Christopher Labos: Is high-level endurance exercise bad for your heart?

Exercise is the best thing you can do to stay healthy. But there is always the worry that some forms of exercise are dangerous and there are always just enough news headlines and studies around to fan the flames of that concern. For example, this month, researchers at the American College of Cardiology conference presented research from the Master@Heart study showing that lifelong endurance athletes had more coronary plaques than non-athletes. The implication was that high level endurance exercise, like marathons, must be bad for your heart.

But before we can make that conclusion, we have to remember that there is a difference between having plaque in your arteries and having a heart attack.

It’s somewhat interesting that marathons became a popular sporting event given that the first person to ever try one was Pheidippides of Athens and he reportedly collapsed and died after running the distance from Marathon to Athens. Despite this inauspicious, and possibly apocryphal, beginning, the marathon has become a staple of the sporting world.

And despite many news reports of people dying during marathons, the rate of sudden cardiac death during these events is very low. When you survey data over multiple years, the prevalence of sudden death during marathons is around 0.002 per cent. Some reports have found even lower estimates.

Marathon runners do sometimes require medical attention. In the Twin Cities Marathon, 25 per 1,000 marathon finishers required medical attention but 90 per cent of these problems were mild and included musculoskeletal injuries, abrasions and blisters. Sometimes runners collapse near the race’s end. Heat stroke and dehydration are also common, especially in hot weather, but cardiovascular events are rare.

But the Master@Heart study is suggesting a more insidious problem — that long-term participation in extreme sports like marathons and long-distance cycling makes heart disease worse. Their study looked at three groups: lifelong endurance athletes, those who came to the sport after age 30, and healthy controls. Both exercise groups did 11 and 10 hours of exercise per week compared to one hour per week for the controls. When researchers scanned their hearts, the two exercise groups had more coronary plaques on their CT scans.

It is tempting to take this study as proof that excessive exercise is bad for you. However, Master@Heart is not the first study to show more coronary calcification in athletes. Previous studies like CARDIA and MESA, which have followed thousands of patients over many years, have also shown that athletes have higher calcium scores. Importantly, they don’t have more heart attacks.

Reconciling these potential contradictions requires us to remember that there is a difference between atherosclerosis and a heart attack. As we age, plaque accumulates in our arteries and the arterial walls start to calcify. But a heart attack is a different beast. A heart attack is a sudden event when an unstable or vulnerable plaque ruptures suddenly and blocks an artery. Old calcified plaques don’t rupture. They can cause chest pain on exertion (i.e. angina), but not heart attacks.

In the Master@Heart study, endurance athletes had more plaques but fewer people had vulnerable plaques (one and two in both exercise groups versus six in the controls). Admittedly, the numbers were overall low regardless because these people were healthy at baseline. But the takeaway is that exercise probably leads to a shift in atherosclerosis towards plaque stabilization that likely leads to less cardiovascular outcomes in the long run.

Nobody likes to hear that they have plaque in their coronary arteries. But there is a danger in focusing on surrogate endpoints like these. What matters is preventing concrete outcomes like heart attacks or strokes — and there is no doubt that exercise reduces your risk of cardiac disease, cancer or death overall. If you want to run a marathon, train properly and you should be OK. And if you don’t feel compelled to run 46 kilometres in one shot, go for a 30 minute walk. The evidence suggests that’s just as good.

Christopher Labos is a Montreal physician and co-host of the Body of Evidence podcast.

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