How Much Physical Activity Offsets the Bad From Prolonged Sitting?

April 23, 2019

It may be appealing to think that standing instead of sitting will counter the damage regular, prolonged sitting inflicts on one's overall survival, but it also might be wrong, suggests an analysis based on a large cohort study.

To do that, it further suggests, one would have to replace the sitting time with exercise of at least moderate intensity, and with vigorous-intensity exercise if the goal is to diminish the excess cardiovascular (CV) mortality risk. But even walking seemed to help in the analysis.

When the numbers were crunched a different way, the excess all-cause and CV mortality risks caused by regular, prolonged sitting — 6 or more hours a day — were seen primarily in people who achieved less than 150 minutes per week of physical activity.

The excess risks were substantially countered in those who maintained the lower guidelines-recommended range of physical activity for improving survival, 150 to 200 minutes per week. And they were all but erased for those achieving the higher recommended physical activity range, at least 300 minutes per week.

"The key message from both of those analyses is that even meeting the lower physical activity recommendation seems to somehow offset the risks that come with sitting," Emmanuel Stamatakis, PhD, University of Sydney, Australia, told | Medscape Cardiology.

Those risks "cannot be seen outside the context of physical activity," he said. "Replacing sitting with standing is not enough. We need to be replacing sitting with movement. Even walking may do the trick in the case of all-cause mortality," said Stamatakis, who is lead author on the study published April 22 in the Journal of the American College of Cardiology (JACC).

TV and Computer Time

Opportunities for replacing sitting time with physical activity might well focus on what another research group described as "screen-based sedentary behaviors," that is, on "time spent sitting watching television or videos and computer use outside school or work."

Estimated total sitting time rose significantly for both adolescents and adults in the United States from 2001 to 2016 in an analysis based on the National Health and Nutrition Examination Survey (NHANES), reported April 23 in JAMA.

But the increase didn't occur across the board for all types of screen watching, observed the authors, led by Lin Yang, PhD, University of Calgary, Alberta, Canada. The estimated prevalence of television-viewing sitting time of at least 2 hours a day remained about stable overall throughout the period.

"Meanwhile, the estimated prevalence of computer use during leisure time of 1 h/day or more significantly increased among all age groups, with a more pronounced increase among adults," the group writes.

"The substantial rise in total sitting time among adolescents and adults appears to be attributable to sedentary behaviors other than television or video watching, which was likely driven in part by the observed increases in computer use."

Replace Sitting With Exercise, But at What Level?

The exercise-for-sitting analysis in JACC was based on about 150,000 women and men in New South Wales, Australia participating in the 45 and Up longitudinal study. Its findings support the lower-range and upper-range physical-activity levels similarly recommended in current guidelines from both Australia and the United States, the authors note.

The participants completed a questionnaire at baseline that reported sitting times and activity levels, among other characteristics.

Over median follow-ups of 8.9 years for all-cause mortality and 7.4 years for CV mortality, there were 8689 deaths, including 1644 from CV disease, the group reports.

Participants were grouped by their reported activity level, which could be "highly active" (≥420 min/week), active at the upper recommended level (300 - 419 min/week), active at the lower recommended level (150 - 299 min/week), insufficiently active (1 - 149 min/week), or inactive (no physical activity).

Within those separate categories, participants were classified as "low sitters" (<4 or 4 to <6 h/day) or "high sitters" (6 to <8 h/day or ≥8 h/day).

With participants who were highly active and who reported sitting less than 4 h/day serving as the index comparator group, those active at least 150 to 299 min/week appeared to be protected against a significant rise in all-cause mortality, at all daily sitting levels except the highest, at least 8 h/day.

That risk was elevated at all levels of sitting for those who were inactive or active only 1 to 149 min/week in a dose-response manner — that is, the longer the sitting time, the greater the risk compared with those in the lowest-sitting, highly active group.

The pattern was similar for CV mortality, with the exception that the lower recommended activity level of 150 to 299 min/week was not protective for participants sitting at least 4 h/day.

For low sitters (6 or fewer h/day), the adjusted hazard ratio (HR) per hour of sitting was nonsignificant for all-cause mortality at 1.01 (95% CI, 0.99 - 1.02) and for CV mortality at 1.02 (95% CI, 0.99 - 1.04).

But they were significantly elevated for high sitters: 1.04 (95% CI, 1.02 - 1.06) for all-cause mortality and 1.07 (95% CI, 1.03 - 1.12) for CV mortality.

In the substitution analysis, replacing an hour of sitting with an hour of standing slightly improved outcomes among low sitters but not high sitters.

For those sitting 6 or fewer hours per day, switching 1 hour of sitting for 1 hour of standing was associated with an adjusted all-cause mortality HR of 0.97 (95% CI, 0.96 - 0.99) and CV mortality HR of 0.94 (95% CI, 0.91 - 0.98).

