The Perils of Passive Sitting

How much do Americans sit?

We Americans sit a lot. A new study used data from the National Health and Nutrition Examination Surveys from 2001 – 2016 to find out just how much. Not counting sitting while at work, in 2001 the average American adult spent 5.5 hours per day watching TV or videos or using a computer. By 2016, that number increased to 6.4 hours per day.

Accurately determining how much time Americans spend sitting is not easy. Self-reports are subject to bias, because being sedentary may be perceived as socially undesirable. Wrist-worn devices have their own problems, including under-counting certain types of movement. Researchers developed a different approach using data from a nationally representative group of 2,640 participants (average age 45 years) in the AmeriSpeak panel. Participants reported time being physically active, being sedentary, and sleeping over the previous 24-hour day. A week or two later, participants were asked to report again on the previous 24-hour day. Half of the participants reported spending 9.5 hours per day (at work plus leisure) in sedentary behaviors, which was 34 percent higher than that from a common surveillance method. Participants aged 70-79 had the highest amount of daily sedentary behavior (10.9 hours). Most of the average daily amount of leisure-time sedentary activity (5.2 hours) involved electronic media (TV, video viewing, computer) in the evening.

Can lots of physical activity cancel the negative effects of lots of sitting?

Overwhelming evidence shows that Keep Moving reduces our risk of chronic diseases and premature death. Considerable evidence also shows that sitting for many hours has the opposite effects. Would people who sit many hours per day and who engage in high levels of physical activity escape the downside health risks of lots of sitting? A recent study answered this question. An international team of researchers conducted an analysis of 16 published reports that evaluated mortality risk in conjunction with amounts of physical activity and sitting time. The studies included over a million subjects with follow-up periods ranging from 2-18 years.

Overall, the risk of premature death increased as the number of daily hours of sitting increased. For the three lowest levels of weekly moderate-intensity physical activity (less than 6 minutes per day, 39 minutes per day, 73 minutes per day), the risk of premature death increased as the number of sitting hours increased. But at the highest level of moderate-intensity physical activity (more than 87 minutes per day), the risk of premature death did not increase as daily sitting time increased to more than 8 hours.

This is huge! We might eliminate the adverse effects of lots of sitting if we get more than an hour and a half of moderate-intensity daily exercise. The analyses were repeated using number of hours of TV watching as a measure of sitting time. The results were similar except that the highest level of physical activity greatly reduced but did not fully eliminate the risk of premature death. Watching TV appeared to be more detrimental than simply sitting. This difference might arise from adverse effects on blood sugar and blood fats when plopping down in front of the TV soon after eating dinner. Especially if TV watching induces eating snack foods. If you watch a lot of TV, do yourself a favor and trade one hour of daily TV time for one hour of brisk walking or some other form of moderate-intensity physical activity.

The above study evaluated the risk of all-cause mortality across ranges of sitting and TV viewing times in relation to levels of physical activity. A follow-up analysis by Stamatakis and colleagues used data for 149,077 participants who were followed for a median of 9 years in the 45 and Up Study in Australia. Like the previous meta-analysis, as sitting increased for participants who did not get the recommended minimum amount of weekly physical activity (150 minutes of moderate-intensity movement), the risk of all-cause mortality increased. But getting at least the minimum amount of weekly physical activity eliminated the association between sitting less than 8 hours per day and risk of mortality. For participants who sat six or more hours per day, replacing one hour of sitting with one hour of moderate- or vigorous-intensity physical activity predicted 20 and 64 percent reductions in cardiovascular-related mortality, respectively. If you sit a lot, please consider trading one hour of sitting time each day for one hour of moderate or vigorous physical activity. You’re likely to live longer and better that way.

An editorial by Charles Mathews in the Journal of the American College of Cardiology commented on the article by Stamatakis and colleagues. Matthews noted that those of us who sit a lot may be able to escape the increased risk of premature death and cardiovascular disease events linked to excessive sitting. Choice A: Increase physical activity up to about 300 minutes per week at moderate to strenuous intensity. Choice B: Decrease sedentary behavior to less than 4 hours per day. Choice C: Both increase physical activity and decrease sedentary behavior. Current evidence suggests that Choice C could cancel the increased risks of premature death and cardiovascular disease events linked to lots of sitting.

Some types of sedentary activities are worse than others

Some types of sedentary behaviors might even be healthful. Researchers in the UK used data from 502,643 participants with an average age of 56 years in the UK Biobank to determine if different types of sedentary behaviors predicted different cognitive responses after an average follow-up of 5 years. Sedentary behaviors included television watching, driving, and non-occupational computer use. Cognitive measures included visual-spatial memory, fluid intelligence, and short-term memory. Cross-sectional data at baseline showed that four or more hours of daily television watching or three or more hours of daily driving predicted poorer cognitive function across all outcomes. Conversely, three or more hours of daily computer time predicted better cognitive function across all outcomes. After adjustment for confounding factors, longitudinal data showed that both more television watching and more driving at baseline predicted modest but significant declines across most cognitive outcomes during follow-up. The reverse was true for three or more hours of daily computer use. The detrimental effects of increased television watching or driving were stronger for older men (who would do well to cut back on TV viewing and spend more time on mentally engaging activities, such as using a computer).

