Time spent in sedentary behavior has been linked to negative health outcomes such as obesity and dislipidemia in children and adolescents. Interrupting sitting time has been shown to improve metabolic health in adults; however no studies have been conducted with pre-adolescent children. Pre-adolescence is the time of steepest decline in physical activity, so this is a high-risk age group. Nevertheless, time spent in sedentary activities over the course of the day varies and this may be due to differences in the measurement of sitting time. Therefore, the first aim of this research was to critically review the objective and subjective measures of time spent in sedentary behavior in pre-adolescent children and to report the associations between time spent in sedentary activity and health outcomes. Understanding current levels of sedentary behavior and associated outcomes may reinforce policy to support interventions aimed at decreasing or interrupting time spent sitting. The review illustrated that pre-adolescent children spend a substantial proportion of time in sedentary behavior (3 – 8 hours per day) and that the measurement of sedentary behavior is complex, with time spent being sedentary being both over- and underestimated.

The second aim of this thesis was to investigate whether interrupted sitting with physical activity increased fat (postprandial) clearance in pre-adolescents after a high fat meal. Previous studies have suggested that intermittent activity may decrease lifestyle disease risk factors. Twelve children underwent two 8.5-hour testing sessions separated by seven days. On day 1 participants interrupted sitting with moderate intensity physical activity every 30 minutes, on day 2 participants remained sedentary. Four meals high in fat were consumed to ensure blood lipids were raised after consumption. Meals selected simulated, with exaggeration, unhealthy eating patterns in children. Blood tests were taken every second hour from 8.30am to 4.00pm to measure triacylglycerol (TAG), glucose, and cholesterol levels. When sedentary activity was interrupted by short bouts of moderate intensity physical activity there was a clear reduction in TAG concentrations in eight out of 12 participants. The combined responses showed higher levels of TAG on day 1 (intermittent sitting) than day 2 (continuous sitting). The difference was -3.2%; (90% confidence limits ±36.5), but the clinical significance of the effect was unclear.

The third aim of this thesis was to discuss the feasibility of methods in measuring postprandial lipemia in children. The protocol in interrupting sitting time with moderate-high intensity activities and frequent blood sampling was deemed feasible with this group of children. However, future studies with larger sample size are necessary to confirm results.