Exercise, like all interventions that improve health, has a dose-response curve. As in most such curves, the initial difference between no treatment (a sedentary or near-sedentary lifestyle) and some treatment (moderate physical activity every day) is quite large. Further increments in activity can add increasing benefits, but ever less as activity time increases further. There is an optimal point at which one can be fairly certain of capturing most of the benefits, even given the usual uncertainties in measurement and variation in the response of individuals. For aerobic exercise, and the average human being, the optimal point is probably a greater amount of time than the 30 minutes daily presently recommended.
Regular moderate- to vigorous-intensity physical activity (MVPA) is associated with a lower risk of cardiovascular disease; certain cancers; and premature death. In addition, the amount of time spent sedentary - distinct from physical inactivity - is associated with a higher risk of death and disease. That may be a result, at least in part, from sedentary behavior displacing physical activity.
Most previous studies have explored the potential effect of sedentary time without considering the physical activity it displaces, leaving a gap in the understanding of the issue. To explore further, investigators analyzed self-reported sitting time, light physical activity, and moderate/vigorous physical activity among 92,541 participants in the ACS's Cancer Prevention Study II Nutrition Cohort.
The analysis reviewed sedentary time and activity levels over 14 years. It found among those who were the least active at baseline (less than 17 minutes/day moderate to vigorous physical activity), replacing 30 minutes/day of sitting with light physical activity was associated with a 14% reduced risk of death, while replacement with moderate to vigorous physical activity was associated with a 45% reduced risk of death.
The investigators found similar but smaller associations among moderately active participants: replacing a half hour of sedentary time with light physical activity was associated with a 6% reduction in mortality among those who were moderately active; replacing 30 minutes of sitting time with moderate to vigorous physical activity was associated with a 17% mortality reduction in this group. However, for the most active (more than 38 minutes/day of MVPA), substitution of sitting time with light physical activity or MVPA was not associated with a reduction in mortality risk.