This article was co-authored by Dr. Michael Joyner, a physiologist and anesthesiologist at the Mayo Clinic.
Is too much exercise bad for you?
The idea that extreme endurance exercise (e.g., exercising over an hour a day, racing multiple marathons in a lifetime, participating in Ironman distance triathlons) might be harmful for long-term heart health has gained traction in recent years. While studies claiming that too much exercise can be harmful are small and rife with anecdotes, they have attracted widespread media attention, leading to headlines in major news outlets like "One Running Shoe in the Grave," "Too Much Exercise May be a Bad for the Heart," and "When Exercise is Too Much of a Good Thing." At a time when nearly 70 percent of the American population is overweight or obese -- leading to a pandemic of chronic conditions that is likely to overwhelm our already fragile health care system, messages that exercise is harmful are counterproductive, especially since the science, even about so called "extreme exercise," tells a different story.
The Science on Extreme Exercise
To date, no large population-based study has supported the "too much exercise" claims. If too much exercise is harmful, then we would expect to see longevity falling and mortality rising with increased volume. However, that is not the case. For example, in Sweden, a socialized medical system enables population-wide epidemiological studies. There, the medical records of nearly 50,000 men and women who compete in long-distance (up to 90 km, or over 55 miles) Nordic skiing races show that the more races one finishes, the lower his or her mortality. Similar results have been seen in the U.S. where a study of 650,000 people yielded no reduction in longevity or mortality for individuals doing up to four times the minimal recommended (30 minutes five times per week) amount of exercise.
The "too much exercise" advocates also point to small-scale investigations that show hardening of the heart muscle and a buildup of calcium in the coronary arteries of some middle-aged endurance athletes. However, these reports typically fail to mention conflicting research that has found the heart muscles of older athletes (i.e., those in the masters category) to be more flexible and youthful than expected, with healthy vascular structures.
Both the population data and conflicting findings from smaller studies lead us to believe that the case for too much exercise being harmful is inconclusive at best.
Unintended Consequences for Public Health
It would be one thing if only the medical community and ultra-endurance athletes followed this issue; but, when the topic is given prime real estate in major publications, it reaches much broader audiences and its impact can be deleterious. Individuals that could benefit greatly from increasing their physical activity may read the headline "Too Much Running Tied to Shorter Life Span, Studies Find" and find it to be the perfect excuse to not start an exercise program. This is particularly true in a culture where many consume news in 140 characters or less, and thus headlines often become the entire content of stories.
The attention given to the unsubstantiated harms of exercise is especially disconcerting given there is considerably more, not to mention definitive, science illustrating the far-reaching advantages of exercise. In addition to the traditional benefits of exercise (e.g., weight-loss, improved cardiovascular health, increased bone density), exercise is also a "keystone habit," or an activity that when people introduce into their routines will lead to numerous benefits in other areas of their lives. A growing body of research demonstrates that those who start exercising regularly also start to eat better, smoke less, become more productive at work, accumulate less debt, and self-report less stress . An extreme example of how this plays out in the real world is "Back on My Feet," a program that has successfully used running as a mechanism to help homeless people turn their lives around.
While it is still important to continue investigating the effects of extreme exercise (which could prove important to the very small portion of the population that are truly extreme exercisers), the overwhelming majority of science suggests that exercise is perhaps the most important pathway to a healthier, better life. Unduly provocative headlines based on inconclusive science work against what should be one of our country's most important public health goals -- getting people to move and exercise more.
 For more on this fascinating topic, see "The Power of Habit" by Charles Duhigg.