Association Between Various Sedentary Behaviours and All-Cause, Cardiovascular Disease and Cancer Mortality

The Multiethnic Cohort Study

Yeonju Kim; Lynne R Wilkens; Song-Yi Park; Marc T Goodman; Kristine R Monroe; Laurence N Kolonel

Disclosures

Int J Epidemiol. 2013;42(4):1040-1056. 

In This Article

Discussion

In this large cohort study, sitting ≥10 h/day, compared with sitting <5 h/day, was associated with an increase in all-cause mortality of 11% for women, but no association was seen for men. However, among both men and women, all leisure-time sitting activities were associated with an increased risk, and the risk was strongest with TV watching. The observed increased risk of all-cause mortality among participants sitting watching TV ≥5 h/day was 19% in men and 32% in women, compared with watching TV <1 h/day. Sitting ≥3 h/day vs <1 h/day in other leisure sitting activities or at meals was also associated with 6–7% and 22% increased risks of all-cause mortality, respectively, among both men and women.

An association between total daily sitting and all-cause mortality was examined in three previous cohorts.[5,13,27] In two studies, risks were increased by 19% (≥9 vs <3 h/day)[5] and 45% (≥11 vs 0– < 4 h/day)[27] for all-cause mortality among healthy men and women who were between 50 and 71 years old[5] or older than 45 years,[27] respectively. Another study, which included men and women aged 18–90 years who had cancer or heart disease, showed an increased risk of 1.54 ('sitting almost all the time' vs 'almost none'), which may have been inflated due to prior disease.[13] There is a lack of consistency in the literature in the categories used for analyses of sitting time, which certainly could have contributed to inter-study differences in the strengths of the observed associations. The subjects in the present study were somewhat older at the time the information on sitting duration was collected than those in other studies which have reported higher HRs than ours.[13,27]

In contrast to the studies that examined total daily sitting, there are several prior prospective studies that examined time spent sitting watching TV.[5,6,11,14–16] Results from a meta-analysis of three prospective studies, including Whites only, showed a 13% increase in the risk of all-cause mortality for every 2 h of TV watching.[9,11,14,16] In three additional cohort studies, all-cause mortality was increased between 17% (≥6 vs <2 h/day) and 61% (≥7 vs <3 h/day) among men and/or women who sat the longest watching TV[5,15] or in leisure time generally, including watching TV.[6] Sitting while watching TV has been associated with unhealthy eating, such as higher intake of high-fat and high-calorie foods, energy-dense drinks and lower consumption of fruit and vegetables.[28] In contrast, although not specifically focused on watching TV, a recent report suggested that sedentary leisure time behavior was an independent cardiovascular disease risk factor, regardless of snacking habits and physical activity.[29] In our study, increased risk of mortality with longer durations of sitting watching TV was observed across levels of diet, smoking and physical activity. It appears unlikely that reverse causality due to undiagnosed diseases inflated the risks in the current study, since the association between mortality and sitting watching TV was observed even after excluding deaths that occurred up to 5 years after entry into the study.

In this study, hazard ratios of all-cause mortality according to daily hours of sitting were weaker in men than in women, largely due to the stronger association between sitting watching TV among women. Previous cohort studies have shown inconsistent results by sex for sitting watching TV: one showed similar results to ours,[6] another found no difference by sex[16] and a third found stronger associations in men than in women.[15] It is possible that men and women behave differently while sitting watching TV in terms of eating, MET level of sedentary activity or number of breaks in sedentary activity.

Some studies showed that prolonged leisure sitting increased cardiovascular mortality after adjustment for various confounders, including physical activity,[5,6,11,16] which is consistent with our finding. A cohort study of Japanese men and women reported on mortality from specific cardiovascular disease and found that mortality from ischaemic heart diseases (51%) and from cerebrovascular diseases (21%) was increased among men who sat longer than 4 h/day watching TV (vs <2 h/day), whereas there was no association among women.[15] This is in contrast to our results that found elevated risks for these causes among men and women.

The relationship between sedentary behaviour and cancer mortality has been inconsistent, with some studies showing no association,[11,13,16] and some larger studies finding increased cancer mortality associated with longer sitting duration watching TV,[5] or time spent at leisure.[6] Our study also found a weak elevated risk of total cancer mortality associated with sitting watching TV, but not with overall sitting. Associations for mortality from specific cancers and sitting watching TV were similar, although a stronger risk with increased sitting was observed for prostate cancer. Results from a prospective study conducted in Japan suggested that risks for some cancers were increased in persons who sat longer watching TV: liver cancer among men and women, lung cancer among men and non-Hodgkin's lymphoma among women.[15]

