age 40 has the same heart benefits of earlier training.