Sedentariness is a behavior characterized primarily by sitting, with associated low levels of energy expenditure (20). Sedentariness is distinct from lack of moderate-to-vigorous physical activity because the former refers to too much sitting rather to too little exercise. In fact, high sedentariness and substantial physical activity may coexist because it is possible for an individual to meet or surpass the recommendations for minimum physical activity and to spend most of the rest of the day seated, either at work, during transportation, or at home (21).
During the last 50 yr, profound societal changes affecting the industrial and services sectors, transportation, urban planning, and communication technologies (e.g., TV, video, and the Internet) have led to increased sedentariness. As a result, the prevalence of excessive sitting time (ST) in most Western countries is very high (4), especially in older adults (15,27). This is important because ST is associated with poor quality of life (3) and higher risk of several chronic diseases (10), independently of the physical activity performed.
One way of measuring the overall effect of sedentary behaviors on health is through all-cause mortality. Recent research has shown that ST is associated with all-cause mortality in several cohorts in Canada (12), the United States (13,16,22), Australia (26), and Japan (11). However, these studies used a single measure of ST at baseline. Thus, they could not directly assess the effect of continued exposure to high sitting, or of changes in ST, on mortality. Information on the change in ST is of particular relevance because it may serve to estimate the effect of interventions aiming to decrease sedentariness.
Accordingly, in a population-based cohort of the Spanish elderly, we prospectively assessed the association of continued sedentariness and of changes in ST for 2 yr with subsequent long-term all-cause mortality.
Study Design and Participants
The study methods have been reported elsewhere (2,3). In short, this was a prospective population-based cohort study. In 2001, information was obtained from 4008 individuals representative of the noninstitutionalized Spanish population 60 yr and older. The information was collected at participants’ homes by trained and certified staff who conducted a health interview and a physical examination with standardized procedures. In 2003, an attempt was made to contact the subjects again, succeeding with 3235 (80.9%) of them. In 2003, information was collected by telephone interview by trained personnel. In 2012, we did a computerized search of the National Death Index, which contains information on the vital status of all residents in Spain, to identify all-cause deaths that had occurred in study participants (17).
We obtained written informed consent from each subject and a family member to participate in this investigation. The study was approved by the Clinical Research Ethics Committee of “La Paz” University Hospital in Madrid.
Sedentary behavior was estimated in 2001 and 2003 by the total number of hours per day spent sitting down based on the following question: “About how much time do you spend sitting down on weekdays? Please add up the total number of hours that you spend sitting down regardless of the activity that you do (eating, listening to the radio, watching television, reading, sewing, driving, etc.).” The same question was asked with reference to a weekend day (21). The number of sitting hours per day was calculated as follows: [(weekday ST × 5 + weekend day ST × 2) / 7]. The main exposure variable was ST from 2001 to 2003. We used the median of ST in the study sample as cutoff to define excessive ST. Accordingly, four categories of ST were defined for the period 2001–2003: consistently sedentary (above the median in 2001 and 2003), newly sedentary (equal or below the median in 2001 and above the median in 2003), formerly sedentary (above the median in 2001 and equal or below the median in 2003), and consistently nonsedentary (equal or below the median in 2001 and 2003). The outcome variable was all-cause mortality from 2003 through 2011.
We used information collected in 2003 on sociodemographic, lifestyle, and health status variables that could be associated with ST and mortality. Specifically, we collected data on age, sex, and educational level (no formal studies, primary, secondary, and university education). Regarding lifestyle, we registered tobacco smoking (never, former, and current smoker), alcohol consumption (never, former, moderate, and heavy drinker), weight, and height. Body mass index (BMI) was calculated as weight in kilograms divided by height in square meter. We also collected data on physical activity during leisure time with the Spanish version of the physical activity questionnaire used in the Nurses’ Health Study and the Health Professionals’ Follow-up Study (14). This questionnaire rates participation in 16 different activities. To determine the METs spent in each activity, the number of hours per week devoted to each activity was multiplied by its specific energy expenditure, taken from the compendium elaborated by Ainsworth et al. (1). Finally, the total volume of MET-hours per week was calculated as the sum of all MET-hours per week for all activities.
