Sitting Kills, in a Dose-Response Fashion, but Regular Physical Activity Attenuates Risks

The hazards of prolonged sitting have increasingly been the source of scientific study, and new research confirms that sedentary time is indeed associated with increased all-cause and cardiovascular mortality among adults who engage in minimal to no physical activity. But in a new twist, investigators also showed that time spent each week doing moderate-to-vigorous activity appears to attenuate and even eliminate this relationship.

“Creating time for exercise daily would be optimal, but we know that this is hard for too many people,” lead author Emmanuel Stamatakis, PhD (The University of Sydney, Australia), told TCTMD in an email. “Fitting a 30-minute brisk walk in someone’s daily or nearly-daily routine would be great; if a dedicated walk is not an option because people are too busy, walking or cycling to work is another great option.

“Overall, taking any opportunity for movement is important—in particular movement that gets people out of breath such as fast walking, stair climbing, walking uphill, [and] carrying shopping,” he continued. “More intense physical activity embedded into everyday life needs no or very little time commitment and can enhance many aspects of people’s health, even if such episodes last for a minute or two only but are repeated a few times a day.”

For the study, published online ahead of the April 30, 2019, print issue of the Journal of the American College of Cardiology, Stamatakis and colleagues analyzed 8,689 deaths over a median 8.9 years from 149,077 participants enrolled in the 45 and Up Study, which measured both sedentary time and levels of moderate-to-vigorous physical activity (MVPA).

Sedentary time was significantly linked with MVPA with regard to all-cause mortality overall (P < 0.001). When participants were stratified by the amount of time per week they spend engaging in MVPA, those who were insufficiently active (1-149 minutes per week) or inactive were at an increased risk for all-cause mortality no matter how much time they spent sitting per week. Among those who met the lowest recommended physical activity time (150-299 minutes per week), only those who also sat more than 8 hours per day were at a statistically significant increased risk of all-cause mortality (HR 1.27; 95% 1.08-1.50).

A similar yet more profound dose-response relationship with sedentary time and cardiovascular mortality was observed among those engaging in less than 300 minutes per week of MVPA. Notably, for those deemed highly active (≥ 420 minutes/week MVPA), sedentary time did not seem to statistically affect either all-cause or cardiovascular mortality rates.

An analysis looking what would happen if sitting time was swapped out with physical activity, standing, and sleep found that each additional hour of daily sitting was associated with an increased risk of all-cause mortality among only those who sat at least 6 hours per day (HR 1.04; 95% CI 1.02-1.06). Spending more time standing was only associated with a small reduction in all-cause mortality risk among those who sit less than 6 hours per day (HR 0.97; 95% CI 0.96-0.99). Also, replacing sitting with walking and vigorous—but not moderate—physical activity led to a reduced risk of all-cause mortality, especially among those who sit a lot. Replacing sitting with sleeping did not seem to have an effect on mortality risk for those who sleep at most 7 hours per day but did increase this risk among those who sleep more than that.

‘Start From 10 Minutes a Day’

“First and foremost, our study emphasizes how important physical activity is for people who have to sit a lot, such as office workers, taxi and bus drivers, and call center workers,” Stamatakis said. “The bad news for high sitters is that replacing sitting with standing seemed to have no benefit. The good news was that even 20-40 minutes of fast walking per day, or half of that amount of strenuous exercise and sports, weakened or even offset the mortality risks of sitting.”

For those who struggle to even fit in 20 minutes of exercise per day, “it is okay to start from 10 minutes a day and at the same time reduce sitting,” he recommended. “Once the 10 minutes become a comfortable lifestyle habit, they [can] try to build up to 20 minutes of activity a day or more.”

Commenting on the study for TCTMD, Felipe Lobelo, MD, PhD (Emory University, Atlanta, GA), said “the take-home message . . . is that, of course, by itself sedentary time is an independent risk factor for all-cause and cardiovascular disease mortality, and obviously the more you sit per day, the higher the risk of mortality—so like a dose-response association, if you will. But you get a multiplicative effect if you're also doing no exercise.”

For someone who sits a lot and also does no physical activity, Lobelo likened the effects of being on a similar magnitude to someone who both smokes cigarettes and drinks alcohol heavily. “You get a double negative effect,” he said.

These results underscore the importance of physicians asking the right questions about physical activity and sedentary behavior of their patients, especially in a primary care setting. “[Ask] questions about: do you walk your dog every day, do you go for a regular bout of exercise every day or per week? And at the same time, what's the average number of hours that you spend sitting at work, sitting in the car, sitting while watching TV?” he advised. “The only group that will sort of get a pass, if you will, for spending too much time sitting are those who are doing a lot of exercise. Otherwise, you need to implement counseling for both—spending too much time sitting and not doing the recommended dose of exercise, that's 150 minutes per week.”

Translational Opportunities

In an accompanying editorial, Charles Matthews, PhD (National Cancer Institute, Rockville, MD), writes that the “findings in this report are consistent with previous studies showing stronger replacement associations for less active adults 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.”

Specifically, Matthews that there are “at least three opportunities to lower risk for inactive adults, with the ultimate choice dictated by personal preferences and a variety of socioecological factors.”

First, he explains, by increasing physical activity to recommended levels (from 0 to 150 to 299 min/week) without changes in sitting time (remain at 8+ hours), patients could “lower mortality risk substantially, but some residual risk due to too much sitting may remain.” Second, by reducing sitting time substantially (from 8+ to < 4 hours) without increasing MVPA, “this change could lower mortality substantially, but some residual risk due to physical inactivity may remain.”

Third, they could increase MVPA to recommended levels (to 150 to 299 minutes per week) as well as reduce sitting from > 8 hours/day. This option would “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,” Matthews writes.

Ultimately, there are “a variety of ways to lower risks associated with physical inactivity and too much sitting,” Matthews concludes, noting that the recently updated Physical Activity Guidelines for Americans have pushed for an increased focus on reducing sedentary behavior and moving more. “Given that sedentary behaviors appear to be vastly outcompeting more healthy physical activity behaviors during our discretionary time,” he says, “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.”