In this large prospective cohort, women who reported sitting for more than 6 hours during their leisure time versus less than 3 hours a day had an approximately 40% higher all-cause death rate, and men had an approximately 20% higher death rate. This association was independent of the amount of physical activity. The combination of both sitting more and being less physically active (>6 hours/day sitting and <24.5 MET-hours/week physical activity) was associated with a 94% and a 48% increase in all-cause death rates in women and men, respectively, compared with those who reported sitting the least and being most active (<3 hours/day sitting and ≥52 MET-hours/week physical activity).
Our findings for time spent sitting are consistent with those from the 3 other studies that have previously examined the association between time spent sitting and mortality (21
). One study included approximately 17,000 Canadian adults with 1,832 deaths, and the authors reported a significant dose-response relation between a qualitative measure of time spent sitting (almost none of the time, one fourth of the time, half of the time, three fourths of the time, almost all of the time) and total mortality (21
). The second study, which included 8,800 Australian adults and 284 deaths, found an almost 50% increase in total mortality with 4 or more hours of television viewing compared with less than 2 hours per day (22
). In both of these studies, associations were strongest for cardiovascular disease mortality (21
). The third study included approximately 83,000 Japanese adults and reported a positive association with sedentary behavior and total mortality among men, but not women (23
Our physical activity findings were similar to those reported from the majority of other studies (1
). Mortality rates were approximately 25% lower among men and women who reported the most versus the least daily physical activity. Although optimal health benefits are achieved at a much higher level of physical activity, death rates were substantially lower even in the second lowest category compared with the lowest category, suggesting a benefit from even relatively light levels of physical activity. As mentioned, the participants in our study were older and engaged in primarily light-intensity activities, such as walking for exercise and gardening. It should be noted that no previous study has examined the combined effects of sitting time and physical activity.
Several factors could explain the positive association between time spent sitting and higher all-cause death rates. First, time spent sitting might be more easily measured than physical activity and/or may reflect a different aspect of inactivity than other indices usually used in epidemiologic studies. However, this potential misclassification of exposure is unlikely to fully explain our findings, because time spent sitting was significantly associated with mortality even among men and women with the highest levels of physical activity.
Second, time spent sitting might be associated with other unhealthy behaviors that are either not captured or incompletely captured through questionnaires. Total energy expenditure is reduced among individuals who are sedentary. However, consistent with previous studies, the present study found no correlation between physical activity and time spent sitting (r
= −0.03). Time spent sitting is also associated with greater food consumption and subsequent weight gain, especially when watching television (16
). Time spent sitting was previously shown to be associated with increased weight gain in this cohort (18
). While residual confounding by obesity could contribute to the association between sitting time and mortality, this association was attenuated but not eliminated by controlling for or stratifying on body mass index.
Third, prolonged time spent sitting, independent of physical activity, has important metabolic consequences that may influence specific biomarkers (such as triglycerides, high density lipoprotein cholesterol, fasting plasma glucose, resting blood pressure, and leptin) of obesity and cardiovascular and other chronic diseases (8
). Animal studies have also shown that sedentary time substantially suppresses enzymes centrally involved in lipid metabolism within skeletal muscle, and low levels of daily life activity are sufficient to improve enzyme activity (36
). Furthermore, substantial evidence in both adults and children from observational studies and randomized clinical trials shows that reducing time spent sitting lowers the risk of obesity and type II diabetes (19
Over the past century, a number of technologic changes have contributed to a decrease in total daily energy expenditure. For example, during the 2006–2007 broadcast year, the average US household reported 8 hours of television watching per day, which is an increase of 1 hour per day of television watching from only a decade ago (43
). Although leisure-time physical activity levels have remained relatively constant over the past few decades (44
), it is well recognized that technologic advances in the workplace have also greatly reduced occupational physical activity. This reduction in overall physical activity, in conjunction with increased time spent sitting and higher caloric intake, has contributed in large part to the rise in obesity and likely influenced temporal trends in cardiovascular disease, type 2 diabetes, and some cancers.
The strengths of our study include the large sample size, prospective design, and ability to control for many potential confounding factors. The lack of occupational physical activity data is a potential limitation; however, we believe this to have minimal impact on daily physical activity levels because the majority of study participants were retired/homemakers (57% of men and 80% of women) and, among those that were not retired, few worked in jobs that involved any activity (21% of men and 7% of women). Because we measured only leisure time spent sitting, the lack of occupational sitting time may have underestimated sitting time among working individuals, since much of their sitting time may have occurred at work. However, adjusting for employment status (employed, retired, or homemaker) did not change risk estimates for time spent sitting or physical activity. Furthermore, we conducted a sensitivity analysis among only men and women who were retired or homemakers, and results were virtually identical to those in the overall cohort. Another limitation is the use of self-reported measures of time spent sitting, physical activity, and all other covariates including height and weight. Although the physical activity and sitting time questions we used are subject to misreporting, they are very similar to those used and validated in the Nurses’ Health Study II, a prospective study with similar participant characteristics, which found a correlation of 0.79 between activity reported on recalls and questionnaire (46
). These measures have also been associated with various cancers in this cohort (47
). Finally, we were not able to differentiate between types of sitting (i.e., while watching television, reading, driving), and the energy expenditure and other behaviors may vary with different types of sitting.
In conclusion, we found that both leisure time spent sitting and physical activity are independently associated with total mortality. Associations were stronger for cardiovascular disease mortality than for cancer mortality. Public health messages and guidelines should be refined to include reducing time spent sitting in addition to promoting physical activity. Because a sizeable fraction of the population spends much of their time sitting, it is beneficial to encourage sedentary individuals to stand up and walk around as well as to reach optimal levels of physical activity.