Sitting Time Not Associated With Risk of Death?

Sitting per se, whether during leisure time, watching TV, or at work, irrespective of physical activity, is not associated with risk of death, shows a new study of 5000 participants published online October 9 in the International Journal of Epidemiologyby Richard Pulsford, PhD, from sport and health sciences at the University of Exeter, United Kingdom, and colleagues.

The study tested the hypothesis that sitting time would predict all-cause mortality risk independently of moderate to vigorous physical activity and associations would vary by type of sitting.

Prior research has suggested that spending extended periods of time sitting down, either at work or during leisure time, is detrimental and contributes to mortality risk regardless of how much physical activity an individual undertakes, explained Dr Pulsford. Sitting has even been dubbed "the new smoking."

In the new study, which ran for 16 years — one of the longest follow-ups in this area of research — 450 deaths were recorded, but no evident associations between any of the indicators of sitting and mortality were found.

Dr Pulsford and colleagues say their results suggest that "policy makers and clinicians should be cautious about placing emphasis on sitting behavior as a risk factor for mortality that is distinct from the effect of physical activity."

"If, as suggested by our data, sitting itself does not represent a risk for early mortality, then perhaps our focus should continue to be to encourage people to be physically active, as only a minority of people are active enough to promote good health," they add.

Moreover, "the results cast doubt on the benefits of sit-stand workstations, which employers are increasingly providing to promote healthy working environments," said senior author Dr Melvyn Hillsdon, also from the University of Exeter, in a statement.

Asked to comment on the new findings, Brigid M Lynch, PhD, from the Cancer Epidemiology Centre, Cancer Council Victoria, Melbourne, Australia, and Neville Owen, PhD, from the Behavioral & Generational Change Program, Baker IDI Heart and Diabetes Institute, Melbourne, had some qualms.

"This cohort study has used questionable items to assess sedentary behavior. The bias — for example, misclassification bias in the self-reported assessment of sedentary behavior — is highly likely to have moved the results toward the null [and] effectively made the associations weaker," they observe.

Need to Examine Different Sitting Behaviors and Mortality

The previous studies have tended to focus either on selected single indicators of self-reported sitting, such as TV viewing, screen time, or traveling in a car, or have examined only total sitting combined and have observed differential associations with mortality, write Dr Pulsford and coauthors.

"As sitting is a behavior that we undertake in a number of different contexts of life, we took the novel step of investigating mortality risk associated with five separate sitting behaviors," explained Dr Pulsford.

These included sitting at work, sitting while watching TV, during leisure time excluding TV viewing, as well as a measure of all types of sitting combined.The new data were drawn from the Whitehall II study, a longitudinal trial of London-based employees of the UK Civil Service. The study, begun in 1985, invited 10,308 civil servants, aged 35 to 55, to participate. For the current analysis, a large cohort of 3720 men and 1412 women were identified with 16 years of follow-up.

The observation period for the current analysis started in 1997, which was the first time point the participants reported their sitting time.

Moderate to vigorous physical activity was recorded, including occupational or leisure-time physical activity — for example, sports, housework, and active transport. All activities that corresponded to an energy expenditure of over 3 METs were summed to give a measure of total moderate to vigorous physical activity.

Analyses were adjusted for age, gender, socioeconomic position, ethnicity, alcohol consumption, smoking status, general health status, physical functioning, diet quality, daily walking time, and moderate to vigorous physical activity. Dr Pulsford and his colleagues ran basic and fully adjusted statistical models.

"Across almost 16 years of follow-up, no prospective associations were observed between five different indicators of sitting time and mortality from all causes," they observe.

They add that the results were inconsistent with previous studies, which have shown positive associations between all-cause mortality risk and TV viewing, sitting at work, and total sitting time.

The authors discuss one possible confounder related to the fact that often, people working in London are relatively active compared with people living and working in other geographical areas of the United Kingdom: "This may have offered them a degree of protection."

This observation "is consistent with previous studies that have shown that when analyses are run separately for people with different levels of physical activity, associations between sitting and mortality in active people are greatly reduced or even disappear completely."

"This is perhaps inconsistent with the idea that sitting provides a risk for mortality that is independent of physical activity," added Dr Pulsford.

"We adjusted our analyses for covariates including walking and moderate to vigorous physical activity, but if levels of physical activity were high across the cohort, then simple statistical adjustment would not account for these," Dr Pulsford pointed out.

Don't Neglect Message on Physical Activity

Drs Lynch and Owen, who have previously published on the relationship between sitting behavior and mortality, remarked that these new Whitehall II findings are preceded by a dozen other epidemiological studies that have consistently identified an association between sitting time and all-cause mortality.

They also question the authors' suggestion that policy makers should be cautious in recommending a reduction in the time spent sitting without also promoting increased physical activity.

"At no time have sedentary-behavior researchers suggested that reducing sitting time alone is sufficient for good health," stressed Drs Lynch and Owen.

"The sedentary-behavior message is always framed within a context that also promotes engaging in the recommended levels of physical activity."

Asked about future plans for work in this field, Dr Pulsford said that the Exeter-based researchers were looking at the exact mechanisms by which small differences in lifestyle related to physical activity or postural allocation can benefit health.

"A better understanding of these things will be an enormous help for researchers like me and hopefully for public-health promotion as well."