To evaluate how the reallocation of time between sleep, sedentary time, light, and moderate-vigorous activities is associated with children's body composition.

Population-based cross-sectional Child Health CheckPoint within the Longitudinal Study of Australian Children (n = 971 11-12 year-olds, 50% boys). Twenty-four hour activity composition via accelerometry (minutes/day of sleep, sedentary time, light, and moderate-to-vigorous physical activity [MVPA]) and 3-part body composition (percentage truncal fat, percentage nontruncal fat, and percentage fat-free mass) via bioelectrical impedance analysis were measured. We estimated differences in 3-part body composition associated with the incremental reallocation of time between activities, using dual-compositional regression models adjusted for sex, age, puberty, and socioeconomic position.

Reallocation of time between MVPA and any other activity was strongly associated with differences in body composition. Adverse body composition differences were larger for a given MVPA decrease than were the beneficial differences for an equivalent MVPA increase. For example, 15 minutes less MVPA (relative to remaining activities) was associated with absolute percentage differences of +0.9% (95% CI 0.7; 1.2) for truncal fat, +0.4% (0.3; 0.6) for nontruncal fat, and −1.3% (−1.7; −1.0) for fat-free mass, and a 15-minute increase was associated with −0.6% (−0.8; −0.5) truncal fat, −0.3% (−0.4; −0.2) nontruncal fat, and +1.0% (0.7; 1.2) fat-free mass. Reallocations between sleep, sedentary time, and light physical activity were not associated with differences in body composition.

Preventing declines in MVPA during inactive periods (eg, holidays) may be an important intervention goal. More MVPA, instead of other activities, may benefit body composition.