Also among low sitters, it took an hour of vigorous physical activity as replacement for 1 hour of sitting to marginally lower all-cause mortality, but it pushed CV mortality down more sharply. Switching in only moderate physical activity didn't lower either kind of mortality in this group.

But high sitters benefitted more from trading an hour of sitting for an hour of activity, showing benefits when the activity was vigorous, moderate, or merely walking.

Hazard Ratio (HR) Effect of Replacing 1 Hour of Sitting With 1 Hour of Different Physical Activities in People Sitting at Least 6 Hours per Day (High Sitters)*
Activity All-Cause Mortality HR (95% CI) Cardiovascular Mortality HR (95% CI)
Standing 0.98 (0.95–1.01) 0.97 (0.92–1.03)
Walking 0.78 (0.70–0.87) 0.90 (0.73–1.12)
Moderate physical activity 0.97 (0.92–1.02) 0.80 (0.70–0.93)
Vigorous physical activity 0.69 (0.56–0.88) 0.36 (0.17–0.74)
*Adjusted for sex, age, education, marital status, urban vs rural residence, body mass index, smoking status, self-rated health, fruit and vegetable consumption, receiving help with daily tasks for a long-term illness or disability, psychological distress, activity class, and total time in all activity classes.

Leisure-time sitting may represent up to half of waking sedentary time in many American adults, whereas less than one-fourth report engaging in leisure time physical activity at least 300 minutes per week, observes Charles E. Matthews, PhD, National Cancer Institute, Rockville, Maryland, in an accompanying editorial.

The current report "provides new and actionable insights for translating their findings to clinical and public health practice," he writes. "These findings clearly demonstrate that there are a variety of ways to lower risks associated with physical inactivity and too much sitting."

Matthews provides an example for someone who doesn't engage in any physical activity and sits "at work, commuting, or watching television" for more than 8 hours per day.

The current analysis suggests that for such a person, "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 proposes, increase physical activity to the lower recommended range of 150 to 299 min/week. "Results from Stamatakis et al. suggest this change could lower mortality risk substantially, but some residual risk due to too much sitting may remain."

Or, "reduce sitting time substantially (reduce from 8+ to <4 h/day) without increasing moderate to vigorous physical activity," he writes. The findings "suggest this change could lower mortality substantially, but some residual risk due to physical inactivity may remain."

Interestingly, Matthews writes: "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 moderate-to-vigorous physical activity recommendations and sitting for 8 or more h/day."

A third choice "would be to increase moderate-to-vigorous physical activity to recommended levels," that is, to 150 to 299 min/week, "and reduce sitting from >8 h/day."

More simply, if there is an overarching message from the analysis, Stamatakis said when interviewed, it may be that "if you have to sit too much, make sure that you find ways to incorporate more moderate to vigorous physical activity into your life."

Screen Time in NHANES

The screen-time analysis in JAMA was based on NHANES data from 2001 to 2016 for children up to age 11 years, and from 2003 to 2016 for adolescents through age 19 years, and adults age 20 years and older. The cohort consisted of 51,896 individuals (50% female) — 10,359 children, 9639 adolescents, and 31,898 adults — after exclusion of anyone with functional limitations.

The mean estimated prevalence of sitting while watching television or videos for at least 2 h/day in data from 2015 and 2016 was 62% for children, 59% for adolescents, and 65% for adults. Within the group of adults, the mean estimated prevalence was 62% for those aged 20 to 64 years and 84% for those 65 and older.

Those prevalence numbers from 2015 and 2016 represented a mean decrease of 3.4% for children (P = .004), no significant change for adolescents or for adults aged 20 to 64, and a 3.5% increase (P = .03) for adults 65 and older compared with data from 2001 to 2004.

In contrast to those "generally stable" trends in television- or video-watching sitting time, the group writes, sitting time of at least 1 h/day involving computer use other than for work or school climbed significantly across the two periods in all age groups.

In children, such computer time rose from 43% in 2001 to 56% in 2016 (P < .001). It went from 53% in 2003 to 57% in 2016 among adolescents (P < .002). And it jumped from 29% in 2003 to 50% in 2016 for all adults (P < .001).

Those differences in computer time from the earliest to the latest periods probably, at least in part, drove the increases in total sitting time observed from 2007 to 2016, the authors propose. Total sitting time went up by 1.1 h/day among adolescents and by 1.0 h/day among all adults (P < .001 for both differences).

Stamatakis discloses receiving an unrestricted grant from PAL Technologies; disclosures for the other authors are in the report. Matthews reports that he has no relevant disclosures. Yang and colleagues reported no industry conflicts.

J Am Coll Cardiol. 2019;73:2062-2072. Full text, Editorial

JAMA. 2019;321:1587-1597. Full text

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