Sitting in your easy chair watching TV may be toxic
Sitting at while typing on your computer may be less so
Spending lots of time being sedentary (often evaluated as either sitting or watching TV) predicts increased risks of premature death and chronic diseases. Given that physical activity is the opposite of being physically inactive, could physical activity counter the adverse effects of being sedentary? A meta-analysis of 16 studies provided a nuanced answer: Yes, but. The analysis showed that, for both sitting and watching TV time, the risk of all-cause mortality increased steadily across four categories of decreasing physical activity. The categories included 1) less than one hour, 2) 1-2 hours, 3) 3-4 hours, and 4) more than 5 hours per day. Compared to those in the two lowest quartiles of physical activity (less than 30 minutes per week of moderate-intensity physical activity), those who sat for more than 8 hours per day had a 59 percent higher risk of all-cause mortality.
But here’s great news: Those who sat more than 8 hours per day and got lots of daily physical activity (at least 60-75 minutes per day at moderate intensity) did not have increased risk of all-cause mortality. TV viewing for 3-5 hours per day predicted increased risk of all-cause mortality except for those who got lots of daily physical activity. Only above 5 hours of daily TV viewing did the risk in all-cause mortality increase significantly regardless of physical activity level. Thus, paring TV viewing to less than 3 hours per day and getting 60-75 minutes per day of moderate-intensity physical activity may negate the adverse effects of sitting and watching TV.
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, 2) 2-4 hours, and 3) more than 4 hours. 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 49 percent significantly 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 spent more than 4 hours per day watching TV did not have a higher risk of all-cause mortality. Getting lots of moderate-intensity physical activity moderated 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-intensity exercise might greatly reduce your risk of premature death.
The studies noted above suggest that sedentary activities are nuanced regarding bad health outcomes. Small but important differences might not appear in studies that rely on self-reported activity. A new study from New Zealand investigated subtle differences in sedentary behavior. Participants included 348 relatively young, apparently healthy women who wore accelerometers that objectively measured waking-hour movements for seven consecutive days. Sedentary activity was subdivided into four categories in order of increasing activity: 1) lying, 2) non-active sitting, 3) active sitting, and 4) standing. Non-active sitting included being plopped down in an easy chair, while active sitting included working on a computer at a desk. Participants’ body measurements and blood samples provided data for physical status and metabolic biomarkers. Total sedentary activity was not related to body composition and metabolic biomarker data.
However, both lying and non-active sitting were correlated with increased body-mass index, body mass, fat mass, and waist-to-hip ratio. On the other hand, active sitting and standing were negatively correlated with these same variables. In addition, lying and inactive sitting were correlated in unhealthy directions with insulin and the hunger hormone leptin. Active sitting and standing were correlated with insulin, leptin, and HDL-cholesterol in a healthy direction. Thus, minor differences in sedentary activities may affect your risk of developing chronic diseases. Participants in this study spent 58 percent of sedentary time spent inactively sitting and only 26 percent of sedentary time actively sitting. Moving a chunk of time from sitting in the easy chair to something else—even working on your computer—might improve your health.