To assess health status, we obtained data on morbidity, health-related quality of life, and functional limitations. Study participants were asked to report any of the following diseases diagnosed by a physician in the period 2001–2003: chronic lung disease, ischemic heart disease, stroke, diabetes mellitus, osteomuscular disease, and cancer at any site. Quality of life was assessed using the 36-item Short Form (SF-36) questionnaire (28). We calculated the physical component summary (PCS), which summarizes the information mainly from five scales: physical functioning, physical role, bodily pain, vitality, and general health. The higher the score in the PCS, the better the health status. Mobility limitation was defined as an affirmative answer to any of the following three questions: 1) Do you experience any difficulty in picking up or carrying a shopping bag? 2) Do you experience any difficulty in climbing one flight of stairs? 3) Do you experience any difficulty in walking several city blocks (a few hundred meters)? 4) Last, agility limitation was defined as a positive answer to the question: Do you experience any difficulty in bending or kneeling? (8)
Of the participants followed until 2003, the following were excluded from the analysis: 245 who died between 2001 and 2003, 333 who did not report ST in 2001 or 2003, and 22 who lacked data on other study variables. Therefore, the analyses were conducted with 2635 individuals.
The association between ST from 2001 to 2003 and all-cause mortality from 2003 through 2011 was summarized with hazard ratios (HR) and their 95% confidence intervals (CI) obtained from Cox regression. Follow-up duration (number of days) was used as the time scale, which started at the date of the health interview in 2003 and continued until date of death or December 31, 2011. People who were consistently sedentary were used as the reference group in the analyses. Cox regression was adjusted for sociodemographic, lifestyle, and health status variables assessed in 2003. All variables were modeled with dummy terms, except age and physical activity (MET·h·wk−1), which were entered as continuous variables.
Given that consistent sedentary behavior and, in particular, increased sedentariness could result from incident disease, obesity, and functional limitations, which might confound the association between ST and mortality, we performed analyses stratified by such health disorders. Also, to rule out the role of preexisting subclinical disease in the study associations, we reran the analyses after excluding individuals who died during 2003 and 2004 (the first 2 yr of mortality follow-up).
Finally, we assessed the relationship between the duration of ST and all-cause mortality. To this end, we calculated the HR of mortality across quartiles of ST, calculated as the average ST in 2001 and 2003. We also obtained a P value for trend by modeling ST as a continuous variable.
We assessed the assumption of the proportionality of mortality hazards both graphically and by testing the significance of interaction terms for ST 2001–2003 and years of follow-up. No evidence was found of departure from the proportional hazards assumption (P > 0.1). Statistical significance was set at two-tailed P < 0.05. Analyses were performed with SAS 9.2 (24).
In the period 2001–2003, 22.9% of study participants were consistently sedentary, 21.5% newly sedentary, 20.2% formerly sedentary, and 35.4% remained consistently nonsedentary. Compared with those who were consistently sedentary, those who remained consistently nonsedentary were slightly younger, had lower BMI, and did more physical activity. They also had a lower frequency of chronic diseases and a better score on the PCS of the SF-36. Individuals who changed their sedentary behaviors showed intermediate values in the sociodemographic, lifestyle, and health status variables (Table 1).
