Routine moderate-intensity physical activity confers numerous cardiovascular benefits and reduces all-cause mortality. However, the health impact of exercise doses that exceed contemporary physical activity guidelines remains incompletely understood, and an emerging body of literature suggests that high levels of exercise may have the capacity to damage the cardiovascular system. This review focuses on the contemporary controversies regarding high-dose exercise and cardiovascular morbidity and mortality. We discuss the limitations of available studies, explore potential mechanisms that may mediate exercise-related cardiac injury, and highlight the gaps in knowledge for future research.
The past few decades have witnessed an increase in participation rates in organized sporting events including marathons, long-distance cycling, and triathlons.2 This increase is largely accounted for by people without any background in competitive sport, including those >40 years old, with risk profiles different from traditional competitive athletes. Achieving competitive race times requires several hours of intense training per day, often >10-15 times the daily recommended dose of physical activity.3 Even outside of these formal endurance events, fitness trends focusing on short periods of high-intensity exercise such as CrossFit (Washington, DC) and high-intensity interval training have gained prominence among the general population. This recent surge in enthusiasm, coupled with controversial data highlighting the potential harm of too much exercise, has put this question once again on top of the agenda—can one exercise too hard, or too much?
The U-Shaped Curve Between Physical Activity and Mortality
It is well established that regular low- and moderate-intensity exercise improves all-cause mortality in a dose-response fashion.4 However, at the upper levels of the exercise dose-response curve, the relationship between exercise and mortality remains incompletely understood. Early studies using professional athletes consistently showed improved survival compared with nonathletic counterparts.5, 6, 7 Conversely, recent data derived from the general population suggest that high doses of exercise may reduce or eliminate the mortality benefit gained from lower levels of exercise exposure. A prospective analysis of more than 1000 ostensibly healthy joggers revealed mortality risk reduction among low and moderate levels of jogging (as defined by jogging pace, duration, and frequency) but no statistical difference in mortality rates comparing “strenuous” joggers to sedentary counterparts.8 Although this finding implies a U-shaped relationship between all-cause mortality and running dose, the “strenuous” jogger subgroup was comprised of only 36 individuals with only 2 recorded deaths. As such, this study was underpowered to assess mortality risk among people who live in the upper end of the exercise dose-response curve. Rather than an uptick in mortality, larger epidemiologic studies have reported a mortality reduction plateau or a trend toward a relative decline in mortality benefit with high levels of exercise.9, 10
These observational data, however, carry inherent limitations. First, cross-sectional datasets are incapable of establishing a direct cause-and-effect relationship between the amount of exercise and mortality. Second, confounding variables not fully considered in these studies, including traditional cardiovascular risk factors, dietary intake, and the principal components of exercise dose: volume and intensity, lead to questions about the mechanistic relationship between exercise and mortality. It is probable that such unmeasured or incompletely measured confounders explain much of the discrepancy in outcomes between professional athletes and those who engage in high levels of exercise in the general population. Future work relying on carefully collected prospective data and detailed phenotypic characterization will be required to delineate the true relationship between longevity and exercise exposure.
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-Prashant Rao, MD, MRCP, Adolph M. Hutter Jr, MD, Aaron L. Baggish, MD
This article originally appeared in the November issue of The American Journal of Medicine.
