Bone Abstracts

Gross motor development is positively associated with bone mineral density in teenagers and is thought to be mediated by lean mass. Age at walking is an accepted milestone in motor development, achieved by 50% of infants by 12 mo of age according to the WHO Motor Development Study.

Objective: To examine if walking within 12 mo of age is related to bone mineral density (BMD) and if this relationship is mediated by lean mass.

Methods: Participants (35 girls; 46 boys) of a randomized dose-response trial of vitamin D (NCT00381914) returned at 3 y of age for assessment of anthropometry, whole body composition, bone mineral content (BMC) and BMD using dual-energy x-ray absorptiometry (Hologic 4500 APEX v13.3:3). Children were term born, appropriate size for gestational age, born to healthy mothers and breastfed. Age at walking was parent-reported and categorized according to: ≤12 mo or >12 mo. Activity was surveyed using the Habitual Activity Estimation Scale. Mixed model ANOVA accounted for maternal education, gestational age at birth and age at follow-up.

Results: At follow-up, n=37 walked on their own before or at 12 mo of age compared to n=44 after 12 mo (mean±S.D.: 10.8±1.0 vs 14.6±1.7 mo, P<0.0001). No differences were observed in weight or gestational age at birth, maternal characteristics, or BMI Z-score or activity level at 3 y of age. After accounting for covariates, walking by 12 mo was associated with greater lean mass (10183±1233 vs 9621±1149 g, P=0.019), BMC (610.22±46.53 vs 591.85±46.61 g, P=0.023) and BMD (0.637±0.040 vs 0.619±0.036 g/cm², P=0.023). No differences were observed in fat mass or percent body fat. The relationships between walking by 12 mo and BMC (P=0.347) and BMD (P=0.195) were eliminated by including lean mass in the model.

Conclusion: These data suggest that in healthy term born children, earlier attainment of walking relates to greater lean and bone mass by 3 y of age and that the relationships to bone are mediated by lean mass.

Funding: Canadian Institutes of Health Research.

Disclosure: The authors declared no competing interests.