Author + information
- Metabolic Epidemiology Branch, Division of Cancer Epidemiology and Genetics, National Cancer Institute, Rockville, Maryland
- ↵∗Address for correspondence:
Dr. Charles E. Matthews, Metabolic Epidemiology Branch, Division of Cancer Epidemiology and Genetics, National Cancer Institute, 9609 Medical Center Drive, Rockville, Maryland 20850.
In the last decade, a strong body of evidence emerged demonstrating that high levels of sedentary behavior, or “too much sitting,” is associated with increased mortality risk (1), particularly for the majority of U.S. adults who are not highly active (2). Stamatakis et al. (3) extend our understanding of the interplay among sitting time, physical activity, and mortality, and rightly continue to probe for the level of physical activity required to minimize risks linked to too much sitting. This line of inquiry remains essential because we spend the majority of our waking day sedentary, and as much as one-half of daily sedentary time may come from leisure-time sitting in older adults (4). Time-use data suggests that leisure time is plentiful for U.S. adults, who report spending an average of 314 min/day in leisure-time pursuits (or >2,000 min/week) (5), while 51% of the population reports 0 min/week of leisure-time physical activity (2). Given that time availability is common barrier to exercise (6), sedentary behaviors appear to be dramatically outcompeting physical activity for our free time.
The report in this issue of the Journal by Stamatakis et al. (3) addresses 2 critical questions: 1) what combinations of physical activity and sitting time are associated with lower mortality; and 2) how much physical activity is required to minimize the risk of too much sitting? At baseline, 149,077 Australian adults reported the amount of time they spent in sitting and in physical activity. They were subsequently followed for an average of 9 years, during which 8,689 total and 1,644 cardiovascular deaths occurred. Sitting time was classified into 4 categories (<4, 4 to 5.9, 6 to 7.9, and 8+ h/day) and moderate-vigorous physical activity (MVPA) into 5 categories of public health relevance (0, 1 to 149, 150 to 299, 300 to 419, and 420+ min/week). In joint analysis, sitting time and MVPA data were classified into 20 distinct MVPA-sitting combinations, and those reporting the most activity (420+ min/week) and least sitting (<4 h/day) were used as the referent group. Additionally, using a reparameterization of standard regression modeling (isotemporal substitution) the authors predicted mortality risks associated with replacing 1 h of sitting time with 1 h of different types of physical activity.
Results from the joint classification revealed a significant sitting–MVPA interaction with all-cause but not cardiovascular mortality. The joint-analysis results are particularly important because they shed light on how 20 different sitting-MVPA combinations may influence longevity (Figure 2 in Stamatakis et al. ). Among less active adults (MVPA <150 min/week), those reporting the least sitting (<4 h/day) were at lower risk than those who sat more during the day. This finding is highly relevant because nearly two-thirds of the U.S. population (65%) report spending <150 min/week in leisure-time physical activity (2) and could benefit from increased MVPA or reduced sitting time. As MVPA increased in the report by Stamatakis et al. (3) out to recommended levels (MVPA 150 to 299 min/week) (7), overall risk was reduced for all sitting groups; yet, excess risk for high levels of sitting (8+ h/day) remained. Excess risk from too much sitting was eventually eliminated for those reporting even higher levels of MVPA (300+ min/week). These findings are broadly consistent with previous studies, best represented in results from pooled analysis from 13 cohorts (8), which also showed that engaging in recommended amounts of MVPA (16 MET-h/week ), was inadequate to eliminate excess risk of higher levels of sitting. Results from Stamatakis et al. (3) finding that excess risk due to sitting may be eliminated at or above 300 min/week is an important step forward that places this amount just above the current primary U.S. recommendation of 150 to 300 min/week (7). Defining the precise amount of MVPA associated with the elimination of excess risk associated with too much sitting is challenging, because the ultimate answer lies in data from multiple epidemiological studies that typically use different questionnaires to measure sitting time and MVPA and sometimes use different units to estimate MVPA (e.g., h/week, MET-h/week). Accelerometer-based studies will soon provide new insight, but they also face methodological challenges even for presumed straightforward measures of MVPA (9), and their capture of the full range of daily activities could reveal different relationships compared with measures of leisure-time MVPA used in many studies. Making sense of the entirety of this evidence will be fascinating.
However, from a clinical and public health perspective, identifying the high level of MVPA associated with elimination of the excess risk of sitting may be a less critical question because only 23% of U.S. adults report doing >300 min/week of leisure-time physical activity (2), while 65% report doing less than the minimum recommendation and may benefit from increasing MVPA, reducing sitting time, or both. Isotemporal substitution results from Stamatakis and colleagues provide new insight into the potential mortality benefits associated with reducing sitting time in favor of replacing that time with walking and/or vigorous intensity physical activities. The authors estimated substitution benefits separately for those who sat less (<6 h/day) and who sat more (6+ h/day) and found stronger more consistent results among those who sat more. Assuming these individuals were less active, findings in this report are consistent with previous studies showing stronger replacement associations for less active adults (10) and replacement benefits for lower-intensity activities, daily walking, household chores, and lawn and garden activities, in addition to purposeful exercise. Collectively, these findings point to important translational opportunities.
From a clinical perspective, it is worth considering how the results of Stamatakis et al. (3) might be translated for patients who want to lower their risk of early mortality, but currently do not exercise (0 min/week MVPA) and sit for 8 or more h/day at work, commuting, or watching television. Results from the 45 and Up Study suggest that there are at least 3 opportunities to lower risk for inactive adults, with the ultimate choice dictated by personal preferences and a variety of socioecological factors (1). The first choice (Choice A) could be to increase physical activity to recommended levels (increasing from 0 to 150 to 299 min/week) without changes in sitting time (remain at 8+ h/day). Results from Stamatakis et al. (Figure 2) (3) suggest this change could lower mortality risk substantially, but some residual risk due to too much sitting may remain. A second choice (Choice B) could be to reduce sitting time substantially (reduce from 8+ to <4 h/day) without increasing MVPA (remain at 0 min/week). Results in Figure 2 suggest this change could lower mortality substantially, but some residual risk due to physical inactivity may remain. Interestingly, the amount of risk reduction for both of these choices are comparable to one another, consistent with previous studies (8). Although reducing sitting time alone may not eliminate excess risk of too much sitting, doing so appears to provide a similar amount of benefit as meeting current MVPA recommendations and sitting for 8 or more h/day. The third choice (Choice C) would be to increase MVPA to recommended levels (from 0 to 150 to 299 min/week) and reduce sitting from >8 h/day. Results from Figure 2 suggest that a combination of changes may produce more benefit compared with Choices A or B alone, and with greater reductions in sitting, the level of benefit nears that of the low-risk referent group. These findings clearly demonstrate that there are a variety of ways to lower risks associated with physical inactivity and too much sitting. The recently published Physical Activity Guidelines for Americans, 2nd Edition (7), has an increased focus on sedentary behavior and encourages us to “move more and sit less” for better health. Given that sedentary behaviors appear to be vastly outcompeting more healthy physical activity behaviors during our discretionary time, it is more important than ever to attend to our daily physical activity and sitting time and to try to optimize both behaviors for better health. To this end, the report from Stamatakis et al. (3) provides new and actionable insights for translating their findings to clinical and public health practice.
↵∗ Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
Dr. Matthews has reported that he has no relationships relevant to the contents of this paper to disclose.
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