Aim: To examine bone mass changes in 321 black and white South African children in relation to habitual physical activity (PA) levels and calcium intakes.
Methods: Children underwent two bone mass scans at ages nine and 10 years using dual X-Ray absorptiometry. PA levels and calcium intakes were assessed using questionnaires. Data were analyzed by regressing change in bone mineral content (BMC) and bone area (BA) from age nine to 10, against BA (for BMC), height and body weight. The residuals were saved and called residualized BMCGAIN and BAGAIN. Residualized values provide good indication of weight, height and BA-matched accumulation rates.
Results: White children had significantly higher PA levels and calcium intakes than black children. Most active white males had significantly higher residualized BMCGAIN and BAGAIN at the whole body, hip and spine but not at the radius, than those who were less active. Most active white females had significantly higher residualized BAGAIN at all sites except the radius than less-active girls. No such effects were seen in black children. There was no interactive effect on residualized BMCGAIN for calcium intake and PA (except at the spine in white girls).
Conclusion: Bone mass and area gain is accentuated in pre- and early-pubertal children with highest levels of habitual physical activity. Limited evidence of an effect of dietary calcium intakes on BMC was found.