The negative effect of sitting time on bone is mediated by lean mass in pubertal children

OBJECTIVE: Effects of time in moderate to vigorous physical activity (MVPA) and sitting (SIT) on bone were tested, hypothesizing that high MVPA would be positively associated with bone size and strength, offset effects of high SIT, and be mediated by lean mass. METHODS: 155 children (79 male, 58 pubertal), 6-20 years (10.2±3.5 yr) were measured by dual-energy x-ray absorptiometry (DXA) and peripheral quantitative computed tomography (pQCT). MVPA and SIT were measured by 7-day activity recall. Regression analyses tested effects of MVPA and SIT on bone. Mediation analysis was conducted to determine if lean mass mediated the effect of activity on bone. RESULTS: In pubertal boys, SIT was negatively associated with tibial endosteal and periosteal circumference as well as bone strength (β= -0.19, -0.14, and -5.68, respectively; all p<0.05). Effects of SIT on bone circumferences and strength were mediated by lean mass. MVPA did not offset the effects of SIT. In pubertal girls, MVPA was positively associated with cortical thickness (β=0.01, p<0.05) and the association was not mediated by lean mass. CONCLUSIONS: Current health communications that encourage increased physical activity should include additional messaging to decrease time spent sitting, especially in pubertal boys.