Among the 2635 study participants, 846 (32.1%) died between 2003 and 2011. Table 2 shows the association between ST in the period 2001–2003 and subsequent all-cause mortality up to 2011. In comparison with consistently sedentary persons, those who remained consistently nonsedentary had a 25% lower mortality (HR = 0.75, 95% CI = 0.62–0.90). Individuals who changed ST experienced an intermediate reduction in mortality: newly sedentary and formerly sedentary individuals had, respectively, a 9% (HR = 0.91, 95% CI = 0.76–1.10) and a 14% (HR = 0.86%, 95% CI = 0.70–1.05) lower mortality than those who were consistently sedentary. Similar results were observed across strata defined by obesity, morbidity, or functional limitations (Table 3). Moreover, the results were similar after excluding the 198 individuals who died in the first 2 yr of follow-up (data not shown).
Figure 1 shows that the reduced mortality associated with consistently nonsedentary behavior and with changes in ST was observed early, specifically, in the first 2 yr of follow-up, and the survival benefit was maintained in the long term.
Figure 2 depicts the joint association between sedentary behavior in 2001–2003, physical activity in 2003, and subsequent all-cause mortality up to 2011. Consistent nonsedentariness was associated with reduced mortality at both below and above the median level of physical activity in the study cohort. Even among individuals with physical activity above the median (21.3 MET·h·wk−1 in men and 12.5 MET·h·wk−1 in women), consistently nonsedentary behavior was associated with a 42% lower mortality (HR = 0.58, 95% CI = 0.45–0.74). Changes in ST were also associated with reduced all-cause mortality in those who were more physically active.
Last, the average ST between 2001 and 2003 showed an inverse dose–response relationship (P for trend <0.001) with all-cause mortality from 2003 through 2011 (Fig. 3A). This relationship held regardless of the level of physical activity (Fig. 3B).
Our results show that in older adults, sedentary behavior for 2 yr is associated with increased long-term all-cause mortality independently of physical activity. In addition, our results suggest that the reduction of ST may lead to lower mortality.
Our study is unique in showing that in a period of only 2 yr, approximately 40% of older adults change sedentary behavior; of note is that 20% halved their ST from 6 h·d−1 in 2001 to 3 h·d−1 in 2003. Future research should identify the reasons for the reduction of ST and test whether this reduction can be achieved through appropriate interventions.
A few previous studies had shown that prolonged total ST is associated with increased all-cause mortality (11–13,16,22,26). However, these studies used only a single measure of ST at baseline, so they had to assume that ST is relatively stable over time. Our study confirms the excess mortality associated with sedentariness without requiring such assumption. It also extends the knowledge in this field because it shows for the first time that mortality in subjects who change ST is intermediate between that of people consistently sedentary and those who remain consistently nonsedentary. The fact that mortality associated with increased ST (from 3 to 7 h·d−1) is similar to that associated with reduced ST (from 6 to 3 h·d−1) suggests that the relevant exposure is cumulative ST, which is similar in those with increased and reduced ST (5.1 and 4.8 h·d−1, respectively).
Because participants in the study were elderly (mean age = 74 yr), it should be noted that the effects of ST, and especially the benefits of reduced ST, are seen from the early years of follow-up and are maintained in the long term. Therefore, most older adults, whose life expectancy is not compromised by a serious health problem, are likely to benefit from reducing ST.
Also important is that the benefits of being nonsedentary are observed even when physical activity is above the median in the study population. In our study, 72.8% of the energy spent in physical activity corresponds to walking, whereas only 5.8% corresponds to going to the gym and 0.4% to running. Because the median physical activity was 21.3 and 12.5 MET·h·wk−1 in men and women, respectively, this is equivalent to walking for 1 h·d−1 (3 MET·h) in men and just over half an hour in women, which meets or even exceeds the recommendations of minimum physical activity in older adults (5,18). Given that in our study the correlation between physical activity and ST was only modest (Spearman rho: −0.28), simply promoting moderate-to-vigorous physical activity should not necessarily result in less sedentariness. All these data suggest that recommendation addressed to older adults should include the reduction of ST (6) in addition to doing physical activity (5,18).
