Maher and colleagues report findings from a secondary analysis of accelerometer data collected as part of the US National Health and Nutrition Examination Survey (NHANES). They found no associations of sedentary time with cardiometabolic risk biomarkers, when controlling for a measure of “total physical activity” that was composed of time spent in both moderate- to vigorous-intensity and light-intensity physical activity (that is, all non-sedentary time), weighted by intensity (accelerometer counts). On this basis, they concluded that sedentary time may not have health effects independent of physical activity. Leaving aside legitimate concerns with interpreting null results in this fashion, a broader conclusion that might be drawn is that there is limited (or no) merit in searching for “independent” effects of behaviours that are unavoidably “interdependent”.

From our program of sedentary behaviour research – including evidence from cross-sectional and prospective observational studies, field experiments and laboratory studies – we conclude that energy expenditure and posture are the two key pathways by which sedentary behaviour may causally impact cardiometabolic health. Neither of these pathways would be detectable in the study by Maher and colleagues.

We postulate that sedentary behaviour (which involves very low energy expenditure) has adverse effects because it reduces total energy expenditure by displacing time that could be spent in alternative activities at a higher energy expenditure (predominantly light-intensity physical activity, because most adults engage in so little moderate-intensity activity and even smaller amounts of vigorous activity). In the case of the Maher et al study, independent effects examined by mutual statistical adjustment (i.e., each category is adjusted for all other categories) ignore effects that may occur because of such displacement.(1)

The potential importance of posture, supported by experimental studies from our group and others, suggests specific pathophysiological responses occur when seated and large skeletal muscle groups are not engaged.(2,3) The authors’ use of the NHANES accelerometer data classifies sedentary time based on intensity of vertical hip movement alone – a crude indicator of energy expenditure from ambulation that is not directly indicative of posture. Hence, these data are inherently limited in their capacity to detect effects that might be attributable to posture.

We agree with Maher and colleagues that previous approaches to sedentary behaviour analyses have limitations. However, their approach to gauging independent effects does not improve, nor does it discount in any material way, the current understanding of the health consequences of sedentary time. We have employed isotemporal substitution analysis to estimate the potential collective impacts of displacing time spent sedentary with time spent in light-, moderate- and vigorous physical activity.(4) These collective effects provide more relevant information than independent effects as, in practice, reduction in sedentary behaviour will naturally occur secondary to an increase in other activities (mostly light intensity physical activity).

Lastly, based on substantial scientific evidence, sedentary behaviour researchers have been meticulous in concluding that reduction of sedentary behaviour (i.e., increase in predominantly light-intensity activity) complements the promotion of moderate- to vigorous-intensity physical activity. This combined approach is acknowledged in physical activity recommendations by leading agencies such as the American College of Sports Medicine.(5) Unfortunately, Maher and colleagues have put forward an equivocal perspective that adds an unnecessary element of confusion to public health thinking.

Contributors to this response: David W Dunstan, Genevieve N Healy, Brigid M Lynch, N Owen, Nicola D Ridgers, Jo Salmon, Phillip B Sparling, Elisabeth A Winkler.

References
1. Mekary RA, Willett WC, Hu FB, Ding EL. Isotemporal substitution paradigm for physical activity epidemiology and weight change. Am J Epidemiol. Aug 15 2009;170(4):519-27.
2. Dunstan DW, Kingwell BA, Larsen R, et al. Breaking up prolonged sitting reduces postprandial glucose and insulin responses. Diabetes Care. 2012;35:976-83.
3. Peddie MC, Bone JL, Rehrer NJ, et al. Breaking prolonged sitting reduces postprandial glycemia in healthy, normal-weight adults: a randomized crossover trial. Am J Clin Nutr. Aug 2013;98(2):358-66.
4. Buman MP, Winkler EA, Kurka JM, et al. Reallocating Time to Sleep, Sedentary Behaviors, or Active Behaviors: Associations With Cardiovascular Disease Risk Biomarkers, NHANES 2005-2006. Am J Epidemiol. Feb 1 2014;179(3):323-34.
5. Garber CE, Blissmer B, Deschenes MR, et al. American College of Sports Medicine position stand. Quantity and quality of exercise for developing and maintaining cardiorespiratory, musculoskeletal, and neuromotor fitness in apparently healthy adults: guidance for prescribing exercise. Medicine & Science in Sports & Exercise. 2011;43(7):1334-59.