COMMENTARY

Is It Possible to Exercise Too Much?

Michael J. Joyner, MD

Disclosures

November 19, 2014

The fall marathon season recently culminated with the New York Marathon earlier this month. Over the past couple of years, high-profile marathons have generated a lot of media attention[1] directed at what I call the "too much exercise hypothesis." What do we know about the exercise dose-response curve and outcomes, where is the newer evidence on this topic pointing, and what questions are out there?

Study after study suggests that almost any amount of physical activity or formal exercise is protective against both cardiovascular disease and all-cause mortality. Current guidelines[1] advocate that adults get 150 minutes per week of moderately vigorous physical activity, or about 7.5 metabolic equivalent (MET)-hours per week. Of importance, this protection starts with just a few minutes of activity daily,[2] which is critical in the United States, where about 50% of adults are absolutely sedentary.[3]

There is wide variability in how each person responds to exercise training, and not everyone achieves the predicted or desired improvements in glucose, lipids, and blood pressure.[4] However, about 40% or more of the protective effects of exercise are not explained by traditional risk factors[5] and may have more to do with blood vessel remodeling and changes in the fight-or-flight part of the nervous system. So, it seems likely that almost everyone derives at least some benefit from being more active, even if their traditional risk factors do not change dramatically. Moreover, high levels of fitness are protective, even when other factors are at work in individual patients.[6]

For apparently healthy people, there simply isn't much evidence of a U- or J-shaped mortality curve, with mortality rising at very high levels of activity. Several recent studies have reinforced this, including a study[7] from Sweden on physical activity and the risk for heart failure, which followed almost 40,000 people for more than a decade and showed that the protective effects of activity were more robust than the differences in traditional risk factors for heart failure might suggest.

In their defense, the individuals who are promulgating the "too much exercise hypothesis" argue that they are talking about a very small subset of people who participate repeatedly in marathons and triathlons and are way out there on the "dose" part of the curve.[8]

A new study[9] from the outstanding exercise cardiology research group in Dallas shows that intensely competitive lifelong master athletes have hearts that remain highly compliant as they age and retain impressive levels of ventricular performance. Earlier studies on ultramarathoners have shown that they have big coronary arteries that also dilate more.[10] These physiology studies would seem to refute worries that too much exercise might make the heart stiff and injure the coronary arteries over time. On the population front, data from Sweden[11] on nearly 75,000 men and women who have participated in long-distance marathon skiing show that those who finished more races (and presumably trained for more years) had lower mortality. So, in terms of mortality in healthy people, data that support the "too much exercise hypothesis" are hard to find.

There are two important caveats to this conclusion. Prolonged intense exercise training for many years appears to increase (modestly) the risk for bradyarrhythmias and lone atrial fibrillation.[12] The extent to which this is the result of remodeling of the heart or cardiac conduction system, rather than a low resting heart rate, is unknown.[13] Furthermore, although it is clear that exercise training in patients with known coronary artery disease reduces their subsequent all-cause and cardiovascular mortality, perhaps there is such a thing as too much exercise in this group.[14] However, the number of cardiac patients at risk for too much exercise pales in comparison with those who are missing the benefits of moderate exercise training. The same point applies to the general population as a whole.

Exercise is not a "vaccine" against coronary artery disease in specific populations or against mortality in general, but it does have powerful protective effects with pretty well-described dose-response curves for mortality and cardiovascular outcomes. Thus, maybe we should all be worrying more about the vast majority of people who do nothing vs yapping about the unproven and hypothetical possibility that a tiny group of people might be doing too much.

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