Dose-response associations between accelerometry measured physical activity and sedentary time and all cause mortality: systematic review and harmonised meta-analysis
BMJ 2019; 366 doi: https://doi.org/10.1136/bmj.l4570 (Published 21 August 2019) Cite this as: BMJ 2019;366:l4570Linked editorial
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We would like to commend Ekelund et al. on their recent systematic review and meta-analysis (1). This study presents evidence to support the dose response relationship between total physical activity, sedentary time and mortality in middle aged and older adults. A key strength of this study is the use of accelerometer measured physical activity and sedentary time. However, we would like to highlight what we believe is an important limitation of this study.
The authors have applied the same accelerometer thresholds for physical activity intensities (sedentary, light, low-light, high-light, moderate-to-vigorous and vigorous) to the whole study population. These thresholds have been derived from calibration studies based on small numbers of healthy young adults, often based on laboratory activity such as treadmill walking and not free-living activity. This limits applicability to the wider population including older adults and chronic disease, likely underestimating absolute physical activity levels in a proportion of the population (2). We recently highlighted this limitation in another study, where the same threshold for moderate-to-vigorous physical activity was applied to individuals with and without chronic disease.(3,4) Defining physical activity intensity for the whole population with a single accelerometer threshold, makes the strong (and we believe inappropriate) assumption that energy expenditure for a given physical activity is the same for every individual, and does not take into account an individual’s exercise capacity, age, or disease state, all of which have been shown to influence energy expenditure and accelerometer counts (5,6). The likely consequence is the misclassification of physical activity. Specifically, accelerometer values labelled as light intensity in low fit populations are more likely to represent moderate intensity preventing any firm conclusions about the benefits of light intensity physical activity.
The authors also failed to arrive at firm conclusions on the effect of bout duration for a given volume of activity. Again, we suggest this may reflect a limitation of current methods for determining the duration of a bout of activity. In the method adopted in the study a 20-minute continuous bout of physical activity at 1951 cpm would be not be classed as a bout of MVPA whereas 8 minutes (allowing for a 1-2 minute interruption) at 1952 cpm would be, even though the former would accumulate a greater volume.
Consensus regarding accelerometer data processing and improved methods for estimating the intensity of physical activity are required. We agree with future research recommendations made by Ekelund et al., for further calibration studies with use of tri-axial accelerometers and the need to consider individuals’ exercise capacity in the understanding and interpretation of physical activity behaviours, not only in the elderly but also chronic disease populations. This will help to better understand the relationships between physical activity and sedentary time with mortality and other health outcomes. This is an important consideration, as current physical activity guidelines consistently recommend the same levels of physical activity for adults irrespective of age and chronic disease, a recommendation which may not be appropriate.
References
1. Ekelund U, Tarp J, Steene-Johannessen J, Hansen BH, Jefferis B, Fagerland MW, et al. Dose-response associations between accelerometry measured physical activity and sedentary time and all cause mortality: systematic review and harmonised meta-analysis. BMJ [Internet]. 2019 Aug 21;l4570. Available from: http://www.bmj.com/lookup/doi/10.1136/bmj.l4570
2. Rejeski WK, Marsh AP, Brubaker PH, et al. Analysis and interpretation of accelerometry data in older adults: the LIFE study. J Gerontol A Biol Sci Med Sci 2016;71(4):521-8.
3. Dibben GO, Taylor RS, Dalal HM, Hillsdon M. One size does not fit all – application of accelerometer thresholds in chronic disease. International Journal of Epidemiology 2019; 48(4): 1380.
4. Barker J, Smith Byrne K, Doherty A, et al. Physical activity of UK adults with chronic disease: cross-sectional analysis of accelerometer-measured physical activity in 96706 UK Biobank participants. International Journal of Epidemiology 2019; 1-8.
5. Evenson KR, Wen F, Herring AH, et al. Calibrating physical activity intensity for hip-worn accelerometry in women age 60 to 91 years: The Women’s Health Initiative OPACH Calibration Study. Prev Med Rep2015;2:750-6. doi:10.1016/j.pmedr.2015.08.021 pmid:26527313.
6. Hall KS, Howe CA, Rana SR, et al. METs and accelerometry of walking in older adults: standard versus measured energy cost. Med Sci Sports Exerc 2013; 45(3):574-82.
Competing interests: No competing interests
Re: Dose-response associations between accelerometry measured physical activity and sedentary time and all cause mortality: systematic review and harmonised meta-analysis
We thank Dibben et al who found our recently published article examining the dose-response associations between sedentary time, physical activity and all-cause mortality of interest (1).
The issue about intensity thresholds for identifying time spent in different intensity levels has been a continuing debate for the last 20 years and there is still no consensus how appropriately defining the most optimal and generalizable intensity cut-points. We agree that defining intensity in absolute terms (METs) rather than in relative terms (i.e. as a proportion of maximal fitness) is a potential limitation, and did note in our manuscript that activity classified as “light intensity” (absolute scale) in the analyses may actually be moderate intensity (relative scale) for some individuals. Reassuringly, the results on the associations of sedentary time and physical activity with mortality from the three populations that included representative samples of younger age (ABC, NHANES, NNPAS) were similar to those from the overall meta-analyses (please see Supplementary Figure 1).
Dibben et al, in their comment argue ‘any firm conclusions about the benefits of light intensity physical activity’ cannot be derived from our study due to misclassification of physical activity (2). We analysed not only light and moderate intensity physical activity in relation to all-cause mortality but also separately defined ‘low’ light and ‘high’ light intensity to fully understand the associations between light intensity and the risk of death. The results presented in Table 2 and Figure 2 in our paper (1) clearly demonstrate the strong dose-response association between low light intensity with mortality. Thus, we respectfully disagree our results ‘prevent any firm conclusions about the benefits of light intensity physical activity.’ since low light intensity activity is likely to be truly low intensity for all individuals. (However, we do acknowledge that data from observational studies alone cannot prove cause and effect; these have to be supported by other lines of evidence, including plausible mechanistic studies.)
We also respectfully disagree with Dibben et al that the results from our study are not applicable to older adults and those with chronic diseases (2). Older adults, of whom many had chronic diseases made up the majority of the participants in our meta-analysis (adjusted for in our Cox regression models). The strongest association between physical activity and mortality was observed for total physical activity (total activity counts / wear time), which includes activity of any intensity. The absolute difference in deaths between the least active (lowest) quarter and the most active (highest) quarter was almost fivefold, suggesting the importance of total physical activity, regardless of intensity, in middle-aged and older adults including those with chronic diseases.
We agree that further research is needed in this area; as science progresses and more data become available, the exact dose-response associations between total physical activity and different intensities of physical activity with mortality, as well as other health outcomes, will emerge and may differ from those in our study (1).
References
1. Ekelund U, Tarp J, Steene-Johannessen J, Hansen BH, Jefferis B, Fagerland MW, et al. Dose-response associations between accelerometry measured physical activity and sedentary time and all cause mortality: systematic review and harmonised meta-analysis. BMJ [Internet]. 2019 Aug 21;l4570. Available from: http://www.bmj.com/lookup/doi/10.1136/bmj.l4570
2. Dibben GO, Taylor RS, Hillsdon M, Dalal HM. BMJ 2019;366:l4570
Competing interests: No competing interests