We found no association of longer sitting at work with all-cause mortality. We did not collect information on specific sitting activities at work, and it is possible that work-related sitting activities contribute to a greater energy expenditure compared with other types of sitting activities. Reported estimates range from 1.5 METs (sitting for light office work) to 2.5 (sitting for moderate work), which are higher values than for other sitting activities such as sitting watching TV (1.3), sitting riding in a car or a bus (1.3) and sitting for eating (1.5).[3] Although some previous studies examined the relation between mortality and occupational physical activity based on job category,[30–32] no prior prospective studies, to our knowledge, have examined the independent role of occupational sitting on mortality in a general population sample. However, longer occupational sitting was correlated with a higher prevalence of overweight and obesity among 1579 full-time employed men and women after adjustment for age, sex, occupation category and physical activity in a cross-sectional study.[33] A challenge in studying the relation of occupational sitting and mortality is controlling for socioeconomic status (SES) and health behaviours, since people of higher SES tend to have healthier lifestyle behaviours and to show a lower risk of mortality, although they also are more likely to work in sedentary occupations.

Increased risk for all-cause mortality with time spent sitting watching TV was consistent among subgroups defined by age, education, diabetes/hypertension prevalence, smoking status, BMI, physical activity and diet. All ethnic groups except Japanese Americans also showed an increased risk for all-cause mortality with time spent sitting watching TV. Previous cohort studies from Japan showed that men who were most sedentary had an increased risk for all-cause mortality, whereas being sedentary was not related to mortality among women.[12,15] It is possible that Japanese Americans might behave differently while watching TV compared with other ethnic groups, in such as taking more breaks or doing light activities while watching TV. This speculation should be examined in further studies.

Strengths of our study included the prospective design, large number of subjects, racial/ethnic diversity and ability to control for many potential confounders, including physical activity. Because of the large sample size and number of deaths, we were able to conduct stratified analyses. Also, since we had collected information on several types of sitting behaviour, we were able to examine the effect of each one separately.

However, a limitation is our reliance on self-reported sedentary behaviours. Questionnaires used to assess physical activity and sedentary behaviours in large-scale epidemiological studies are known to contain substantial errors.[34] Also it is possible that the time categories provided on the questionnaire were too broad to categorize individuals' activities well. Validation of the MEC activity questionnaire was done by comparison of its estimate of energy expenditure with that from doubly-labelled water. We are unable to validate sitting times specifically using this technique. However, as sitting activities occupy an average of 47% of the time in a typical day in our population, it seems unlikely that good agreement on total energy expenditure would be seen without agreement on total sedentary/sitting time or if the duration categories were too broad to distinguish activities. We also compared the responses to this questionnaire with those to another study conducted by direct interview and administered at about the same time to several overlapping participants. The interviewers were trained to probe about different types of activities to ensure a full day was covered, so the level of enquiry was more intense. We found reasonable agreement between responses on sitting activities between the two separate methods. Therefore, indirect evidence points to the sitting times being reasonably accurate, with total sitting time likely to be complete.

Another limitation of this study is that we did not assess details about current employment status at baseline, factors which would affect the association between sitting at work and mortality. A weak inverse relation between hours sitting at work and all-cause mortality was found among men and women younger than 65 years at baseline when those who reported 'zero' hours sitting at work were included in the reference group, suggesting a healthy worker effect where those who have work longer at sedentary jobs are healthier and at lower risk of death. Therefore, in an effort to minimize classification bias of employment status, we included only subjects who reported some sitting time at work in that analysis. Although we were able to adjust for many potential confounders, such as smoking, education, physical activity and other sitting activities, we cannot rule out the possibility that residual confounding could lead to biased results, due to unknown confounders or imprecise adjustment, such as for SES.

Previous reports suggested that resting can suppress lipoprotein lipase activity in skeletal muscle,[35–37] resulting in metabolic consequences such as dyslipidaemia, insulin resistance, hypertension and obesity.[38–42] In addition, survey results have shown that leisure sitting time increased an average of 15 min/day from 2003 to 2009 in the USA,[10] but that physical activity levels did not increase over the same period.[10,43] More research is essential on not only how to increase physical activity but also how to reduce the amount of leisure sitting time,[5,6] and on whether the pattern of sitting makes a difference.[44] Intervention studies to reduce and to break up sedentary time are reported to be effective in reducing total sedentary time and increasing energy expenditure,[45,46] and in decreasing postprandial glucose and insulin levels.[47] It seems likely that future investigations which incorporate both self-reports and device-based measures will improve assessments of time, postures and energy expenditure associated with various sitting activities.[34,48,49]

In conclusion, we found that longer time spent sitting, especially sitting watching TV, was associated with an increased risk of all-cause and cardiovascular disease mortality, but not of cancer mortality, after adjusting for physical activity and several other potential confounders. The association with sitting watching TV was consistent in men and women; in Whites, African Americans, Native Hawaiians, and Latinos; and in individuals at different BMI and physical activity levels. Interestingly, longer sitting at work or in a car or bus was not associated with increased mortality risk. Our results suggest that the amount of time spent in voluntary sitting should be limited.

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