Understanding the Effects of Exercise on Your Heart

For the past few years, a debate about whether too much exercise can be bad for your health has been playing out in popular media. Maybe you’ve seen some of the articles—they’ve appeared in the New York Times, Sports Illustrated, the Wall Street Journal, U.S. News and World Report, and many other places. With titles like “The Great Fitness Debate: Is It True That You Can Exercise Too Much?” and “Can Too Much Exercise Harm the Heart?” they instantly grab the attention of many readers. After all, we’ve been told for decades that exercise is the best medicine for your heart. Now there’s a chance it’s harmful? What is a person to believe?

Sometimes the authors of these articles pit one camp against another, stating things like “the too-much-exercise advocates believe…,” as if this were a political debate, with one side being right and the other side being misinformed at best and dangerous at worst. Occasionally the authors suggest that those who believe there can be too much exercise are verging on alarmism. (Let us emphasize that we are anything but alarmists. In fact, all of us are lifelong endurance athletes with a penchant for riding hard and suffering often. That is, until we were made to slow down after developing heart arrhythmias.)

Some authors may cite studies that look at Olympians or professional cyclists who’ve competed in the Tour de France and find no lasting negative effects on the heart. Are you an Olympian? Have you ever raced in the Tour de France? Could it be that the athletes who have reached such heights in sports are genetically different from you and me? Maybe they’re not the best examples for understanding what is happening in the hearts of the general population. (There’s also the fact that Olympians and professional cyclists tend to relax after they’ve retired, more often than not. It’s just the middle-aged people who think they’re training to race the Tour who can’t seem to stop themselves from pushing so hard for so long.)

This is a topic massive in scope and complexity, without simple answers. Almost nothing about this subject is black and white. There’s a good chance the aforementioned articles suffer from oversimplification. That’s not to say we have all the answers, either. Far from it. Much more research is needed to settle the issue of just how much exercise is safe, and to better understand the links between exercise dosage and heart health. Scientific research in this area not only is relatively new but is hard to conduct given the limited number of people who fall into the mold of the longtime endurance athlete.

It cannot be stated enough: Exercise is extremely beneficial for heart health. Dozens of large epidemiological studies have found that people who exercise in any amount, whether five minutes a day or two hours a day, are much less likely to develop or die from heart disease than people who are inactive. (That being said, exercise does not make you immune to every heart problem that exists, especially if you have a history of unhealthy living, eating poorly, or smoking or are genetically predisposed to conditions that affect the heart.) These benefits are especially important in this era. The Western world now suffers from a near epidemic of chronic diseases wrought by the toxic combination of too much food and too little exercise.

No one in the health field doubts that regular exercise promotes and maintains physical, mental, and emotional health. Structured exercise can even be used to treat disease. Examples include cardiac rehabilitation in patients with coronary artery disease and heart failure, and pulmonary rehabilitation for patients with emphysema.

Recently, a group of researchers from Adelaide, Australia, showed that overweight and sedentary patients with a heart arrhythmia called atrial fibrillation who exercised enough to gain fitness ended up with less arrhythmia burden. (Their hearts no longer had as many periods of abnormal heart rhythm.) Exercise in this study behaved like an antiarrhythmic drug.

As we’ve already observed, if exercise were a marketable pill or procedure, it would be a blockbuster. That’s why it’s difficult for us to write about the possibility that exercise can be harmful. But there are many studies suggesting just that.

The Limits of Exercise Science

Although the evidence confirming the health benefits of low- to moderate-dose exercise is strong, the science that explores high-dose exercise is much more speculative and controversial.

An important reason is the type of studies used in exercise science. The strongest evidence in all of medicine comes from the blinded randomized controlled trial. In this type of study, one group of individuals is randomly selected to have treatment X and another group gets treatment Y. Randomization is used to even out any differences in the two groups for things like age, gender, and socioeconomic status. In the experiment, researchers strive to make the two treatments the only difference between the two groups in the study. Blinding makes it impossible for anyone to know which treatment was received. That way, if there is a difference in a subject’s outcome, it can only be attributed to the treatment.

