Watson A, Maher C, Tomkinson GR, et al. - Given that increased fatness and decreased fitness in children is seen during the summer holiday period (by emerging evidence), and these patterns may be attributable to alterations in diet and/or time use (eg, less activity, more eating, and more screen time may be observed in children during the school holidays vs the in-school period), therefore, researchers have planned this investigation to measure rates of shift in fatness and fitness of children, originally in Grade 4 (age 9 years) across three successive years and to associate these shifts to alterations in diet and time use between in-school and summer holiday periods. Invitation for participation will be sent to Grade 4 Children visiting Australian Government, Catholic and Independent schools in the Adelaide metropolitan region, to select 300 students. The Automated Self-Administered 24-h Dietary Assessment Tool will be used to determine diet reported by parents. By means of 24-h wrist-worn accelerometry and self-report by children using the Multimedia Activity Recall for Children and Adults (eg, chores, reading, sport), they will ascertain time use. At the commencement (Term 1) and close (Term 4) of each school year and during the summer holiday period, measurement of diet and time use will be obtained. At the start and close of each school year, measurement of fitness (20-m shuttle run and standing broad jump) and fatness (body mass index z-score, waist circumference, %body fat) will be obtained. Novel and efficient avenues for intervention efforts directed at addressing childhood fitness and fatness may be informed by revelations from this project. Interventions directed on the home environment, or alternatively extension of the school environment may be justified.