Is too much exercise bad for your heart?

Even for people who have had a heart attack, regular aerobic activity helps the heart work better, and today the majority are encouraged to participate in cardiac exercise rehabilitation programs just weeks after having an event.

Still, questions remained about whether you can have too much of a good thing.

While moderate amounts of exercise benefit the heart, recent studies found that people who exercise a lot, at an intense level, may have an increased risk of heart conditions including atrial fibrillation, the most common abnormal heart rhythm and a major cause of stroke in Australia, and of coronary calcification, the buildup of plaques which are the main cause of heart attacks.

When unstable plaques break off, a blood clot can form around them, blocking the flow of blood and oxygen to your heart. But experts thought that perhaps the plaques in highly fit folk were different to those in sedentary people, the group most likely to have a buildup.

“There was a lot of speculation that ‘well, these are athletes, we think they have lower risk of heart attack. There must be something special about their plaque’,” says Professor Jason Kovacic, the director and CEO of the Victor Chang Cardiac Research Institute.

That was until the publication of this new paper.

Finding the sweet spot

For the study, researchers from the University of Texas and the Cooper Center took more than 26,000 adults (72 per cent of them men) in their 50s and divided them into groups based on physical activity.

The least active group did about 30 minutes a week, the moderate group did between 2.5 to five hours a week, while the high-volume group averaged 10.5 hours per week.

Following up 20 years later, they found that the moderate exercisers had the lowest risk of cardiovascular events. The low and high-volume groups, however, had similar levels of coronary artery disease.

Additionally, 34.5 per cent of the most active participants and 28.5 per cent of the least active had high coronary calcification scores, correlating with a greater likelihood of coronary artery disease, regardless of whether they were active or not.

“This suggests that the coronary artery calcification observed in active individuals is not benign and has the same clinical significance as subclinical atherosclerosis in less active subjects,” wrote Paul Thompson, the chief of cardiology at Hartford Hospital, in an editorial about the paper.

Being fit and lean, having low cholesterol and low blood pressure doesn’t seem to compensate for having a high coronary artery calcification score, adds honorary professor Kieran Fallon from the Australian National University: “The public health recommendation is … a moderate level of physical activity is probably best for cardiac health.”

And although the most active group had the lowest overall mortality, they did not split the group into those with high and low calcification scores. If exercising a lot doesn’t impact your heart health, then it is associated with greater risk reduction across a range of chronic diseases.

“I would underscore … that no one here is saying anybody should stop exercising,” Kovacic stresses. “And no one is saying three to five hours a week isn’t a good thing.”

He points to the first ever clinical trial, published in the New England Journal of Medicine this week, to show that a structured exercise program reduced cancer recurrence or death by 28 per cent, among patients with colon cancer who had recently completed chemotherapy.

Why then would exercise, when it’s so helpful to heart health, harm the heart?

There are many theories, including that long-term high volumes of intense exercise increase inflammation; that the sheer stress of the bloodflow against the vessel wall is a factor; that a lot of endurance athletes consume sugary drinks and glucose gels, so it relates to their dietary habits; or that it’s to do with an increase in circulating adrenaline and noradrenaline.

“All of that is speculation,” says Kovacic, himself a former elite rower who now exercises for about five hours a week.

Regardless, he says that for people who do a high volume of intense exercise but do not have any significant calcification in their arteries, there is no reason to change what they are doing.

For those with a high score, however, before this paper he hadn’t taken the step of telling them to consider reducing the volume of exercise. “And now I have.”

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