Everyone knows that exercise is good for them, but is it possible to get too much of a good thing? While exercise provides many health benefits, at some point working out too hard or too long can increase the risk of injury and have other adverse effects. That usually means intense exercise like training for marathons or endurance activities. Such challenging activities are perfectly fine, maybe even laudable, endeavors, but if you're looking for optimal health benefits, research suggests that more moderate workouts are generally the way to go.
In recent years a handful of studies, all observational, have gotten lots of press for highlighting the drawbacks of exercising too much (in terms of duration and intensity). Newspaper headlines about them—such as "Fast Running is as Deadly as Sitting on Couch, Scientists Find" or "One Running Shoe in the Grave"—may have given some sedentary readers a feeling of schadenfreude. But bear in mind that overdoing exercise, however that is defined, is a problem relatively few Americans need to worry about, since inactivity is far more common and also more harmful. And not all the research on "extreme" exercise has found increased risks.
Searching for the sweet spotHere's a look at some of the latest research:
- In a British study of more than a million healthy middle-aged women in Circulation, those who were physically active at least once a week were less likely to have heart attacks, strokes, or venous thromboembolic events (blood clots in legs or lungs) over a nine-year period than inactive women. Moreover, any type of physical activity (including gardening and housework) had pretty much the same effects as strenuous activity. But what made headlines was that the women who exercised every day didn’t benefit more than the less frequent exercisers—in fact, they were at greater risk. The accompanying editorial noted the study's U-shaped risk curves, in which exercising two to six times a week is at the bottom of the curve (lowest cardiovascular risk), compared to exercising rarely or every day.
- Similarly, a U-shaped curve was found in a large Danish study in the Journal of the American College of Cardiology, which compared the mortality rates of joggers and sedentary people. Those who did light to moderate running (in terms of pace, duration, and frequency) were less likely to die over a 12-year period than non-exercisers. But strenuous joggers (fast pace, more than 2 1/2 hours a week, or more than 3 times a week) had a mortality rate similar to sedentary people. "The U-shaped association suggests...higher doses of running are not only unnecessary but may also erode some of the remarkable longevity benefits conferred by lower doses of running," the study concluded.
- Another type of curve found in many exercise studies is the "reverse J," which shows increased risk at both ends, but higher on the left side, which represents sedentariness. For instance, a German study in the journal Heart looked at people who had stable coronary heart disease and found that both those who were sedentary and those who exercised every day (or did more than about 15 hours of strenuous activity a week) were more likely to die over a 10-year period than those who exercised several times a week—but the risk was greatest for the couch potatoes, resulting in a reverse J-shaped curve.
- A study in Mayo Clinic Proceedings analyzed data from the decades-long National Runners' Health Study and National Walkers’ Health Study, focusing specifically on heart attack survivors. Once again, those who walked or ran were less likely to die from cardiovascular disease than sedentary people, with the benefit increasing steadily up to a very high level of exercise. Beyond that point—running more than about 30 miles a week or walking more than 46 miles a week—much of the benefit disappeared, again producing a reverse J-shaped curve.
Because of the nature of observational research (which can demonstrate only correlation, not causation), as well as gaps in the data gathered, differences in the way activity levels were categorized, and the relatively small percentage of people who exercised at the highest levels, these studies leave many questions unanswered. Notably, is there something about people who overdo strenuous endurance exercise that harms their health? Perhaps they are overly competitive or compulsive and tend to overdo other things as well. In contrast, do people who exercise moderately also do other things in moderation, which is generally a healthy way to live? Researchers try to control for such factors, but the likelihood that some such factors are not identified or adjusted for remains a concern.
How might excessive endurance exercise be harmful, especially when done every day? Besides causing wear and tear on the body and increasing the risk of injuries, it can depress the immune system and increase inflammatory processes. Taking off a day or two a week gives the body time to recover from the stress of exercise. Moreover, some studies of endurance athletes, usually marathoners, have found coronary changes that may increase the risk of arrhythmias, sudden death, and other problems.
Keep in mind, however, that running and other aerobic exercise—no matter how strenuously they're done—help improve many cardiovascular risk factors, such as blood pressure, cholesterol, and body weight. Thus it's possible that "high doses" of exercise can benefit some aspects of cardiovascular health while negatively impacting others.
But not so clear-cutBefore you start feeling sorry for your neighbor who runs marathons or spends many hours sweating in the gym, you should know that quite a few other studies over the years have found that strenuous endurance exercise is as beneficial as moderate activity or even more so. For instance, a study in the Lancet, which followed more than 400,000 people in Taiwan for eight years, found that the more time people spent exercising, the lower their mortality rate, until it plateaued at about 100 minutes a day. Vigorous exercise was even more beneficial, though its benefit plateaued at about 45 minutes a day. And there was no decrease in benefits among those who exercised the most.
Words to the wise
As Hippocrates put it 2,400 years ago, "If we could give every individual the right amount of nourishment and exercise, not too little and not too much, we would have found the safest way to health." But "too much" varies from person to person. Some of us can exercise strenuously for decades and not suffer any major physical or cardiovascular problems, while others are more susceptible to them, partly for genetic reasons, but also possibly because of differences in training. There’s no way to say exactly what is the upper limit of exercise for everyone.
Simply put, any exercise is better than none, and more—in terms of duration or intensity—is generally better, within reason. But you certainly don’t have to keep intensifying your exercise regimen to stay healthy. For most people, moderate exercise is the sweet spot. For more on exercise guidelines in general, see Is Intense Aerobic Exercise Best?