July 27, 2016

expert reaction to study investigating the effects of activity and sitting time on mortality

Publishing in The Lancet journal a group of researchers have carried out a meta-analysis reviewing the effects of sedentary behaviour and physical activity and report that around 60-75 minutes of moderate intensity activity can mitigate the increased risk of death associated with high sitting time.

Prof. Steven Cummins, Professor of Population Health, London School of Hygiene & Tropical Medicine, said:

“This important study shows that it is possible to reduce the effects of prolonged sitting by undertaking 60-75 minutes of moderate physical activity per day. Though measures of physical activity used are self-reported, the prospective nature of the analyses along with the consistency of the findings makes a good case for a possible causal link.

“The reality is that most of us have relatively sedentary occupations, which makes undertaking the amount of moderate physical activity recommended by the authors seem like a challenge. However, there are simple and effective ways to increase everyday physical activity. For example ditching the car and using more active ways of getting to work, through using public transport or walking or cycling, has been shown to increase physical activity and improve cardio-metabolic health.”

Dr David Nunan, Departmental Lecturer and Senior Researcher at the Centre for Evidence Based Medicine, University of Oxford, said:

“This new study by Ekelund and colleagues reports that the increased risk of early death associated with too much sitting is mitigated in adults who say they do 60 to 75 minutes of moderate vigorous intensity physical activity (MVPA) per day, suggesting that with enough of the right intensity of daily physical activity there is little risk of prolonged sitting for early death. The study does however suggest that the WHO recommendations of 30 minutes MVPA per day may not be sufficient to eliminate this risk.

“However, there are a number of strengths and weakness associated with the study as well as further nuances that need to be taken into account when considering the wider adoption of the findings. For example, the authors found that specific types of sitting activity, such as TV viewing time, was still found to increase people’s risk of early death regardless of how much daily MVPA people said they performed, although this risk was attenuated with higher levels of activity. Furthermore, paradoxically, the authors fail to point out that no amount of physical activity was found to provide a protective effect (reducing the risk of early death).

“That said, one of the strengths was that the study performed was a systematic review of prospective cohort studies including over one million participants and follow-up of 2 to 18 years. Additional strengths include the re-analysis of original data, thus reducing the methodological and statistical heterogeneity between studies, and the use of official registries for ascertainment of mortality outcomes.

“However, there are also weaknesses. For example, it is possible that the authors may have missed relevant studies by excluding non-English studies and using only one reviewer for screening titles and the quality of included studies was not discussed in detail.

“The effect sizes referred to throughout are relative estimates and it is important to consider the absolute effects. For example, whilst the authors report a 59% relative increase in risk of death in people who state sitting for over 8 hours a day and doing less than 2 hours of MVPA compared with people sitting for less than 4 hours a day and doing 35.5 hours of MVPA a week, this confers an absolute increase in risk of 3.1%, meaning 32 people would need to be sedentary and inactive to this degree for there to be one additional death.

“Given the nature of included observational data, there is likely to be residual confounding, something that is acknowledged by the authors. For example, socioeconomic status has a big impact in cohort studies such as those included in this review; only two of the included studies appear to control for this factor in their analysis. As acknowledged by the authors, reverse causality is an additional confounding factor that was not controlled for. This would likely result in exaggerated findings.

“As the authors acknowledge, there is an issue of recall bias by the use of self-report to assess physical activity and sedentary time. Indeed, some of the surveys used asked people to recall their average daily sitting time from the previous year. It is known that using these methods people often underestimate sedentary time and overestimate physical activity level when compared with objective measures.

“Given the above, any discussion of attenuating benefit of physical activity at the individual level must be tempered by the observational nature and quality of included data and the absolute benefit. However, in consideration of its strengths and weaknesses, it is encouraging that this study suggests the negative associations of too much sitting with early death due to all-causes, cardiovascular disease and cancer can be attenuated, and in some cases mitigated, by participation in daily physical activity of 30 minutes or greater. As the greatest gains were observed for highest physical activity and lowest sitting and TV-watching time, we should continue to support promotion of daily physical activity alongside strategies to reduce time in sedentary activities.”

