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We thank Chaput and colleagues1 for their comments on our recent review2 that examined the timeliness of sitting guidelines and provided a critical overview of the sedentary behaviour evidence base. To recap for the reader: we argued that evidence of sitting and health is limited and we urged caution to prevent enthusiastic but premature guideline development. Chaput and colleagues argued that the sedentary behaviour evidence base is adequate and that a provisional screen time-based benchmark is better than no benchmark at all. We see four main areas for debate and discuss them in turn.
Dispute #1: ‘activity mixes’ and ‘reconceptualisation’ of physical activity in public health
Initially, our respected international colleagues1 proposed that the field needs to move towards ‘activity mixes’ that take into account the 24-hour compositional nature of the physical activity, sedentary behaviour and sleep data. Consider a weekend day when a person goes for a 3-hour bike ride. That reduces potential sitting time by 3 hours, so these physical activity elements, moderate to vigorous physical activity (MVPA) (bike ride), and sedentary behaviour are inter-related. It is beyond doubt that the use of this novel and promising statistical approach is a positive development and we acknowledged in our review2 that 24-hour compositional data might become an important addition to the evidence base, once (a) we have enough longitudinal studies to interpret; and (b) methods are known for translating such data into meaningful public health messages. However, it is still early days. Chaput et al base their case on four cross-sectional studies with surrogate endpoints1 as the evidence base for a quantitative guideline on sedentary time. With very few exceptions,3 this summarises much or the compositional data analysis evidence base to date, to our knowledge. Besides, these studies largely replicate what is already known about the relationships between sedentary time, physical activity and health.4 Stronger epidemiological evidence also requires long follow-up times and hard clinical endpoints, diverse populations, reliable and valid measures, replication of the findings across studies and settings, combined with underpinning mechanistic and experimental data.
Dispute #2: how realistic is 1 hour of MVPA per day?
The authors further argue that the amounts of physical activity required to offset the harms of sitting are excessive and unrealistic and that only 5% of US adults do even 30 mins/day. We speculate that this 5% was probably based on objective assessment of PA (no citation was provided). In the Ekelund Lancet study5, around 25% of the participants self-reported >60 min of MVPA a day, supporting the feasibility of 1 hour of MVPA per day for large segments of the population. Besides, studies by both the Ekelund et al (joint analyses)5 and Ekelund et al (stratified analyses)6 showed that associations between sitting time and mortality are attenuated with lowered levels of MVPA than 1 hour/day. For example, the association between sitting and cardiovascular disease mortality in the second MVPA quartile (25–35 min/day MVPA) was limited to the top sitting group (>8 hours/day), and there was little evidence for dose response.6
Dispute #3: are screen time guidelines the way forward?
We were surprised that the main argument put forward by Chaput et al 1 in favour of sedentary guidelines focused on screen-based guidelines. First, different screen-based behaviours appear differentially associated with health and TV watching is the only screen behaviour consistently associated with adverse longitudinal outcomes. As we elaborated in our review,2 TV time is a very poor marker of sitting time and is likely to be heavily influenced by confounding factors. Besides, in the fast-changing media landscape, which involves multiple screen devices, TV-based evidence is outdated and almost obsolete. As such, we feel that the points raised about screen time as a starting point for sedentary behaviour guidelines are unsubstantiated.
Dispute #4: low-risk guidelines or guidelines that will bring benefit?
Finally, Chaput et al suggest that public health guidance on sitting is ‘low-risk’. We respectfully disagree with the argument that something is better than nothing because too much information, too many guideline changes, can overwhelm those who should take action. An unproven and fragile sitting guideline will distract policy-makers and the public alike from the true culprits: the absence of movement in people’s lives in general and the absence of opportunities to get out of breath. The solution will comprise interventions, environments and systems7 that support more incidental physical activity and regular exercise.
We conclude that a ‘provisional benchmark’1 on-screen time based on a handful of cross-sectional compositional studies and the weak evidence base from TV viewing studies,2 an increasingly obsolete behaviour, is premature and will cause confusion. Our standpoint is consistent with the 2018 Physical Activity Guidelines for Americans,8 which includes a non-quantitative recommendation for sitting time and recognises that the sedentary behaviour evidence base needs to develop further. The recommendation for move more, sit less and accumulate 150–300 min of MVPA per week is based on the best available evidence.
Contributors ES and UE conceived the idea and agreed on key points. ES: drafted the manuscript. All authors redrafted parts of the manuscript and approved the final version before submission.
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.