Sedentary behaviour and obesity: review of the current scientific evidence

1. Sedentary behaviour is not simply a lack of physical activity but is a cluster of individual behaviours where sitting or lying is the dominant mode of posture and energy expenditure is very low. 2. Sedentary behaviours are multi-faceted and might include behaviours at work or school, at home, during transport, and in leisure-time. Typically, key sedentary behaviours include screen-time (TV viewing, computer use), motorised transport, and sitting to read, talk, do homework, or listen to music. 3. Total time spent in sedentary behaviours can be captured by objective monitoring devices, such as accelerometers and inclinometers. The former can quantify the amount of time spent below a predetermined threshold of movement, and its temporal patterning across the day. Inclinometers can quantify time spent in different postures by distinguishing between lying, sitting and standing. 4. Self-reported sedentary behaviour instruments can ask respondents to report frequency and duration of time spent in different behaviours, such as TV viewing and computer game playing, over a specific time frame. 5. UK self-report data suggests that the majority of young people have ‘acceptable’ levels of TV viewing, but about one-quarter to one-third watch 4 hours per day or more, levels generally considered excessive. 6. Data on computer game playing by young people show more variability, but with up to 60% playing for more than 1 hour/day. These trends are changing rapidly and it is increasingly the case that technologies are converging. 7. According to accelerometer data, UK youth appear to spend about 420460 minutes per day in sedentary behaviour, which is about 60-65% of measured time. 8. Self-report estimates of sedentary behaviour show that approximately two-thirds of adults spend more than 2 hours per day watching TV and using the computer. 9. Significant proportions of adults report sitting for more than 5 hours per day (including work and leisure-time), and adults report spending between 3-4 hours per day sitting during their leisure-time. 10. Sedentary behaviours appear to track from childhood to adolescence or adulthood at low to moderate levels, with the strongest tracking shown for TV viewing. 11. The technological landscape is rapidly changing and evolving (for instance TV viewing on computers or internet access on TVs). This has implications for the interpretation of results from studies that may become rapidly dated. 12. Some countries have guidelines for sedentary behaviour. However, there is little or no justification given in the vast majority of recommendation documents for any time limit concerning sedentary behaviour. 13. There is a greater risk of obesity in young people with high amounts of sedentary behaviour and TV viewing at a young age being predictive of overweight as a young adult. 14. There is a positive association between sedentary time and markers of metabolic risk in young people. 15. Sedentary behaviour for adults is associated with all-cause and cardiovascular mortality, diabetes, some types of cancer and metabolic dysfunction. 16. The prospective association between sedentary behaviour and gain in body weight or the development of obesity is less clear. 17. Variables that are associated with screen-viewing in young children, and may be possible to change, include family TV viewing, snacking, body weight, parent viewing, and having a TV in the bedroom. 18. Higher BMI and depression are associated with screen-viewing in adolescents. 19. Screen-viewing tends to differ in young children by age, gender and SES; for adolescents by age, gender, ethnicity, SES, parent education; for young people by age, SES, single parent household, and ethnicity. 20. Sedentary behaviours in adults are associated with age, gender, socioeconomic conditions, occupation, weight status, and some characteristics of the physical environment. These relationships are independent of level of overall physical activity. 21. TV viewing in young people and adults is associated with a higher energy intake and poorer diet. 22. Interventions to reduce sedentary behaviour in young people, with or without the goal of changing weight status, show promise. However, given the paucity of evidence on modifiable correlates of sedentary behaviour, clear strategies to bring about successful behaviour change are still not known. 23. There is almost no evidence concerning sedentary behaviour interventions with adults. 24. Four recommendations suggest that the UK summary statements on physical activity: 1). should contain a specific recommendation that children and young people, adults, and older adults should aim to minimise the time they spend being sedentary each day; 2). should not set a quantified target for sedentary time (for people of school age and above) but should emphasize minimising time spent being sedentary each day; 3). should include specific recommendations for limiting sedentary time among children of pre-school age. These should be developed and agreed by the early years expert group; 4). should suggest the strategies to reduce sedentary behaviour.