ESTRUCTURA DE LA OPERACIÓN a) Descripción general

Tremblay et al. (35) conducted a large scale systematic review in 2011, synthesising the evidence from 232 studies in school aged children. The main finding from this review was that more than 2h/day of TV viewing was unfavourably associated with body composition, fitness, self-esteem, pro-social behaviour and academic achievement. Despite the large volume of studies, few were of high quality with the vast majority being cross-sectional, used self-report or proxy-report measures of sedentary time which are prone to bias (49,128), and most of which capturing TV viewing, missing other SBs. Mitchell and Byun (129) reviewed studies exploring SB and health outcomes in children and youth (6-18 years old) from 2008-2012. The review included a diverse set of study designs (observational and experimental) that explored several health outcomes using both self-report and objective measures of sedentary time. Screen-based SB was positively associated with obesity and had an attenuated association with cardio- respiratory fitness and insulin sensitivity, mostly independent of MVPA. The links between screen-based SB and obesity was independent of dietary intake, and the links between screen-based SB and cardio-respiratory fitness and insulin were both independent of obesity. While screen time was consistently associated with several health outcomes like in the Tremblay et al. (35) review, objectively measured SB demonstrated inconsistent associations with health outcomes across study designs and particularly when controlling for MVPA. This review provided some of the first synthesised evidence in children demonstrating disparities between not only screen time and total sedentary time but also between self-report and objectively-measured sedentary time evidence in relation to health effects.

More recently, Carson et al. (42) conducted a comprehensive systematic review, exploring a holistic range of health outcomes, with different measures of SB, study designs, different types of SB, and different dimensions of sedentary time accumulation (total time, bout durations, frequency of breaks). The review included 235 studies with 1,657,064 children and adolescents across 71 countries. Like in the previous two reviews mentioned, TV viewing and/or screen time was found to be detrimental to almost all aspects of health examined. Conversely, non-screen-based reading and

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doing homework were beneficial to academic achievement. Computer and video game use was not consistently associated with any physical health indicator but positively associated with emotions and social health indicators. Taken together these findings further support the notion that different types of SB have different effects on physical and mental health outcomes and thus should be treated as individual behaviours. A gradient effect of screen time and health outcomes was observed in 73 studies and generally supported the current 2h/day screen time limit recommended for children in some countries (37,75,77). Consistent with Mitchell and Byun (129), in 35 identified studies using objective measures of sedentary time, there were no consistent associations found with total sedentary time, breaks in sedentary time or in bout lengths of sedentary time in any health outcome. Consequently, the authors stated that it was difficult to draw any conclusions on the maximum dose of total sedentary time for optimal health in young people. However, all studies measured sedentary time with hip-worn accelerometry which cannot distinguish between sitting and standing postures (23) and consequently it is unlikely that sedentary time will have been measured accurately. Furthermore, very few studies had explored patterns of sedentary time accumulation. Across all SB outcomes, the review identified largely observational studies, most of which were cross-sectional, and all low to moderate quality, with just two experimental studies identified. A large proportion of studies used self-report measures of SB that were not tested for reliability or validity (42). Furthermore, as screen time is continually changing via portable hand-held devices such as tablets and mobile phones, the nature of current screen-based behaviours may contrast to those measured within many of the studies in this review, somewhat reducing the validity of the evidence.

Another recent review (43) exclusively explored the associations of objectively measured sedentary time with health and development in 2-18 year olds. In 88 identified studies, all of which were observational (most were cross-sectional), 20-50% found an association in each outcome (adiposity, cardio-metabolic outcomes, fitness, bone/musculoskeletal health, psychosocial, gross motor skills, and cognitive outcomes). The authors concluded that there was ‘limited available evidence demonstrating that total sedentary time is associated with health and development in children and young people, particularly when accounting for MVPA or studies with low risk of bias’. All but one study used accelerometry to measure sedentary time and therefore the evidence is limited by the accuracy of this method. The authors also concluded that ‘without further

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experimental evidence testing subtle shifts from sitting to standing or LPA, it is premature to conclude that excessive SB does not adversely impact on health and development in children and adolescents’ (43).

These reviews highlight that, as with adults, screen time demonstrates a stronger association with health outcomes compared to total sedentary time in children and youths. The same potential screen-based mechanisms outlined for adults also apply to young people. TV viewing (particularly advertisements) as well as passive video game play would appear to influence an increase in calorie intake in children and adolescents (130,131) which will have implications for adiposity and cardio-metabolic outcomes. There are several possible reasons for the inconsistencies between sedentary time and health across studies in younger ages. Many studies are of low quality and cross- sectional (35,42), whereas in the adult literature, more high quality studies of longitudinal and experimental design have been implemented (102,106). Another theory is that children accumulate sedentary time differently to adults, often in shorter bouts, and frequently interrupting sitting periods compared to adults, which may therefore reduce the potentially hazardous influence of sedentary time (94). Furthermore, young people for the most part are relatively active, less sedentary, have had less years of exposure to SB or physical inactivity and are mostly free of NCDs compared to adults. Therefore, weaker associations should perhaps be expected between SB and health indicators at this stage of life (15).

It is worth noting that most research exploring the impact of SB on health have included nationally representative samples of children and adolescents (29) in western countries. These participants, who may be largely of White-European ancestry, are of reduced risk of cardio-metabolic outcomes compared to some ethnic groups (e.g. South Asian) (132,133). Furthermore, individuals with a family history of chronic conditions may have a more attenuated SB-health prospect (134). Consequently, higher health risk groups may have stronger associations between total sedentary time and health outcomes but are likely to be only a small proportion of samples in reviewed studies. There is also evidence that youths with less favourable health profiles have benefitted more from SB interventions compared to healthy individuals (135). In a recent systematic review and meta-analysis (135), SB interventions designed to reduce BMI in children and adolescents observed the greatest reductions in overweight populations, which the

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authors stated as ‘likely to be clinically significant at a population level.’ In terms of disease prevention policy, ‘proportionate universalism,’ whereby strategies are implemented for all children but with greater resources focused on higher risk groups (136), may be the most prudent approach in tackling SB for population health gain.

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1.9. Tracking and trends of sedentary behaviour, physical activity