These sorts of trials are impossible in sports medicine. It’s easy to see how a randomized controlled study would never be able to answer the too-much-exercise question. First, athletes cannot be blinded to their exercise exposure. Second, there are many variables that affect the occurrence of heart disease. Things like exercise duration and intensity, other stressors in life (divorce, illness, job), family history, and diet all factor into the development of heart disease.

The lack of controlled trials weakens the evidence base in sports medicine. It means we must rely on less rigorous types of studies. These include observational, mechanistic, and animal studies.

Observational trials are problematic because without randomizing the groups and controlling all the trial’s factors, one cannot determine cause and effect. For instance, an observational study could suggest long-term endurance exercise associates with heart problems, but it cannot easily say long-term endurance exercise causes heart problems.

The reason for this important difference is that when you observe nonrandomized groups, you cannot exclude confounding factors and biases. A common bias is that only athletes with problems seek medical attention; the majority of people who compete could be doing well. This is called selection bias.

Mechanistic studies pose problems because they merely provide a plausible way in which endurance exercise could damage the heart. For instance, we describe studies that show modest increases in the cardiac enzyme troponin in athletes right after finishing a major long-distance race. Troponin is typically released during heart injury (e.g., heart attack). This type of study, therefore, supports the hypothesis that repeated bouts of exercise that are intense enough to release an enzyme associated with heart damage could, over time, lead to heart disease. But it’s just a hypothesis—a plausible one, but a hypothesis nonetheless.

Animal studies are limited for obvious reasons: Animals are not people. That doesn’t mean animal studies are useless; in fact, they can be quite helpful. But it does mean that they are largely speculative.

As you can see, exercise science faces a few challenges. That doesn’t mean the results of the emerging research should be ignored or marginalized as inaccurate or inconsequential. It does mean that it’s much harder to prove conclusively that one thing causes another—that high-dose exercise causes heart arrhythmias, for example. Thus, the debates will continue to stir in popular media. And researchers will continue to seek answers.

Exercise Dosage

Even the best of things can be overdone: Water and oxygen can be lethal in high enough doses. Too much water leads to hyponatremia (low sodium in the blood), and prolonged exposure to high levels of oxygen can damage lung tissue. The old saying holds true: “everything in moderation.” Sometimes, there truly can be too much of a good thing.

There is also a growing body of evidence to suggest that long-term endurance exercise can have negative consequences for your heart. Let’s be very clear about what we mean by that: We’re talking about a highly elevated level of exercise that is not only extremely intense but often competitive and is performed for years, if not decades.

This is not a “normal” or “regular” exercise dosage. But you may very well be one of those “abnormal” folks who partakes in this kind of activity, and may have for decades. You’re not alone.

The dose of exercise that promotes health is surprisingly small. A study of more than 13,000 men and women who were followed for eight years showed that although death rates decreased with greater levels of fitness, the largest reduction occurred between the sedentary group and those with low levels of fitness. How low? Mortality benefit in this study plateaued at levels of fitness that represent half of what is expected from a trained athlete.

A larger, more recent study confirmed the plateau effect of exercise. In a 12-year study of Taiwanese subjects, researchers also found lower death rates (both from heart disease and cancer) with increasing levels of daily physical activity, but the degree of benefit lessened after 30–60 minutes of exercise per day.

The take-home message from these findings is that if health is your goal, you need not exercise more than 30–60 minutes each day. Of course, most who participate or compete in endurance sports far exceed these levels of exercise.

The question is, does exercise have an upper-dose benefit limit? Does the health benefit of exercise reverse at higher doses of exercise? Is there a U-shaped curve?

Adapted from The Haywire Heart by Chris Case, Dr. John Mandrola, and Lennard Zinn, with permission of VeloPress.

The Haywire Heart