Dr Erika Borkoles, Senior Lecturer in Sports Science, Bournemouth University, said:

“The research is of excellent quality. The hypothesis for the meta-analysis is a good one. It employed harmonised methodology, which is still rare in such research and it makes the findings powerful. It is good to see that sitting time and TV viewing is separated and analysed accordingly in relation to physical activity intensity. The categorisation of physical activity is somewhat arbitrary, but the four categories are related to daily recommended physical activity guidelines across the world. The conclusions are backed up with solid data, but causality cannot be established, only associations.

“The paper made a significant contribution to the existing literature and showed that there is an independent association between physical activity and mortality (in all that were examined). Highest bands of physical activity are significantly associated with reduced risks of mortality, even though sedentariness is present. The unique point of this paper is that they don’t treat activity separately from sedentariness, but authors acknowledge that they co-exist (e.g. one may have a very sedentary job – drivers or office workers), but being physically active (high band) significantly attenuates health outcomes.

“The authors acknowledged the cofounders. Given the amount of variables they had to deal with and harmonised, they appear to account for confounders presented in the 16 data sets. It would have been good to know mental health and job descriptions (they both contribute to high sedentariness).

“The strength of the paper is also that it is acknowledged that reducing sedentariness doesn’t necessarily equate increasing physical activity. The paper does provide robust evidence based on sample size, methodology employed and stringent inclusion criteria for the study, including collaboration with the study authors.

“The real implication is that being active at moderate levels but for sufficiently long time (60-75 per minute a day) is associated with reduced health risks. However, sedentariness also needs to be tackled (not just helping people become more active), such as reducing sitting and TV viewing time with other activities (e.g. playing a guitar, or any other leisure activities that are not just involve sitting).

“People who sit for longer hours should be concerned and could take the findings of this research as an opportunity to be proactive. If they are in a job that requires prolonged sitting, exiting research suggest that they should stand up and walk about at least once an hour if they can.

“People who have problems, such as recovering from stroke (e.g. balance) or other neurological diseases should be advised to try to walk about and do as much as they feel they can standing despite balance and fatigue problems. The same for mental health patients, although they should seek advice from their doctor. However, those who’s daily job requires 8 hours or more sitting, should have time to walk about at least once in an hour. Also standing desks can be used or any other activities the person prefers to break up the sitting time.”

Dr Carolyn Greig, Senior Lecturer in Nutrition and Ageing, University of Birmingham, said:

“This is a very interesting study which uses a considerable amount of data to try and address the question of whether high physical activity can offset the (mortality) risk associated with increased sedentariness. The answer would appear to be that yes, in middle aged and older adults in particular environments, it does. However the finding that moderate intensity for one hour per day does not completely eliminate the risk associated with substantial amounts of TV viewing time, provides some very interesting insights into this particular, possibly more risky type of sitting behaviour and points to further mechanistic studies to investigate the metabolic consequences of excessive TV watching as well as interventions to actively reduce it.

“It is well known that sedentariness increases with advancing age and there are many middle-aged and older adults who simply are not sufficiently active to completely offset the risk of prolonged TV viewing, which seems to be a particularly ‘risky’ form of sitting time. What is encouraging is that there is still some protection to be gained in those who are doing less, but still enough to meet the current physical activity guidelines, and this provides more support to existing public health messages to be both more active and less sedentary, including in older age.”

‘Does physical activity attenuate, or even eliminate, the detrimental association of sitting time with mortality? A harmonised meta-analysis of data from more than 1 million men and women’ by Ekelund et al. published in The Lancet on Wednesday 27th July. 

Declared interests

Prof. Steven Cummins: No conflicts of interest

Dr David Nunan: Dr Nunan is a lead member of the Royal College of General Practitioners (RCGP) steering committee to support the new Physical Activity and Lifestyle clinical priority. Dr Nunan has received funding for research from the NHS National Institute for Health Research School for Primary Care Research (NIHR SPCR) and the RCGP for independent research projects related to physical activity and dietary interventions. The views expressed are those of the author and not necessarily those of the NHS, the NIHR, the Department of Health, or the RCGP.

Dr Erika Borkoles: No conflicts of interest.

Dr Carolyn Greig: No conflicts of interest.