Sit less for better cardiometabolic health

The Coronary Artery Risk Development (CARDIA) study enrolled participants aged 18 to 30 years at four centers around the US as part of a long-tern investigation. Follow-up examinations occurred every 2-5 years thereafter including years 20 and 30. The coronary artery risk factors included high waist circumference, systolic and diastolic blood pressure, glucose, insulin, triglycerides and low HDL-cholesterol. Participants wore accelerometers for a week at years 20 and 30 to measure physical activity. From year 20 to 30, the average participants’ sedentary time increased by 37 minutes per day as did all six cardiovascular risk measures. Researchers asked if substituting 30 minutes of sedentary time each day for 1,922 participants at year 20 with 30 minutes of physical activity each day would decrease cardiovascular disease risk at year 30. In fact, substituting 30 minutes of light physical activity for 30 minutes of sedentary activity predicted significantly lower waist circumference and serum insulin. When either moderate- or strenuous-intensity physical replaced sedentary time, both serum insulin and triglycerides dropped significantly. This study provides more reasons to move more and sit less. Even light physical activity provides health benefits.

Sitting at work, sitting otherwise

We Americans sit a lot during our waking hours. More time sitting predicts worse health outcomes. It’s not clear how the health effects of leisure time sitting compare to sitting at work. A new study addressed this issue for 3,592 Black American adults enrolled in the Jackson Heart Study. Researchers categorized daily television viewing as 1) less than 2 hours per day, 2) 2-4 hours per day, and 3) more than 4 hours per day. The categories of occupational sitting included 1) never or seldom, 2) sometimes, and 3) often or always. Compared to participants who watched less than 2 hours of TV per day, those who watched more than 4 hours per day had a significant 49 percent higher risk of all-cause mortality, independent of a host of confounding factors. In contrast, occupational sitting did not predict risk of all-cause mortality. Interestingly, participants who engaged in more than 150 minutes per week of moderate-intensity physical activity per week and who watched more than 4 hours of TV per day did not have a higher risk of all-cause mortality. Getting lots of moderate physical activity reduced the harmful effects of lots of TV viewing. Cutting back TV viewing to less than 4 hours per week and devoting the equivalent amount of time to moderate exercise might greatly reduce your risk of premature death.

Sitting and dementia

Increased sedentary behavior predicts increased risk of all-cause mortality and cardiovascular disease. The associations between sedentary behaviors and dementia are less clear. Researchers investigated links between certain types of self-reported sedentary behavior and risk of dementia irrespective of amounts of self-reported physical activity. Data came from 146,651 participants in the UK Biobank with an average age of 65 years and free of dementia at baseline. Sedentary behaviors were categorized as either watching TV or spending time on a computer. After an average follow-up of 12 years and after adjusting for confounding factors (including time spent in physical activity), more time spent watching TV predicted a significant 4 percent greater risk of developing dementia. On the other hand, more time spent on a computer predicted a significant 15 percent lower risk of developing dementia. Thus, sitting time as such may not be the boogey man. Rather, passive sitting (watching TV) with little mental stimulation may be detrimental, whereas engaging in mentally stimulating activity while sitting at a computer may be healthful. Thus, swapping an hour of TV viewing for an hour of computer time may reduce your risk of dementia.

People who sit a lot have greater risks of health problems including mental deterioration. Researchers used data from 49,841 participants with an average age of 67 years in the UK Biobank to see if being highly sedentary increased the risk of dementia. Wrist-worn accelerometers measured the amount, lengths, and frequencies of daily sedentary behaviors. The risk of dementia increased sharply with more than 12 hours of daily sedentary behavior. Specifically, compared to participants with the median daily amount of sedentary behavior (9 hours), participants with 10, 12, and 15 or more hours of daily sedentary behavior had significant 8, 23, and 221 percent greater risks, respectively, of developing dementia over 7 years of follow-up. The lengths and frequencies of bouts of sedentary behavior did not predict dementia risk after accounting for the daily amount of sedentary behavior. While this study could not determine if being highly sedentary caused dementia, a prudent response would be to find ways to reduce your sitting time.

What's a healthy person to do?

Given the negative consequences of lots of passive sitting, I suggest that you trade an hour of passive sitting time (outside of work) each day for something healthier. How about replacing an hour of TV watching each day with brisk walking in three 20-minute segments after each meal? Your cardiometabolic health will likely improve plus you'll feel better.

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