Evidence is growing that moderate-to-vigorous physical activity and sedentary behavior may act through different biological mechanisms, which could explain their independent effects on health (9,25). Specifically, sitting may be harmful because of the prolonged absence of muscle contractile activity in the legs. Prolonged sitting has been shown to produce alterations in metabolic risk factors (e.g., reduced insulin sensitivity, lower HDL cholesterol, and elevated triglycerides) (9,25). Sedentariness has been also linked to mitochondrial dysfunction, increased exposure to reactive oxygen species, and increased inflammation in older adults, which may lead to accelerated cellular death (23).
Given that the deleterious effect of sedentariness on mortality is biologically plausible, it is worth considering whether this association meets other criteria of causality. First, our study concurs with previous research (11–13,16,22,26) in that it is not likely that the association is due to systematic errors. In fact, it is independent of many potential confounders, including physical activity. Moreover, it seems not to be explained by preexisting subclinical disease leading to increased sedentariness; our study was able to assess change in ST and subsequent mortality, which may rule out reverse causation. Furthermore, the results were maintained among strata according to obesity, morbidity, or functional limitations and after excluding those who died in the first 2 yr of follow-up. Second, the observed association is of moderate magnitude. Similar to previous studies (11,12,16,22,26), we found a mortality reduction of approximately 25% in those with consistently nonsedentary behavior. Third, we observed an inverse dose–response relationship between cumulative ST and mortality. Therefore, it is reasonable to think that this association may be causal. Given that our study was not a randomized controlled trial, future research should include experimental studies to test if a reduction of ST is possible (7) and whether it leads to reduced mortality.
This study has several strengths and limitations. Among the former is that the data were taken from a population-based cohort representative of the older adults of a whole country, which allows for a wide generalization of results. Other strengths are the long follow-up, the complete ascertainment of vital status in the cohort, and the fact that the analyses were adjusted for many confounders. Among the limitations, the most important was that ST was self-reported. Unfortunately, no data are available on the reliability and validity of the ST questions that were used in this study. Moreover, comprehension and recall errors might have occurred and, in particular, ST could have been underestimated. However, its effect on study results could have been palliated by using the median sample to defined excessive ST. Also, a recent study using objectively measured ST has reported a direct association between sedentariness and mortality in adults (13), which is consistent with studies using self-reported ST. Moreover, in our own cohort, ST has been associated with poor health-related quality of life (3), which in turn predicted higher mortality (19). Notwithstanding this, our results should be confirmed in future research using validated questions and objective measures of ST. Another limitation was that the question on ST did not reveal the specific activity performed while sitting (eating, watching TV, reading, etc.); thus, our study cannot make specific recommendations to reduce sedentary behavior. Moreover, we lacked data on historical exposure (e.g., in middle age) to ST and physical activity, which might influence the study association. Lastly, in stratified analyses, the sample size was relatively small in some categories of ST, which has led to relatively imprecise estimations.
Our results have practical importance. First, in older adults, prolonged ST and specifically increasing ST should be a warning of a worse vital prognosis and should stimulate healthcare professionals to investigate the possible reasons for such sedentary behavior. Second, in contrast to the inevitable increase in sedentariness with age observed in cross-sectional studies, our findings show that approximately 20% of older adults reduce ST for 2 yr. If new research confirms that reducing sedentariness leads to lower mortality, this would encourage the design and evaluation of interventions for reducing ST in the elderly.
This work was funded by Fondo de Investigación Sanitaria (FIS) grants 11/01379 and 12/1166, FP7-HEALTH-2012 proposal no. 305483-2 (FRAILOMIC Initiative), and by the Cátedra UAM de Epidemiología y Control del Riesgo Cardiovascular. DMG had a “Juan de la Cierva” contract from the Spanish Ministry of Science.
The authors declare no conflict of interest.
The results of the present study do not constitute endorsement by the American College of Sports Medicine.
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Keywords:© 2013 American College of Sports Medicine
SITTING TIME; MORTALITY; OLDER ADULTS; SPAIN