At Any Dose?
Physical inactivity has been labeled a pandemic due to its increasing global prevalence and its health, economic, environmental, and social consequences. More than half of US adults fail to meet the 2008 physical activity recommendations of 30 minutes of moderate-intensity exercise daily (eg, brisk walking, dancing, and gardening) or 75 minutes of vigorous-intensity exercise weekly (eg, running, fast cycling, and competitive sports). Hence, increasing physical activity is essential for public health because it improves primary and secondary disease prevention across the population.
The benefits of exercise are indisputable and the current perception is that a curvilinear relationship exists between the amount of physical activity and the related health benefits. Many studies have demonstrated that physical activity is associated with reduced risk of cardiovascular diseases, diabetes, cancer, and dementia in a dose-dependent fashion. However, recent studies suggest that high doses of exercise can be harmful and are associated with increased risk of cardiovascular mortality.1– 4 These findings were interpreted to mean that exercise may harm the heart at a certain dose. The “too much exercise hypothesis” may cause confusion about the benefits of exercise and the optimal dose of physical activity to prescribe in clinical practice. What dose of exercise is needed to promote health? Do larger doses elicit larger health benefits? What is the cutoff between the beneficial and possibly detrimental effects of exercise?
LOWEST EFFECTIVE DOSE
Several studies have assessed the minimum amount of physical activity needed to provide cardiovascular benefits. A large Taiwanese study demonstrated that individuals exercising 15 minutes a day at a moderate intensity had a reduced risk of all-cause mortality (eTable in the Supplement) and a 3-year longer estimated life expectancy compared with their inactive peers.5 These data were reinforced by a pooled analysis of 6 US and European cohorts.6 Individuals performing physical activity in doses below the physical activity recommendations had a reduced risk for mortality over 14.2 years compared with physically inactive peers. Runners participating in the Aerobics Center Longitudinal Study running only 5- to 10-minutes a day had a reduction in cardiovascular mortality and all-cause mortality over 15 years of follow-up.7 These studies suggest that even short bouts of physical activity can have substantial health benefits. Although lack of time is a critical barrier to engaging in physical activity, these results5,7 suggest that even the busiest individuals should have time for this lowest effective dose of physical activity.
Increasing from the minimum effective levels of moderate (15 minutes a day)5 or vigorous intensity (8 minutes a day)6,7 physical activity to the national physical activity guidelines of 30 minutes a day of moderate-intensity exercise or 75 minutes a week of vigorous-intensity exercise appears to be associated with increased health benefits. Every additional 15 minutes of moderate-intensity physical activity was associated with a 4% further reduction of all-cause mortality over 13 years in the Taiwanese population.5 These benefits were independent of age, sex, and cardiovascular history. The greatest benefit was obtained by the most active individuals (63-88 minutes a day), with higher mortality reductions for vigorous-intensity vs moderate-intensity exercise.5
Similar findings were observed in US and European cohorts. Individuals performing physical activity at a dose of 3 to 5 times the current recommendations reported the lowest mortality rates over 14.2 years.6 Higher physical activity doses, up to 10 times the recommended physical activity levels, were associated with mortality rates comparable with the 3 to 5 times group. In the National Walkers’ and Runners’ Health studies, cardiovascular mortality over 10.4 years was reduced among survivors of myocardial infarction who exercised 8 to 24 minutes a day compared with patients who exercised at lower doses.2 The largest reduction in cardiovascular mortality was observed among patients who exercised 38 to 96 minutes a day. Data from the Cooper Institute confirmed that 7 minutes a day of high-intensity physical activity was associated with reduced cardiovascular mortality over 15 years, but revealed that running 51 to 176 minutes or more a week was not associated with additional benefits.7 Similarly, the Copenhagen City Heart Study demonstrated that all-cause mortality was lower over 12 years among joggers exercising less than 60 minutes a week than in nonjoggers.1 More jogging (up to >240 minutes/week) was not associated with additional mortality benefits. These data suggest that higher doses of moderate-intensity physical activity are related to larger health benefits, whereas physical activity beyond the lowest effective dose for high-intensity exercise is not associated with further reduced mortality rates.